Vitamin c

VITAMIN C

Vitamin C

• Scurvy: Sore gums, painful joints and hemorrhages.

• Described by: Eber papyrus (1150 B.C.) and Hippocrates (420 B.C.)

• Prevalence mainly among seamen:– 1498: Portuguese, Vasco da Gama lost 60% of

crew– 1535: French, J Cartier, most crew developed

scurvy

Chemistry

• L-ascorbic acid (MW 176) and its oxidized derivative L-dehydroacorbic acid.

• They form a reversible redox system• Quencher of free radicals, reduce iron and

other metals and superoxide radical.• DHAA react with several amino acids to

form brown color• Stable in acidic condition

VITAMIN C

OH

OOH

HO

HO

HO

Sources

• Fruits, vegetables and organ meats (liver, kidney)

• Loss due to oxidation, in the presence of O2, heat, metal ions, neutral and alkaline conditions.

• Cooking, loss due to heating and water• Quick heating can protect by inactivating of

oxidases

Synthesis

• All Plants can synthesize Vitamin C• Most animals can synthesize vitamin C

except : Humans, Guinea pigs, red-vented Bulbul, fruit eating bat, rainbow trout, Coho salmon

• They lack: L-gulono-γ-lactone oxidase

The biosynthesis of L-ascorbic acid. α-D-glucose

glucose-6-phosphate

uridine diphosphate glucose

uridine diphosphate glucuronic acid

D-glucuronic acid-1-phosphate

D-glucuronic acid NADPH·H+

L-gulonate pentose phosphate NADP pathway L-gulono-γ-lactone O2

H2O2 L-gulono-γ-lactone oxidase

2-keto-L-gulonolactone

spontaneous isomerization

L-ascorbic acid

Absorption

• Occurs primarily by active transport– Saturable and dose dependent

• Simple diffusion and carrier-mediated contribute to small extent

• Prior to absorption, ascorbic acid may be oxidized to dehydroascorbate

• Within enterocytes, dehydroascorbate reduced to ascorbic acid – Dehydroascorbate reductase• Requires reduced GSH

Absorption

• Occurs primarily in distal portion of SI– % absorbed decreases with increased vitamin intake.• 16% at high intakes (6 g) and 98% at low intakes (<

20mg)• Over range of 20 to 12 mg/d, get 80-95%

absorption• Potential Factors Impairing Absorption

– Pectin (14.2 g/d)– Zinc (9.3 mg/d)– High iron content in GI can lead to destruction of AA

Absorption

• 200mg/day is a particular upper limit: give 12mg/L plasma level• 2,500mg/day: increase plasma level to

15mg/L• Actively absorbed–98% efficiency at 20mg/day–16% efficiency at ~12g/day

Absorption and transport

• Absorption: Active transport system, Na and ATP dependent.

• Efficiency of absorption decline with dose above 1g.

• Transport in plasma as ascorbic acid• Human cells become saturated at

100mg/day.• Cellular uptake by Active transport system,

Na and ATP dependent. • Uptake: glucose inhibits, insulin stimulates

Vitamin C• Transport across basolateral membrane – Sodium-independent carrier-mediated

transport • Transported in plasma in free form– Albumin may also transport some ascorbate

and dehydroascorbate (5% seen in circulation)• Ascorbate moves freely into cells– Concentration of ascorbate however is much

higher in some tissues• Adrenal gland, pituitary gland and eye

– May be actively transported into these tissues

Ascorbic Acid

• Dehydroascorbic acid– Taken up by red blood cells, lymphocytes and

neutrophils• Reduced to ascorbic acid within cells

• Tissue and plasma level reflect intake until intake exceeds ~90mg/day.

Tissue distribution

• In vital organs with active metabolism• Total body pool size about 1500mg• Half life about 20 days• Turnover rate 1mg/kg/day• Daily utilization breakdown is 0.2mg/kg fat

free weight

Metabolism• Occurs primarily in liver• Vitamin C is oxidized (removal of 2 electrons

and 2 protons) to dehydroascorbate– Follows the formation of

semidehydroascorbate radical.– Oxidized form may be reduced back to

ascorbate by GSH, NADH or NADPH.– Oxidized form may be further oxidized to 2,3-

diketogulonic acid.• Diketolulonic acid is cleaved into oxalic acid and 4

or 5 carbon sugars.

