Lead Toxicity and Heme Biosynthesis

transcript

Lead Toxicity and Hem e Biosynthesis

M IC H A EL M. LUBRAN, M.D.

Harbor-UCLA Medical Center, Torrance, CA 90509

ABSTRACT

L ead intoxication results in a d isturbance of hem e biosynthesis, its degree d ep en d in g on the severity and duration of exposure to lead. A m ild secondary, sideroblastic anem ia is common; basophilic stippling may occur, especially in severe lead poisoning. Increased excretion in the u rine of delta-am inolevulin ic acid and coproporphyrin II I may occur; po rphob ilin ogen excretion is not usually increased. D elta-am inolevulinate dehydratase, coproporphyrin oxidase, and ferrochelatase activities are reduced; delta- am ino levu linate synthetase activity is increased . E rythrocyte pro toporphyrin (F E P and ZPP) is increased . R ecen t know ledge of th e hem e b iosynthetic enzym es is rev iew ed and the significance of F E P and ZPP discussed. A b rie f history is given of the re lationship of lead toxicity to the porphyrins.

H istorical In troduction

L ead has b een w idely used since anc ien t tim es, probably because of its low m elting po in t and ease of working and the abundance o f lead-con tain ing m inerals and ores. L ead poisoning also has an anc ien t history.2’5 T he first association of exposure to lead w ith abdom inal colic and skin pallor (effects of chronic lead poisoning) has b een a ttribu ted to H ippocrates (ca. 370 B.C.). T he victim was a m etal extractor. By the first century A.D., the re lationship b e tw een chronic lead ingestion and abdom inal colic, skin pallor and “ sw elling of the body” was well-known. In m edieval tim es, lead poisoning, know n as saturnism (saturn was the alchem ists’

nam e for lead) and the occurrence of “ pals ies” was recognized. T he first com plete clinical descrip tion of lead poisoning was given in 1839,55 and the anem ia of lead poisoning was first directly verified by counting the red cells in 1840.4 Punctate basophilia of the red cells was first recorded in 1899.8 This observation followed s h o rtly a f te r th e in tro d u c tio n o f th e R om anowsky stains and was considered for m any years to be a sensitive and im portan t diagnostic criterion of lead poisoning. It m ust be appreciated that, un til the d ev e lo p m e n t o f s e n s it iv e m e th o d s for m easuring blood lead, the diagnosis of lead poisoning was based on a history of chronic exposure to lead and the characteristic signs and symptoms, includ ing the

0091-7370/80/0900-0402 $01.80 © Institute for Clinical Science, Inc.

LEAD AND HEME BIOSYNTHESIS 4 0 3

lead b lue line of the gums, rarely seen today. Punctate basophilia is frequently found in florid cases of lead poisoning.5

O ccu rren ce o f red co lo red u rin e in some cases o f lead poisoning was an old observation. T he color was a ttribu ted to blood. Stokvis54 (1899) appears to be the first to have iden tified “ hem atoporphyrin ” in the u rine and feces of a patien t su ffe ring from sev e re le ad po ison ing . H em atoporphyrin, p roduced by treating hem oglobin w ith strong su lphuric acid, is now know n no t to occur in u rine in health or d isease . T he p ig m en t d esc rib ed by Stokvis and m any la te r w orkers w ere probably a m ixture o f coproporphyrin and uroporphyrin. T eleky56 (1919) described hem atoporphyrinuria as an early diagnostic sign of lead poisoning, b u t the recognition of the true nature of the urinary pigm ent was no t possib le un til the chem ical s tru c tu re o f th e p o rp h y rin s h ad b e e n w o rk e d o u t by K iis te r34 (1920). G rotepass25 (1925) show ed that the u rinary porphyrin was coproporphyrin III, identical w ith the naturally occurring coproporphyrins o f blood. Van den Bergh and G rotepass (1933)59 described patients w ith lead poisoning, who had an increase of red cell porphyrin , type III , w ithout p o rp h y r in u r ia . T h e y re c o m m e n d e d m easurem ent of red cell porphyrins in the investigation of lead poisoning. T he technical difficulty of this m easurem ent restricted to its use to specia lized centers. T h e sy n th e s is o f th e p o rp h y r in s by F ischer over a period o f 20 years (described in his book o f 1937)19 m arked the beginning of the m odern era of the study of p o rphyrin chem istry . A pplication of th is k n o w le d g e to th e in v e s tig a tio n of d isease was m ad e p o ss ib le by th e d e v e lo p m e n t o f n ew a n a ly tic a l tech n iq u es .15,24,44,62

T he biochem istry of the porphyrias was investigated in m any centers, and some of the results w ere app lied to the diagnosis of lead poisoning. M ethods, although still complex, w ere sim plified for clinical use,

a n d f re e e ry th ro c y te p ro to p o rp h y r in m easurem ents becam e possible. W atson an d co -w o rk ers63 d em o n stra ted an in c rease in F E P in c h ild re n w ith lead poisoning. A great advance in the study of lead poisoning in ch ild ren was m ade possib le by the in troduction by P iom elli45(1973) of a sim ple, rapid, m icrom ethod for free erythrocyte protoporphyrin. It m easu red coproporphyrin as well, b u t fortunately (for the analyst) this porphyrin is at the m ost only slightly increased in lead poisoning, w hereas free erythrocyte protoporphyrin is often m arkedly increased. O ther m icrom ethods follow ed,16,48 b u t the next (and m ost recent) major advance was the discovery by Lam ola and co-workers(1974) tha t free erythrocyte pro toporphyrin was essentially zinc pro toporphyrin .53 A new instrum ent, the H em atofluorom e- te r,9,10 was designed and is now com m ercially availab le .* Z inc p ro to p o rp h y rin concentration can be m easured by its use in a drop o f blood, w ithout prior chem ical treatm ent.

W hile the chem istry of the porphyrins was b e in g e luc ida ted , the b iochem ical synthetic pathw ay was also b e in g investigated. Progress was rapid after isotopes (both rad ioac tive and non-rad ioactive) had becom e available. By 1954, the essen tial features o f hem e biosynthesis w ere know n50 and the in vitro effects of lead on th e en z y m e s h ad b e e n in v e s tig a te d . H ow ever, the difficulty of assaying the enzym es has p rev en ted their w idespread application to the investigation o f lead poisoning. In fact, only one en zy m e— d e lta -a m in o le v u lin a te d e h y d ra ta se or ALA-D— has b e e n ex tensive ly in v es tigated and has b een shown to decrease in activity as blood lead rises.22 T he difficulties of m easurm ent arise because the en zym es m u st b e m easu red in re d c e ll hem olysates or leucocyte extracts. M ost of

* ESA model 4000, (Environmental Science Associates, Bedford, MA 01730); Aviv (Aviv Associates, Lakewood, NJ 08701); and Bromberg (Dr. Bromberg, Weston MA 02193).