Urinary Excretion

• Oxidation: ascorbic acid, mono-dehydro-ascorbic acid, dehydro-ascorbic acid.

• Excretion: 20-25 % ascorbic acid and DHAA, 20% diketogulonic acid, 40-45% oxalate

• Dehydroascorbate, diketoglulonate, oxalic acid and excess ascorbate excreted in urine– 25% of vitamin C intake is excreted as oxalic acid– Amount of Vitamin C filtered and then reabsorbed by

kidneys depends on plasma vitamin C concentrations– Plasma levels above 1.4 mg/dL exceeds renal threshold

and vitamin C will not be reabsorbed.

Degradation of ascorbic acid

-2H

Ascorbic acid Dehydroascorbic acid +2H

+H2O

Ascorbate-2-sulphate Diketogulonic acid -C02

+ H2O Lyxonic acid

+O

+ H2O

Oxalic acid + Threonic acid -C02

-CO2 + H2O

+2H Xylonic acid

Xylose

Source: Basu and Schorah,1982.

Excretion

• Excretion reduced when intake is low• Urinary excretion–Body pool <1,500mg leads to only

metabolites in urine–Body pool >1500mg leads to

proportionately more ascorbate in the urine (can mask clinical tests)

Interactions With Other Nutrients

• Vitamin C increases intestinal absorption of nonheme iron– Reduces Fe3+ to Fe 2+ or forms a soluble complex

with the iron• Excessive iron in presence of vitamin C can

accelerate the oxidative catabolism of vitamin C

• Vitamin C aids incorporation of iron into ferritin

Interactions with Other Nutrients

• Vitamin C may increase absorption and excretion of heavy metals– Form chelates with metals

• Vitamin C intakes above 600 mg/d may interfere with copper metabolism

• Vitamin C helps keep folate in its reduced and active form.

Metabolic functions

• Electron transport• Antioxidant functions• Prooxidant properties• Enzyme cosubstrate functions:– Collagen synthesis– Neurotransmitter metabolism– Carnitine synthesis– Drug and steroid metabolism– Tyrosine metabolism

Metabolic functions

• Metal ion metabolism• Antihistamine reactions• Health effects:– Immune function– Wound healing– Cardiovascular disease– Diabetes, cataracts– Pulmonary function, Cancer

Vitamin C Functions and Mechanisms of Action

• Antioxidant and Pro-oxidant Activity– Reducing agent (antioxidant) (AH-)• Donate electrons and hydrogen ions• AH- may react with free radicals and reactive oxygen

species

– Reactive oxygen species• OH (hydroxy radical), O2

- (superoxide radical), H2O2 (hydrogen peroxide), and HO2 (hydroperoxyl radical)

• Attack phospholipids and protein embedded in membranes• Oxidize LDL and red blood cells

Antioxidant

Ascorbate (AH-) + OH.

semidehydroascorbate radical (A-) + H2O

AH- + O2- + H+ A- + H2O2

AH- + H2O2 A- + H2O

Regeneration of Ascorbate

2 semidehydroascorbate radicalsascorbate + dehydroascorbate

2 semidehydroascorbate radicals (A-) + 2 GSH

2 ascorbate (AH-) + GSSG

Dehydroascorbate (A) + 2 GSHascorbate (AH-) + GSSG

2 semidehydroascorbate (A-) + NADH + H+

2 ascorbate (AH-) + NAD+

Interconvertibility of ascorbic acid by oxidation and reduction

ascorbate

[O] oxidase [H20]

Ascorbic acid Dehydroascorbic acid

GSSG glutathione 2 GSH

dehydrogenase

Vitamin C as Pro-oxidant

Ascorbate (AH-) + Fe+3 or Cu+2

semideydroascorbate radical (A-) + Fe+2 or Cu+1

The products Fe+2 and Cu+1 can proceed to cause cell damagegeneration of reactive oxygen species and free radicals.

Fe+2 or Cu+1 + H2O2

Fe+3 or Cu+2 +OH- + OH.

Fe+2 or Cu+1 + O2

Fe+3 or Cu+2 + O2-

Collagen Synthesis• Most abundant protein found in body–Major component of most connective tissue• Skin, bone, cartilage, tendons, ligaments• All collagen (n~19) have a triple helical structure

• For the collagen molecule to aggregate into its triple-helix configuration selected proline residues must be hydroxylated forming hydroxyproline– Requires di-oxygenase enzymes, alpha KG, reduced

iron (Fe+2), ascorbate– Ascorbate functions to reduce iron (cofactor) back to

its ferrous state.

O C

CH

Proline

CH2

CH2

HCCH2

N

HN

NH2(CH2)4

Lysine

(CH2)2

COOHCO2

α-ketogutarate

O C

COOH

OC (CH2)2

COOH

COO*H

Succinate

O C

CH

Hydroxyproline

CH

CH2

HCCH2

N

HN

(CH2)4

HydroxylysineO C

OH

CH2 NH2CH

Dehydro-ascorbate

Fe+3O2

Ascorbate

Fe+2

Ascorbate functions in the hydroxylation of peptide-bound proline and lysine in procollagen.

*OH

Collagen Synthesis

• Vitamin C also required for hydroxylation of lysine residues

• Hydroxylysyl residues permit cross-linking or collagen and other post-translational modifications

• Vitamin C may also influence mRNA levels needed for collagen synthesis.

Carnitine Synthesis

• Vitamin C required for 2 reactions in the synthesis of carnitine from trimethyllysine– Trimethyllysine conversion to 3 hydroxy-

trimethyllysine requires• Trimethylhydroxylase (dioxygenase), alpha KG, Fe+2 and

ascorbate

– 4-butyrobetaine to carnitine requires• 4-butyrobetaine hydroxlyase (dioxygenase), alpha KG,

Fe+2 and ascorbate

(CH2)2

COOHCO2

α-ketogutarate

COOH

OC (CH2)2

COOH

COOH

Succinate

Dehydro-ascorbate

Fe+3O2

Ascorbate

Fe+2Trimethyl

lysinehydroxylase

CH2

CH2

CH2

CH2

H3C CH3

CH3

+NH3

COO-

+N

CH

Trimethyl lysine

CH2

CH2

CH2

HC – OH

H3C CH3

CH3

+NH3

COO-

+N

CH

3-OH-Trimethyl lysine

The function of vitamin C in carnitine synthesis (1)

CH2

CH2

CH2

HC – OH

H3C CH3

CH3

+N

3-OH-Trimethyl lysine

+NH3

COO-

CH

4-butyrobetainealdehyde

Glycine

+NH3

COO-

CH2

CH2

CH2

CH2

HC

H3C CH3

CH3

+N

O

NAD+

NADH4-butyrobetaine

α-ketogutarateSuccinate

CO2

Dehydro-ascorbate

Fe+3O2

Ascorbate

Fe+24-butyrobetaine

hydroxylaseCH2

HC – OH

CH2

COO-

H3C CH3

CH3

+N

Carnitine

Serine hydroxymethyltransferase-PLP-dependent

The function of vitamin C in carnitine synthesis (2)

Steps in carnitine biosynthesis Lysine 6-N-Trimethyl lysine

6-N-Trimethyllysine Hydroxylase

3-Hydroxy-6-N-Trimethyl lysine Glycine

γ-Butyrobetaine Carnitine

γ-Butyrobetaine Hydroxylase

Carnitine

Tyrosine Synthesis

• Hydroxylation of Phenylalanine – Requires phenylalanine mono-oxygenase

(hydroxylase), Fe+2, O2, tetrahydrobiopterin, NADPH, vitamin C• reducing power is supplied ultimately by NADPH but

immediately by tetrahydropterin• Vitamin C may function in regeneration of

tetrahydrobiopterin from dihydrobiopterin.

CH2 – CH – COO-

+NH3

CH2 – CH – COO-

+NH3

CH2

O

CH2 – CH – COO-

+NH3

HO

O2 H2O

Tetrahydro-biopterin

Dihydro-biopterin

NAD(P)+ NAD(P)H

*

Phenylalanine hydroxylase Fe2+

COO-C

HO

H2O

O2

Dihydro-

biopterin

Tetrahydro-

biopretin

NAD(P)H

NAD(P)+Tyro

sine m

ono-oxygenase

/hydroxylase-

Fe2+

Phenylalanine Tyrosine

α-keto-glutarate

glutamate

Tranaminase-vitamin B6-dependent

HO

HO 3,4-dihydroxyphenylalanine(DOPA) P-hydroxyphenylpyruvate

The role of vitamin C* in the phenylalanine and tyrosine metabolism, including norepinephrine synthesis