4 0 4 LUBRAN

the enzym es are labile and m ust be reactivated. Irreversib le losses of enzym e activity may occur during the extraction procedures, and lead contam ination may occur during these and the analytical stages.

S ince 1950, a n ew porp h y rin in the b iosynthetic pathw ay has b een discovered (harderoporphyrin , 1974)32 and a previously postu lated enzym e has been shown to exist (protoporphyrinogen deh y d ro g en ase29,30). In sp ite of th e vast am ount of work on lead poisoning, little is k now n ab o u t th e m ech an ism o f lead toxicity, apart from its effects on hem e biosynthesis in the bone m arrow and depression of globin synthesis.23,24,47 C urrently, w hole blood lead and zinc (or free) erythrocyte protoporphyrin are m easured as indicators o f lead toxicity, b u t they may very w ell no t be the m ost appropriate ones.

B iosynthesis of H em e

To fo llow th e v ario u s s tep s in th e biosynthesis of hem e, a b rie f account of the structure of the porphyrins w ill be given. T he p aren t com pound, porphyrin, (figure 1) is an essen tially p lanar com pound m ade of four pyrrole units linked by CH (m ethene) groups. T he pyrro le rings are le tte red A,B,C,D. Side chains are attached, two in each ring, in positions 2,3; 7,8; 12,13; and 17,18. In the porphyrin

com pounds, there are only two hydrogen atoms a ttached to the four nitrogen atoms; in the porphyrinogen com pounds, each n itrogen has a hydrogen atom and the m e th e n e b r id g e s a re r e d u c e d to m eth y len e (C H 2). T he porphyrinogens are converted to the porphyrins by non- en zy m atic ox idation ex cep t for p ro to p o rp h y r in IX. T h e s id e -c h a in s a re a c e tic ( —C H 2C O O H or A), p ro p io n ic ( - C H 2C H 2C O O H or P), v in y l ( - C H = C H 2 or V) and m ethyl (—C H 3 or M). T h e side chains of the po rphyrins in volved in hem e synthesis are given in tab le I. It w ill be seen that they all have P in positions 13 and 17. T he naturally oc- curing porphyrin in this pathw ay are type III, i.e., the side chains or rings A and B are in an inverted order, e.g., A,P : P,A. In the I series, they are in cyclic order, e.g.,A ,P :A ,P . T h e fin a l p o rp h y r in in th e series is p rotoporphyrin IX, the num ber referring to one of the stereoisom ers w ith the sam e m olecular formula. It is still a type II I porphyrin . H em e form ed by in sertion o f a ferrous (F e 2+) atom into protoporphyrin IX, is p roperly called hem e b (figure 2); hem e a and hem e c, in w hich differen t side chains occur in rings A andB, are found in some of the cytochrom es, w hich are also hem e proteins.

T he m ain stages in the biosynthesis of hem e are the form ation of a pyrrole, com bination of four pyrroles to form a porphy

FlGURE. 1. (A) The parent porphyrin structure, with modern num bering of the atoms. (B) Heme b. (C)Porphobilinogen. The carbons at 5,10,15,20 are m ethene (CH) in the porphyrins and m ethylene (CH2) in the porphyrinogens.

LEAD AND HEME BIOSYNTHESIS 405

rin, conversion of the acetic ac id side chains in rings A and B to m ethyl, and then conversion of the acetic groups in rings C and D to m ethyl, to form protoporphyrinogen IX. This is oxidized to protoporphyrin IX, iron is inserted , and hem e results. Every step is the resu lt of the action o f o n e or m o re e n z y m e s , w h ic h are situated in the m itochondrion or cytosol. H em e synthesis can therefore only occur in cells contain ing m itochondria; in particu lar, it can n o t occu r in th e m atu re erythrocyte. M ost cells synthesize hem e, b u t o n ly e ry th ro c y te p re c u rso rs can synthesize the globins requ ired for the fo rm ation o f h em o g lo b in . O th e r ce lls syn thesize ap p ro p ria te apopro te ins for the formation of o ther hem e proteins. T he outline of hem e synthesis is given in figure 3, w hich also indicates the location of the enzym es. T he essential features of the synthesis have b ee n confirm ed in the hum an by study of patien ts w ith porphyrias ow ing to enzym e deficiencies.

St a g e O n e

T he first stage of the biosynthesis of hem e is the condensation of glycine and su c c in y l-c o e n z y m e A to y ie ld d e lta - a m in o lev u lin ic ac id (SALA), N H 2C H 2 C O C H 2C H 2C O O H . S u cc in y l-C o A is a re a d ily a v a ila b le su b s tra te fo rm ed from su c c in a te an d co e n zy m e A by th e enzym atic ac tion o f succinyl-C oA synthetase, w ith the aid of nucleosidetri- phosphate (NTP) and m agnesium ions. It is a lso fo rm e d from m e th y lm a lo n y l- coenzym e A through the action of methyl- m alonyl iso m erase an d co en zy m e-B 12 (in vitam in B12 deficiency in man, m ethylm alon ic ac id ex c re tio n is in c reased ). T he condensation of glycine and succinyl- CoA is cata lyzed by 5-am inolevulinate synthetase (ALA-S). Pyridoxal phosphate obligatory for the reaction.42

NH2CH2COOH + HOOC.CH2CH2CO(SCoAH> NH2CH2COCH2CH2COOH + CoASH + C 02

In th is re ac tio n , pyridoxal p h o sp h a te , w hich is bound to the enzym e, first com bines w ith glycine to form a Schiff base (com bination of the -C H O of pyridoxal and the N H 2 — of glycine to form —C H = N —). As a result, the usually stable glycine becom es h ighly reactive, forms a carban- ion (i.e., a carbon atom in the com pound bears a negative charge), and com bines w ith succinyl-CoA.1

ALA-S is the rate-lim iting enzym e of h em e sy n th esis .50*52 It is an in d u c ib le m itochondrial enzym e norm ally p resen t in low concentration. Its p roduction is stim ulated w hen porphyrins accum ulate for various reasons, and it is in h ib ited by the end product, hem e (negative feed back ). A lth o u g h ALA-S ac ts in th e m ito c h o n d rio n , th e en zy m e is sy n th e s iz e d in th e c y to p la sm .26 I t can a lso b e in d u c e d by h y p o x ia 60 an d ery th ropo ie tin .11

St a g e T w o

T he second stage involves the condensation of tw o m olecules of S-ALA to form porphobilinogen , the structural u n it o f the porphyrins, w ith the elim ination of two m olecules o f w ater (figure 3). T he cys- to so lic en z y m e , 8 -a m in o le v u lin a te dehydratase(ALA-D), catalyzes the reaction. T he m echanism of this rem arkable c o n d e n s a tio n has b e e n e x te n s iv e ly studied, b u t has b een only partially e lucidated .51 ALA-D has a m olecular mass of280,000 and consists of e ig h t iden tical subunits, each of m olecular mass 35,000 occupying the corners of a cube. Activity of the enzym es requ ires SH groups. T he enzym e also contains about four to six Z n2+ atoms p e r m olecule. T he enzym e com bines w ith its substrate S-ALA forming a Schiff base through the e-am ino group of lysine. O nly four of the eigh t s u b u n its form S c h iff b a se s . A n o th e r am inoacid, p robably h istid ine ,58 is also involved in the enzym atic reaction.

It has b een p roposed6 th a t a functional d im er (two subunits o f ALA-D) is form ed

406 LUBRAN

by several in teractions, e.g., ionic and hydrogen-bonding, and covalent linkage of SH groups w ith Z n2+. Two m olecules of S-ALA are sandw iched b etw een the subunits; one m olecu le of S-ALA forms a Schiff base b e tw een its carbonyl (CO) group and the e-N H 2 of lysine. H istid ine, w hich forms part of the active cen ter, transports a proton from the C H 2 next to th e CO, c rea tin g a reac tiv e carban ion (C~ H). A series of reactions th en takes place, resu lting in the com bination of two m olecules of S-ALA and the formation of porphobilinogen. ALA-D is deactivated by oxidation o f its SH groups. Activity may be restored in vitro by reduction w ith sul- phydryl reducing agents, such as glutath io n e , d i th io th re i to l an d m ercap - toethanol. Activation by zinc ions occurs only w h en th e enzym e is in its reduced state. I t is p robab le that in vivo the enzyme is m ain tained in its reduced state by g lu ta th io n e r e d u c ta s e 14 an d p o ss ib ly other enzym e systems.

S t a g e T h r e e

T he th ird stage of the biosynthesis, the enzym atic polym erization of four m olecules of porphob ilinogen (PBG) to form uroporphyrinogen (UPG) takes place in th e c y to so l.20 A m m onia is p ro d u c e d ; h en ce a d eam in ase is invo lved . UPG

TABLE I

Structure of Porphyrins Involved in and Related to Heme Biosynthesis

Compound Side A

Chains in Rinqs B C D 7 8 12 13 17 18

Uroporphyrinogen I A P A P A P A PUroporphyrinogen III A P A P A P P ACoproporphyrinogen III M P M P M P P MHarderoporphyrinogen III M V M P M P P Mprotoporphyrinogen IX M V M V M P P MPemptoporphyrin M H M V M P P MPorphyrin S - 411 M Acr M P M P P M

A = c h 2c o o h V = c h= c h 2P = CH2CH2COOH Acr = CH=CHCOOHM = CH^ H = Unsubstituted hydrogen

(table I) is a cyclic tetrapyrrole occurring in the body in tw o isom eric forms, UPG I and U PG I I I . T h e fo rm er occurs in trace am ounts in norm al subjects b u t is in creased in ery thropoietic porphyria , in w hich d isease there is a deficiency o f uroporphyrinogen II I cosynthetase. UPG III is the p aren t of the naturally occurring hem e pro teins in man. C onversion of PBG to UPG II I in the cytosol is affected by two' en z y m e s a c tin g to g e th e r (n o t in s e quence), porphobilinogen deam inase and uroporphyrinogen II I cosynthetase. T he enzym e p a ir is som etim es ca lled por- ph ob ilinogenase7 and can be assayed as such. U PG I is form ed by a sequentia l head to tail condensation of four m olecules o f PBG and the elim ination of four m olecules of am m onia. T he resu lting te trapyrrole has eight side chains in the cyclic order A,P,A,P,A,P,A,P. In this condensation, th e C H 2 o f the C H 2N H 2— side chain o f PBG com bines w ith the carbon atom (m arked *, figure 3) of a second PBG m olecule, thus preserving the cyclic ord er. I f ac tin g a lo n e , PB G d ea m in a se catalyzes head to tail condensation, resu lting in U PG I.

In head to head condensation, two PBG m o le c u le s c o m b in e th ro u g h th e ir C H 2N H 2 side chains, y ield ing a dipyrrole w ith the side groups inverted (A,P,P,A). W hen this condensation is effected chem ically, am m onia and form aldehyde are produced. T he side groups are in the in verted o rder A,P,P,A in ring D of UPG III, suggesting tha t head to head condensation has occurred. In the enzym atic production o f U PG III, form aldehyde cannot be detec ted . C osynthetase enters into a loose association w ith the deam inase and changes its m ode o f action so that the m ixed enzym e reaction results in head to head condensation, a dipyrrole is form ed and an active m ethyl group p roduced . F u r th e r co n d en sa tio n o f a PBG takes place at this active m ethyl group giving a tripy rro le and active m ethyl group; finally, a fourth PBG is added and a closed ring tetrapyrro le formed. T he products of

LEAD AND HEME BIOSYNTHESIS 4 0 7

the in term ediate reaction do not escape from th e en zy m e (z ip p e r effect). T he cosynthetase is always in great excess of the deam inase, thus assuring continuous form ation of U PG III. T he deam inase- cosynthetase enzym e system is thus functionally a m ethyl transferase.

St a g e F o u r

T he fourth stage o f the biosynthesis also occurs in the cytosol. UPG II I is partially decarboxylated enzym atically to y ield coproporphyrinogen I I I (C PG )30 (table I). U ro p o rp h y rin o g en d ecarboxy lase converts th e four acetic (a) side chains to m ethyl (M) groups sequentia lly , giving rise successively to porphyrins containing sev en , six, five an d fin a lly fou r ac id groups. In health , only trace am ounts of the seven, six and five forms are found b u t co n s id e rab le q u a n titie s m ay occu r in some of the porphyrias, particularly in porphyria cu tanea tarda, in w hich there is a deficiency of UPG decarboxylase. O f the 24 theoretical routes o f decarboxylation b e tw een UPG II I and C PG III, the one preferred biologically is clockw ise, starting in ring D and progressing through rings A,B and C. T he m echanism of the enzym atic decarboxylation is unknow n.