CH2 – CH – COO-

+NH3

HO

HO

3,4-dihydroxyphenylalanine(DOPA)

CO2

DOPAdecarboxylase – Vitamin B6(CH2)2 – NH2HO

HO

Dopamine

O2

H2O Cu2+

Cu1+Dopaminemono-oxygenase

Dehydro-ascorbate

Ascorbate*

CH – CH2 – NH2HO

HOOH

Norepinephrine


O2 CO2

Cu2+ Cu1+P-hydroxy-phenylpyruvatehydroxylase

Dehydro-ascorbate

CH2

O

COO-C

HO

P-hydroxyphenylpyruvate

Ascorbate*

OH

CH2 – COO-

HO

Homogentisate

O2

Cu2+

Cu1+P-hydroxy-phenylpyruvatehydroxylase

Dehydro-ascorbate

Ascorbate*

-OOC – CH = CH – C – CH2 – C – CH2 – COO-

OO

4-maleylacetoacetate


Tyrosine Catabolism

Tyrosine

p-Hydroxyphenylpyruvic acid *

Homogentisic Acid *

Maleylacetoacetic Acid

Fumarylacetoacetic Acid

Acetoacetic Acid

Acetoacetyl CoA

p-hydroxypenylpyruvate dioxygenase

Homogentisate dioxygenase

Ascorbate, Cu2+

Ascorbate, Fe2+

Tyrosine Metabolism

Tyrosine

3,4-dihydroxyphenylalanine(Dopa)

Dopamine

Norepinephrine

Cu1+

Vitamin C

DopamineMono-oxygenase

Amidation of Peptides with C-terminal glycine (hormone activation)

• Peptidylglycine amidating oxygenase– Requires Cu+1, ascorbate, O2

– Functions to cleave the carboxyl-terminal through use of molecular O2. • Amino group is retained as terminal amide while

rest is released as glyoxylate• Many of amidated peptides resulting from this

reaction are active as hormones, hormone-releasing factors and neurotransmitters– E.g.Gastrin, CCK, oxytocin, corticotropin, calcitonin,

thyrotropin, vasopressin

Proline HO-Proline Dopamine Norepinephrine

Proline Dopamine

Monooxygenase Monooxygenase

Semihydroascorbate Ascorbate Semidehydroascorbate Ascorbate

Ascorbate Semihydrosascorbate

O Amidating O O(Inactive Hormone)R-C-N-CH2COOH Enzyme R-C-NH2 +HC-COOH (Amidated Hormone)

H

Fe2+ Fe3+ Cu+ Cu2+

Serotonin Synthesis

• Serotonin can be synthesized from tryptophan• Hydroxylation of Tryptophan to 5-hydroTrp– Requires tryptophan mono-oxygenase• O2, tetrahydrobiopterin, vitamin C and NADPH

• Decarboxylation of 5-OHTrp to Serotonin

Serotonin synthesis

Tryptophan mono-oxygenase-Fe2+

O2 H20

CO2

Tetrahydrobiopterin Dihydrobiopterin

(5-hydroxytryptamine)

*

NADP+ NADPH+H+

*Vitamin C may function in tetrahydrobiopterin regeneration

TryptophanTryptophan 5-hydroxytryptophan5-hydroxytryptophan

Serotonin

Enzymatic reactions involving ascorbic acid-dependent dioxygenases

Reaction Substrate Enzyme Product

1

2

3

4

5

6

p-OH-phenylpyruvate hydroxylase1

Peptidyl L-proline

Peptidyl L-proline

Peptidyl L-lysine

6-N-Trimethyl L-lysine

4-N-Trimethyl aminobutyrate

p-OH-phenylpyruvate

Prolyl 4-hydroxylase2

Prolyl 3-hydroxylase2

Lysyl hydroxylase2

6-N-Trimethyl L-lysine hydroxylase2

4-N-Trimethyl amino-butyrate hydroxylase2

Homogentisate

Peptidyl 4- transhydroxyl-L-prolinePeptidyl 3- transhydroxyl-L-prolinePeptidyl5-erythrohydroxy- L-lysine

Erythro-3-hydroxy-6-N-trimethyl –L-lysine

3-Hydroxy-4-N-trimethyl amino-butyrate

1 α-Ketoglutarate is not required as a cosubstrate.2 α-Ketoglutarate is required as a cosubstrateSource: modified from Englard and Seifter (1986)