St a g e F iv e

In the fifth stage, coproporphyrinogen II I is converted enzym atically to protoporphyrin IX (PP IX)31 (table I). T he reactions take place in th e m itochondrion. Two enzym es act sequentia lly , the first in two steps. C oproporphyrinogen oxidase catalyzes the oxidative decarboxylation of C PG II I to protoporphyrinogen IX (PBG IX); protoporphyrinogen dehydrogenase com pletes the oxidation of protoporphyrin IX (PP IX).29 O xidative decarboxylation converts the propionic acids (P) o f rings A and B to vinyl (V). T h e P and V conversions take p lace in sequence, the first occurring in ring A to y ield a tripropionic acid p o rp h y rin ca lle d h ard ero p o rp h y -

rinogen. T his porphyrin is found norm ally in sm all am ounts in b ile, bone marrow, and erythrocytes (as harderoporphyrin). Its isom er (V in ringB ) is not found in man. Tw o o ther recently d iscovered porphyrins are degradation products of harderoporp h y rin , w h ic h do n o t o cc u r in th e b iosynthetic pathw ay o f hem e. T hey are pem ptoporphyrin , in w hich V in ring A of harderoporphyrin is rep laced by H , and porphyrin S 411 found in m econium , in w hich the V o f ring A is rep laced by an acrylic group ( — C H =C H C O O H ). In the decarboxylation process, CPG II I is attached to the enzym e through the P side chain of ring A. Conversion of P to V th en occurs. T he harderoporphyrin form ed rem ains attached to the enzym e b u t rap id ly rotates anticlockw ise to bring the P side chain of ring B into the active cen te r of the enzym e. T he second oxidative decarboxylation now takes place and the p roduct (PPG IX) no longer having P groups is re leased from the enzym e. C PG oxidase has strict substrate specificity, the groups M, M, P, M being requ ired in tha t cyclic order on the pyrroles, although the first M can be rep laced by V, H, or ethyl (E).

P ro toporphyrinogen d eh y d ro g en ase29 is a recently d iscovered enzym e w hich oxidizes PPG IX to PP IX. I t w ill also o x id iz e , b u t m u ch less e f f ic ie n tly , harderoporphyrinogen and PPG X III, bu t PPG IX is the p refered substrate.

St a g e Six

T he final stage of hem e biosynthesis is the insertion of F e 2+ into protoporphyrin IX to form h em e37,61 (figure 2). T he en zym e involved is ferrochelatase, and the reaction takes place in the m itochondrion. In v itro experim ents on hepa tic ferrochelatase have show n that its activity is in h ib ited by d ivalen t cations such as Z n 2+, C o2+, and M nz+ in am ounts about equal to th a t of th e PP. F u ll enzym e activity is re stored by C u 2+ in m uch low er concentrations. Ferrochelatase activity requ ires reducing conditions provided in v itro by

4 0 8 LUBRAN

thiol reducing agents and ascorbate and in v ivo p ro b ab ly by re d u c e d g lu ta th io n e (GSH). T he enzym e probably has sim ilar requirem ents in the erythrocyte p recursors. F errochelatase activity is subject to substrate inh ib ition by PP and to feedback inh ib ition by hem e. A lthough ALA-S is the rate lim iting enzym e o f hem e production , fe rro ch e la tase p lays a regu la to ry role.52

Specific inh ib ition of ferrochelatase by D D C (3 ,5 -d ica rb o x y e th y l-l,4 -d ih y d ro - collidine) and m etals such as lead results in induction of ALA-S and accum ulation o f som e po rphyrin in term ed iates. I t is probable that copper is req u ired in vivo for the full enzym atic activity o f ferroc h e la ta se . I t is b e l ie v e d th a t p y ri- doxal p h o s p h a te is a co fac to r o f fe rro ch e la tase . Som e of th e effects of pyridoxine deficiency in m an involving iron, e.g., ringed sideroblasts and accum ula tio n o f PP in th e s e c e lls a n d in siderocytes, can be explained if pyridoxal phosphate is involved in ferrochelatase activity and is bound to the enzym e. The cofactor can th en chela te ferrous iron and m ain tain it in th e red u ced state. L ead m ight com pete w ith iron in this process, or m ight in terfere w ith the b ind ing o f the coenzym e by the enzym e.

T he en zy m es, su b s tra te s and in te r m ediates involved in hem e biosynthesis m ust no t be considered in isolation. H em e synthesis occurs in cells undergo ing a w ide range of o ther m etabolic activities. In particular, hem e is incorporated into hem e proteins (hem oglobin, m yoglobin, cytochrom es, catalase, etc.) and is d e stroyed by hem e oxygenase to produce b ile pigm ents. Some of the enzym es require su lphydryl com pounds; therefore, enzym es concerned w ith g lu tath ione m etabolism m ay be ind irectly involved in hem e biosynthesis (e.g., g lu tath ione re ductase , g lu ta th io n e syn thetase , g lu tath io n e p e ro x id a s e 28,40). H y d ro g e n peroxide and “ ac tiv e” oxygen are produced during m itochondrial activity and

m ust b e rem oved by appropriate oxidases, peroxidases, and superoxidases. I t is possible th a t som e of the effects of toxic m etals, such as lead on hem e biosynthesis, m ight be due to th e ir in teraction w ith these o ther enzym e systems.

L ead Toxicity and H em e B iosynthesis

T h e hem ato log ical m anifesta tions of chronic lead po ison ing vary w idely in th e ir severity and do not correlate w ell w ith blood lead concentrations. Anemia, although com m on, is usually of m ild seve r ity an d h e m o g lo b in c o n c e n tra tio n s rarely fall below 9 g p e r dl. T he red cells a re u s u a lly n o rm o c y tic an d n o rm o ch ro m ic , b u t m ay b e m ic ro c y tic an d hypochrom ic, especially in children. T he m ean corpuscular hem oglobin concentration (M CHC) is only m oderately reduced . T here is usually some polychrom asia and a m ild reticulocytosis (2 to 7 percent). B a so p h ilic s t ip p lin g is o cc as io n a lly found; how ever, w hen it is presen t, it is no t pa th o g n o m o n ic o f lead po ison ing . T h e re m ay b e som e an isocy tosis an d p o ik ilocy tosis, an d occasionally a few nuclea ted red cells are seen. T he osm otic fragility o f the red cells is decreased, b u t th e ir m echan ical fragility is increased , and th e ir life-span decreased. T he bone m arrow shows an erythroid hyperp lasia and stipp ling o f m any nuclea ted red cells, som e o f w hich are ring sideroblasts. T he anem ia o f lead poisoning may thus be c lassified as a secondary sid ero b lastic anem ia. T h e hem ato logical changes of lead poisoning are m ore pronounced in subjects having iron deficiency.

T he chem ical findings in the u rine in lead poisoning, as th ey re la te to hem e b iosynthesis, a re 5,25,53: increased delta- am in o lev u lin ic ac id (8-ALA) excretion (g rea te r th an 20 m g p e r 1); in c re ased c o p ro p o rp h y rin I I I (C P) e x c re tio n (greater than 0.5 m g p e r 1); norm al or slightly increased porphobilinogen (BG) excretion (normal: less than 2 m g p e r 1).

H ow ever, as 8-ALA and CP excretion may be w ith in norm al lim its in subjects w ith defin ite lead poisoning, these tests are of lim ited value in th e diagnosis o f lead p o iso n in g or in th e a sse ssm en t o f its severity.

L ead has b een show n to depress the activity of th ree of th e enzym es of the h e m e b io s y n th e t ic p a th w a y , n am e lyS-am inolevulinate dehydratase (ALA-D), co p ro p o rp h y rin o g en oxidase (CPG -O ) and ferrochelatase.14 ALA-synthetase activity is increased .14 T he o ther enzym es (UPG II I synthetase activity has not b een in v e s tig a te d ) a re u n a f fe c te d by le ad poisoning. T he m itochondrial enzym es (ALA-S, CPG -O , ferrochelatase) are assa y e d in e x tra c ts o f le u c o c y te s , th e cytosolic enzym es in red cell hem olysates.12 Suitable assay m ethods are availab le, bu t, w ith the exception of ALA-D, are too involved for use in the routine clinical laboratory. T he sensitivity o f ALA-D to ex tran eo u s lead co n tam in a tio n m akes m easurem ent of the activity o f this en zym e un re liab le , un less stringent p recau tions are adop ted to exclude lead.

In h ib itio n o f fe rrochela tase explains th e in c re a sed p ro to p o rp h y rin IX (PP) seen in the red cells in lead poisoning; in h ib it io n o f ALA-D and in c re a se in ALA-S account for the increased excretion o f 8-ALA; inh ib ition of CPG-O explains the increase in CP found in red cells and urine. H ow ever, it is not conven ien t to assay h em e enzym es ro u tin e ly in th e diagnosis of lead poisoning because of technical difficulties and because all of the enzym es w ould have to b e m easured to estab lish the pattern characteristic of lead poisoning. As a practical alternative, atten tion has b een focused on the m easu rem en t o f ery th rocy te pro toporphyrin (EPP) as an indicator of lead poisoning.

E rythrocyte P rotoporphyrin

It is now w ell estab lished that E P P is in c reased in lead poisoning subjects.47

T he m ethods u sed for m easu ring E P P vary in detail, b u t essentially involve extra c t io n o f p o rp h y r in s in to an e th y l acetate-acetic acid m ixture and extraction o f the porphyrins from the organic solvent w ith hydrochloric acid. A ccording to the conditions used, varying am ounts o f p rotoporphyrin IX, coproporphyrin II I , and u roporphyrin I are extrated and, possibly, o th e r p o rp h y r in s . P ro to p o rp h y r in is m easu red fluorim entrica lly by su itab le c h o ic e o f e x c ita tio n a n d e m is s io n w a v e le n g th s . T h e p o rp h y r in s w e re nam ed free erythrocyte pro toporphyrin (FEP) in the b e lie f that they are m etal-free (iron protoporphyrin does no t fluoresce u n d er these conditions). P iom elli’s m icrom ethod for F E P ,45 w hich is w idely u sed in its original form or w ith som e m odificat io n s ,16-43’48 m easu res PP an d C P , a lthough the latter is a relatively m inor constituent. C oproporphyrin I is u sed as a standard and an em pirical correction factor is em ployed to convert C P into PP flu o re scen ce . A lthough th e m e th o d is s im p le , i t does no t give re p ro d u c ib le an sw ers in d iffe re n t h an d s , p ro b a b ly because of incom plete extraction of porphyrins, the p resence of o ther porphyrins b e sides PP, and the variability o f the correction factors w ith th e fluorom eter used. F E P m ethods m ust therefore be conside red as screening m ethods. In a recen t interlaboratory com parison o f E P P using extraction procedures,31 the coefficient of variation ob ta ined from 45 laboratories was 16.8 p ercen t for a low value (0.72 mg p e r 1) and 18.1 p ercen t for a h ig h er value (1.94 m g p e r 1), although the m eans w ere close to those of the re ference laborato ries . T h e c.v.s. for th e sev en re fe rence laboratories were, respectively , 9.4 p ercen t and 9.3 percent. I t is c lear tha t considerable im provem ent in m ethodology is req u ired if F E P is to b e u sed as a diagnostic test.

As has b een m entioned in th e h istorical in t ro d u c tio n , L am o la an d co w o rk e rs show ed th a t th e ery th rocy te p ro to p o r

410 LUBRAN

p h y r in in le a d p o is o n in g w as z in c p ro to p o rp h y rin (Z P P )39 and d ev ised a sim ple analytical m ethod for its m easu rem ent.36 Because the zinc is readily re m oved in the usual extraction p rocedures, it is convenien t to m easure Z PP in red cell hem olysates or w hole b lood w ithou t prior extraction . T h ree H em ato fluo rom eters are com m ercially availab le , em ploying the prin icip les of front surface illum ination re flec tance spec tro fluo rom etry .9,10 Excitation is at 420 to 430 nm and em ission is m easured at 580 to 680 nm. T he standard is a stable dye (Rhodam ine B dissolved in Krylon Black E nam el) deposited on a slide. In c id en t light illum inates in tu rn , for a fixed p erio d , a b lan k slid e (background), the standard, and the sam ple. T he in tensities o f the em itted light (I0, Is, and I b, respectively) are m easured and R = Rs (I„ - ID) / (Is - I„) calculated. Rs is the calibration constant of the instru m ent, d e term in ed by the m anufacturer u s in g b lo o d s o f k n o w n Z P P c o n c e n trations. T he in tensity of the ligh t em itted from the sam ple depends on the am ount of ligh t it absorbs and the am ount em itted by the fluorescent porphyrins. I t is in reality proportional to the ratio of Z PP to hem oglobin in the sam ple. H ow ever, to conform to the m ethod used in reporting results of extraction m ethods, the instrum ent d isplays the ZPP concentration in fig p e r dl of blood, assum ing a hem atocrit of 35 p e rcen t (for children) and p resum ably a norm al hem o g lo b in co n cen tra tio n . In th e in terlaboratory com parison re fe rred to, the c.v.s w ere 11.9 p ercen t and 9.9 p ercen t at the low and high concentrations of ZPP for 34 laboratories using Hematofluorome te rs , b u t th e m ean s w e re a b o u t 10 percen t low er than the reference values.