Vitamin C

• Vitamin C and The Common Cold– 1 g Vit C /d = 50mg/d • Number of colds, severity and duration

– 1 g Vit C/d decreases duration and severity of symptoms

• Vitamin C and Cancer– Controversial• Epidemiologic studies suggest inverse relationship

between Vitamin C and cancers of oral cavity, esophagus, and uterine cervix

Vitamin C and Cancer

• Clinical Studies– Some researchers have shown that survival

time in cancer patients may be prolonged;– Others have not shown this

• Possible Protective Mechanisms– Ability to act as a reducing agent– Detoxify carcinogens• Vitamin C ingested with nitrates or nitrites can

prevent formation of nitrosamines or nitrosamides

Vitamin C

• Atherosclerosis:– Negative relation with vitamin C– LDL lipid peroxidation

• Cataracts– Negative relation with vitamin C

• Bone density– Positive relation with vitamin C

• Wound healing and connective tissue metabolism

Recommended dietaRy allowances (Rda)

For adults: 60-70mg/dayFor pregnancy & lactation: 20-40% more than the normal RDA

Deficiency• Vitamin C intakes < 10 mg/d result in scurvy

– See when total body pool is < 300 mg– Symptoms

• bleeding gums, small skin discoloration due to ruptured blood vessels, easy bruising, impaired wound and fracture healing, joint pain, loose and decaying teeth, hyperketatosis of hair follicles.

• Scurvy rare in US– Low plasma vitamin C levels observed in

elderly

Vitamin C Deficiency

Corkscrew Hair


Perifollicular Petechiae


Scurvy

Vitamin C (Ascorbic acid)

Gum changes in infant scurvy:

The swelling and hemorrhages are confined to the areas of the gum surrounding the erupting teeth.


Gums in scurvy:

The gums are blue-red and glossy swollen in this patient with severe scurvy. The earliest changes are swelling of the internal dental papillae and tendency to bleed easily. Lesions occur only in relation to teeth and so in young infants and edentulous adults they are absent. In advanced cases there is usually an element of infection and antibiotics as well as vitamin C are required for healing


Very advanced gum lesions in scurvy


Orbital hemorrhage:

This is a dramatic but infrequent sign of scurvy. There is complete clearing with treatment.


Splinter hemorrhage:

In this unusual sign in scurvy the hemorrhages are arranged in a semicircular lattice involving nail beds. They are more extensive than those in sub-acute bacterial endocarditis.


Perifillicular petechiae:

Minimal bleeding into the hair follicles is pathognomonic of vitamin C deficiency and is often the earliest clinical manifestation. In vitamin K deficiency, thrombocytopenia and other conditions, petechiae are situated in areas of skin unrelated to the hair follicles. In perifillicular hyperkeratosis, there is no bleeding and hyperkeratosis is present. Ecchymoses develop in more advanced deficiency and are the most frequent sign in “workhouse” scurvy in old men. Wound healing is markedly delayed.

Scurvy

• After 45-80 days of stopping vitamin C intake• The 4Hs:

– Hemorrhagic signs– Hyperkeratosis of hair follicles– Hypochondriasis (Psychological)– Hematological (impaired iron absorption)

Toxicity

• Toxicity more likely with ingestion of several large (1g) doses than one single dose– Remember absorption is saturable and dose

dependent– Kidney stones?

• oxalic acid + calcium make up kidney stones– People predisposed to kidney stones should avoid high

intakes

– Urate crystals and urate kidney stones• Vitamin C competes with uric acid reabsorption

Toxicity• Toxicity is rare• Increased intake and B6 and B12 utilization• Chronic high doses of vitamin C may be

unsafe for those unable to regulate absorption of iron– Hemochromatosis

• May interfere with clinical tests– Tests for glucose in urine

• Decrease intake gradually to avoid scurvy-like symptoms

Assessment of Nutriture

• Blood, serum or plasma levels most commonly used.

• Change in response to recent dietary Vitamin C intakes

• Plasma or serum levels most sensitive indicators of deficiency

• White blood cell content reflects body stores– Measurement is technically difficult.

Pharmacological uses

• Megadoses for fighting infections• Vitamin C and common Cold.• Vitamin C and stress.. chickens

Date post:	15-Jul-2015
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Vitamin c

Health & Medicine