Errors arising from d ifferen t hem atocrit and hem oglobin concentration can be corrected , b u t others may occur. For example, the b lood m ust be fully oxygenated (reduced hem oglobin and m ethem oglo- b in absorb d ifferen t am ounts o f light from oxyhem oglobin); otherw ise, a m arked re

duction in apparen t ZPP concentration is reported . M any substances may in terfere w ith fluorescence m easurem ents. In particular, b iliru b in has b een reported to give erroneously high values o f Z P P .13 Using the b est com bination of filters, each mg p e r dl of b iliru b in increases the apparent Z PP concentration by 6 fig p e r dl,37b u t the erro r m ay be la rg e r w ith som e in stru m ents. No doub t o ther sources of error w ill be repo rted as the instrum ents b ecom e m ore w idely used. N evertheless, they are of great value as field instrum ents in th e d e te c t io n o f p o s s ib le lead poisoning.

In terp reta tion of F E P and ZPP C oncentrations*

F E P is, to a sm all extent, age and sex dependen t, being h ig h er in young child ren and fem ales.33 M ost investigators agree on a range in adults o f 35 to 65 fig per dl of red cells, w ith 80 fig p e r dl as the th ree standard deviation u p p er lim it and values g reater than 150 fig p e r dl indicative of lead poisoning (assum ing a normal h em o g lo b in c o n c e n tra tio n 18,57). In 20 norm al in fan ts (hem og lob ins b e tw e en11.0 and 13.1 g p e r dl) F E P ranged betw een 11.6 and 35.4 fig p e r dl of w hole blood; m ean, 23.1; s.d., 5.4 fig p er dl, corresponding to an u p p er lim it of about 100 fig per dl of red blood cells.46 A b e tte r index is th e ratio o f F E P (or ZPP) to hem oglobin. This ratio had a m ean value of 1.9 fig F E P p e r g o f H b in the sam e 20 infants. Ratios g reater than 17.5 fig p e r g are found in severe lead poisoning and values over 5.5 fig p e r g are suggestive of it.46

F E P and Z P P and th e F E P /H b 46 or F E P /H e m e35 ratio are increased in many o ther conditions besides lead poisoning,

* FEP refers to the erythrocyte protoporphyrin m easured using conventional extraction procedures. ZPP refers to the erythrocyte protoporphyrin measured (as described above) directly in red cells or w hole blood, or after the special extraction techniques.16

esp ec ia lly iron d efic ien cy an em ia .46-57 F E P is ra ised in the rare hered itary disorder, ery thropoietic protoporphyria,21 and also in febrile chronic infections in w hich values as h igh as 400 /xg p e r dl o f red cells m ay b e found.33 Values may b e ra ised in the post hem orrhagic state. In sheep , high values of F E P have b een reported in vitam in B123 and copper defic ien t an im als3 and in rabbits w ith vitam in E deficiency.17 R ece n tly , e le v a te d v a lu es have b e e n found in patien ts suffering from F ried re ich ’s ataxia and som e o ther ataxias.41

In essence, erythrocyte pro toporphyrin increases in conditions in w hich th ere is increased ery thropoiesis49 and as a resu lt of inh ib ition o f certain enzym es of the hem e synthetic pathw ay, e ith e r because o f an inborn error of m etabolism or b e cause of toxic substances. In the case of lead poisoning, there is a reciprocal re lationsh ip b e tw een blood lead concentration and F E P and Z PP concen tra tion . H ow ever, because of the large experim ental errors involved in the m easurem en t of F E P and ZPP, and also of blood lead, b o rderline elevations m ust be in terp re ted w ith caution. T he im portance of erythrocyte protoporphyrin is tha t its concentration reflects th e con tinued action of lead on the hem atopoietic system ; blood lead is m ore re la ted to the intake o f lead and its transport to various locations. F E P and Z PP are therefore m ore likely to be re la ted to the severity of lead poisoning than is blood lead. E xperience has show n th a t e ry th ro cy te p ro to p o rp h y rin m easu rem en t is a m ore sensitive ind icator of lead poisoning than is the m easurem ent of urinary porphyrins or delta-aminolevulinic acid and is easier to carry out than the assay of ALA-D activity.

References

1. Ak h t a r , M., Ab b o u d , M. M., Ba r n a r d , G., J o r d a n , P., and ZAMAN, Z . :M echanism and stereochemistry of enzymic reactions involved in porphyrin biosynthesis. Phil. Trans. Roy. Soc. Lond. B. 273:117-136, 1976.

2. Al d e r s ON, J.: Effects of lead on the organism. Lancet ¿¿:73-75 and 95-98, 1852.

3. Al l e n , S. H.: The effect of vitamin B12 deficiency and copper deficiency on the concentration of free protoporphyrin in the erythrocytes of sheep. Biochem. J. 63:461-469, 1956.

4. An d r a l and G a v a r r e t : Recherches sur les m odifications de proportion du sang. Ann. Chim. Physiol 75:309, 1840.

5. Aub, J. C ., F a ir h a l l , L . T., M in o t , A. S., a n d R e z n i k o f f , P.: L e a d p o is o n in g . M e d ic in e 4:1-250, 1925.

6. Ba t l l e , A. M. d e l C . and St e l l a , A. M.: Delta aminolaevulinate dehydratase: its mechanism of action. Inter. J. Biochem. 9:861-864, 1978.

7. Ba t l l e , A. M. d e l C ., d e Xif r a , E . A. W., and St e l l a , A. M.: A simple method for measuring erythrocyte porphobilinogenase, and its use in the diagnosis of acute interm ittent porphyrin. Inter. J. Biochem. 9:871-875, 1978.

8. B e h r e n d : Discussion on M. Litten: U eberen- doglobuläre Einschlüsse roter Blütkörpchen. D eutsche M ed. W chnschr 25 (suppl.):254, 1899.

9. B l u m b e r g , W. E., E is in g e r , J., L a m o l a , A. A., and Z u c k e r m a n , D. M.: The hematofluorometer. Clin. Chem. 23:270-274, 1977.

10. B l u m b e r g , W. E ., E is e n g e r , J., L a m o l a , A.A., and Z U C K E R M A N , D. M.: Zinc protoporphyrin level in blood determ ined by a portable hematofluorometer: a screening device for lead poisoning. J. Lab.Clin. M ed.89:712-723,1977.

11. B o t t o m l e y , S. S. and S m it h e e , G. A.: Effect of e ry th ro p o ie tin on bone m arrow A-aminolevulinic acid synthetase and heme synthetase. J. Lab. Clin. Med. 74:445-452, 1969.

12. B r o d ie , M. J., T h o m p s o n , G . G ., M o o r e , M. R ., B e a t t i e , A. D ., and G o l d b e r g , A.: Hereditary coproporphyria. Demonstration of the abnorm alities in haem biosynthesis in peripheral blood. Quart. J. Med. 46:229-241,1977.

13. Buhrmann, E., Men tzer , W. C., and Lubin , B.H.: The influence of plasma bilirubin on zinc protoporphyrin m easurem ent by a hemato- fluorimeter. J. Lab. Clin. Med. 92:710-716,1978.

14. C a m p b e l l , B. C ., B r o d ie , M. J., T h o m p s o n , G.G ., M e r e d i t h , P. A., M o o r e , M . R., and G o l d b e r g , A.: Alterations in the activity of enzymes of haem biosynthesis in lead poisoning and acu te h ep a tic po rphy ria . C lin . Sei. 53:335-340, 1977.

15. C a r t w r ig h t , G. E ., H u g u l e y , C . M ., andA S H E N B R U C K E R , H.: Studies on free erythrocyte protoporphyrin, plasma iron and plasma copper in normal and anemic subjects. Blood 3:501-525, 1948.

16. C h is h o l m , J. J., J r ., H a s t in g s , C . W., and C h e u n g , D. K.: Microfluorometric assay for protoporphyrin in acidified acetone extracts of whole blood. Biochem. Med. 9:113-135, 1974.

17. C h o u , A. C ., B r o w n , G. O., and F i t c h , C . D.: A bnorm alities o f iron m etabolism and ery-

412 LUBRAN

thropoiesis in vitamin — E deficient rabbits. Blood 52:187-195, 1978.

18. E is in g e r , J., B l u m b e r g , W. E., F is c h b e in , A., L i l i s , R., and Se l ik o f f , I. J.: Zinc protoporphyrin in blood as a biological indicator of chronic lead intoxication. J. Environ. Path. Toxicol. 1 ¡897-910, 1978.

19. F is c h e r , H. and O r t h , H.: Die Chem ie des Pyrroles. Akad. V erlagsgesellsch. L eipsig , 1937.

20. F r y d m a n , B., F r y d m a n , R. B., Va l a s in a s , A., L e v y , E. S., and F e in s t e in , G.: Biosynthesis of u roporphyrinogens from porphob ilinogen : mechanism and nature of the process. Phil. Trans. Roy. Soc. Lond. B. 273:137-160, 1976.

21. G a j d o S, A. and G a j d o s - T 6 r o k , M.: The specific enzyme deficiencies in five of the six know n v a rie tie s o f po rphy rias. In te r . J. Biochem. 9:917-920, 1978.

22. G r a n ic k , J. L., Sa s s a , S., G r a n ic k , B., L e v e r e , R. D., and Ka p p a s , A.: Studies in lead poisoning. II. Correlation betw een the ratio of activated to inactivated S-aminolevulinic acid dehydratase of whole blood and the blood lead level. Biochem. Med. 8:149-159, 1973.

23. G r ig g s , R. C.: Lead poisoning: hematologic aspects. Prog. Hemat. 4:117-137, 1964.

24. G r in s t e in , M. and W a t s o n , C. J.: Studies of p ro toporphyrin : p h o to e lec tr ic and fluoro- photometric methods for quantitative determ ination of protoporphyrin in blood. J. Biol. Chem. 247:675-684, 1943.

25. GROTEPASS, W.: Zur Kenntnis des im H am auf- tre te n d e n P o rphy rin s b e i B le iverg iftung . Zeitschr. Physiol. Chem. 205:193-197, 1925.

26. H a y a s h i, K., Yo d a , B., and KlKUCHI, G.: Diffe ren ces in th e m o lecu la r sizes of 5-aminolevulinate synthetases in the soluble and m itochondrial fractions of rat liver. J. Biochem. (Tokyo) 67:859-861, 1970.

27. H e il m e y e r , L: Disturbances in Heme Synthesis. Springfield, IL, Charles C Thomas, 1966.

28. H o w a r d , J. K.: Human erythrocyte glutathione reductase and glucose 6-phosphate d eh y drogenase activities in normal subjects and in persons exposed to lead. Clin. Sci. 47 :515-520,1974.

29. Ja c k s o n , A. H., E l d e r , G. H., and Sm it h , S. G.: The metabolism of coproporphyrinogen—III into protoporphyrin— IX. Inter. J. Biochem. 9:877-882, 1978.

30. J a c k s o n , A. H ., Sa n k o v i c h , H. A ., F e r r a m o l a , A. M ., E v a n s , N ., G a m e s , D . E ., Ma t l in , S. A., E l d e r , G . H., and Sm it h , S. G.: Macrocyclic interm ediates in the biosynthesis of porphyrins. Phil. Trans. Roy. Soc. Lond. B. 273:191-206, 1976.

31. J a c k s o n , K. w .: Interlaboratory comparison of results of erythrocyte protoporphyrin. Clin. Chem. 24:2135-2138, 1978.

32. Ke n n e d y , G. Y., Ja c k s o n , A. H., Ke n n e r , G. W., and Su c k l in g , C. J.: Isolation, structure and synthesis of a tricarboxylic acid porphyrin from the harderian glands of the rat. FEBS Lett. 6:9-12, 1970.

33. Kr a m m e r , A., C a r t w r ig h t , G. E., and W in -

T R O B E , M. M.: The anemia of infection: XIX. Studies on free erythrocyte coproporphyrin and protoporphyrin. Blood 9 :1 8 3 -1 8 8 , 1954.

34. K ü s te r , W .: Beitrage zur Kenntis der Prosth e tisch en G ru p p e des B lu tfarbsto ffs . Z. Physiol Chem. 2 0 9 :1 0 8 -1 1 6 , 1920.

35. L a b b e , R. F ., F in c h , C. A., Sm it h , N . J., D o a n , R. N., SOOD, S. K., and M a d a n , N.: Erythrocyte protoporphyrin/hem e ratio in the assessm ent of iron studies. Clin. Chem. 2 5 :8 7 -9 2 , 1979.

3 6 . L a b b e , R. F. and N i e l s e n , L .: C lin ical- biochem ical in te rp re ta tions of ery throcyte p ro topo rphy rin . F e rro ch e la ta se -p y rid o x a l phosphate studies. Porphyrins in human diseases. 1st International Porphyrin M eeting, Freiburg/Br. 1975. Karger, Basel, 1976, pp. 1 4 1 -1 4 7 .

37. L a m o la , A. A., E is i n g e r , J., a n d B lu m b e r g , W. E .: B iliru b in s e n s itiv ity o f z in c p ro to p o rp h y r in h e m a to f lu o ro m e te r . J. L ab . C lin . M ed . 9 3 :3 4 5 -3 4 8 , 1979.

38. L a m o la , A. A., E is i n g e r , J., B l u m b e r g , W . E ., K o m e ta n i , T., and B u r n h a m , B. F.: Quantitative d e te rm in a tio n o f e ry th ro cy te zinc protoporphyrin. J. L ab . Clin. Med. 8 9 :8 8 1 -8 9 0 , 1977.

39. L a m o l a , A. A. and Ya m a n e , T.: Zinc protoporphyrin in the erythrocytes of patients w ith lead intoxication and iron deficiency anemia. Science 186:9 3 6 -9 3 8 , 1974.

40. M e is t e r , A.: G lutathione synthesis. The Enzymes, vol. 10, 3rd ed. Boyer, P. D ., ed. New York, Academic Press, 1974, pp. 6 7 1 -6 9 7 .

41. M o r g a n , R. O., N a g l ie , G., H o r r o b in , D . F., and Ba r b e a u , A.: Erythrocyte protoporphyrin levels in patients with Friedreich’s and other ataxias. Can. J. Neurol. Sci. 6 :2 2 7 -2 3 2 , 1979.

42. N e u b e r g e r , A.: Aspects of the metabolism of glycine and of porphyrins. Biochem. J. 7 8 :1 -1 0 , 1961.

43. O r f a n o s , A. P., M u r p h y , W. H., and G u th m a , R.: A simple flurom etric assay of protoporphyrin in erythrocytes as a screening test for lead poisoning. J. Lab. Clin. Med. 8 9 :6 5 9 -6 6 5 ,1.977.

44. P a p e n d ie c k , A. a n d BONATH, K.: Ü b e r Por- p h y r in e au s B lu tfa rb s to f f Z. p h y s io l. C h e m . 144:6 0 -6 7 , 1925.

45. P lO M E L L l, S.: A micromethod for free erythrocyte porphyrins: the F E P test. J. Lab. Clin. Med.: 8 2 :9 3 2 -9 4 0 , 1973.

46. P io m e l l i , S., Br ic k m a n , A., and C a r l o s , E.: Rapid diagnosis of iron deficiency by measurem ent of free erythrocyte protoporphyrins and hemoglobin: The FEP/H em oglobin ratio. Pediatrics 5 7 :1 3 6 -1 4 1 , 1976.

47. R u b in o , G. F.: The role of lead in porphyrin metabolism. Symposium on the normal and pathological metabolism of porphyrins. Turin 1961. Torino, Panminerva.

48. Sa s s a , S., G r a n ic k , S., and Ka p p a s , A.: Effect of lead and genetic factors on heme biosynthesis in the human red cell. Ann. N. Y. Acad. Sci. 2 4 4 :4 1 9 -4 3 9 , 1975.

49. Sc h w a r t z , S. and W i k o f f , H . M.: The relation of erythrocyte coproporphyrin and protopor

p hy rin to e ry th ro p o ie sis . J. Biol. C hem . 194:563-573, 1952.

50. SHEMIN, D.: The biosynthesis of porphyrins. Harvey Lecture 50:258-284, 1954-1955.

51. SHEMIN, D.: 5-A m inolaevulinic acid dehydratase: structure, function, and mechanism. Phil. Trans. Roy. Soc. Lond. B. 273:109-115,1976.

52. Sin c l a ir , P. R. and G r a n ic k , S.: Heme control on the synthesis of delta-aminolevulinic acid synthetase in cultured chick embryo liver cells. Ann. N. Y. Acad. Sci. 244:509-518, 1975.

53. St a n k o v ié , M ., D jU R lé , S., M i l i é , S., and DJORDJEVIC, V.: The effects of lead on porphyrin biosynthesis with special consideration on uroporphyrin excretion. Environmental Health Aspects of Lead. Proc. Int. Symp. Amsterdam1972. pp. 507-515. Publ. Commission of the European Communities, CID , Luxembourg,1973.

54. S t o k v i s , B. J.: Z ur P a th o g en ese d e r Hámatoporphyrinurie. Z. Klin. M ed ic in 2 8 :l-9, 1895.

55. T a n q u e r e l D e s p l a n c h e s , L .: Traites des maladies de plomb, ou saturines. Paris, 1839.

56. T e l e k y , L.: D ie Frühdiagnose der Bleivergif- tung. Schr. G esam tgebeit d. G ew erbehyg. 5:1-27, 1919.

57. T h o m a s , W . J., Ko e n ig , H. M., L ig h t s e y , A. L., a n d G r e e n , R.: Free erythrocyte protopor

phyrin: hemoglobin ratios, serum ferritin, and transferrin saturation levels during treatm ent of infants w ith iron-deficiency anem ia. Blood 49:455-462, 1977.

58. T s u k a m o t o , I., Yo s h in g a , T ., and Sa n o , S.: Evidence for histidine as another functional group of 8-am inolevulinic acid dehydratase from beef liver. Biochem. Biophys. Res. Com- mun 67:294-300, 1975.

59. Va n d e n B e r g h , A. A. H. and G r o t e p a s s , W.: P orphyrinäm ie ohne P o rp h y rin u rie . Klin. Wschr. 12:586-597, 1933.

60. W a d a , D., Sa s s a , T a k a k u , F ., Ya n o , Y ., U r a t a , G ., and N a k a o , K.: D ifferent responses o f th e h ep a tic and e ry th ro p o ie tic 8-am inolevulin ic acid syn thetase o f m ice. Biochem. Biophys. Acta 248:585-587, 1967.

61. W a g n e r , G . S., D in a m a r c a , M .L., and T e p h l y , T . R.: Studies on ferrochelatase activity: ro le in regu la tion of h ep a tic hem e biosynthesis. Porphyrins in human diseases. 1st Int. Porphyrin M eeting, Freiburg/B r. 1975. Karger, Basel, 1976, pp. 111-122.

62. WATSON, C. J.: Erythrocyte coproporphyrin; variation in respect to erythrocyte protoporphyrin and reticulocytes in certain of anemias. Arch. Int. Med. 86:797-809, 1950.

63. W a t s o n , C. J., D e c k e r , E., and L ic h t m a n , H. C.: Hematologic studies of children with lead poisoning. Pediatrics 21 :40-46, 1958.

Lead Toxicity and Heme Biosynthesis

Documents