By :

H.BaniamerianKermanshah university of medical science

Enzymes are known as markers of cellular damage, and their measurement is an important function of clinical laboratories.

Enzymes

Not all intracellular enzymes are equally valuable as indicators of cellular damage .

For example, isocitrate dehydrogenase (ICD) activity is high in heart muscle ,

but after a myocardial infarction, it is inactivated rapidly on entering the vascular compartment..

LIVER, CARDIAC, ANDSKELETAL ENZYMES

Enymes in this category include the aminotransferases(AST,ALT), creatine kinase(CK), and lactate dehydrogenase(LDH) , alkaline phosphatase(ALP).

LIVER ENZYMES

Enzymes in this category include alanine and aspartate aminotransferases(AST,ALT), glutamate dehydrogenase (GLD), ALP, 5'-nucleotidase (NTP), ɣ-glutamyl transferase (GGT),

glutathione S-transferase (GST), and serum cholinesterase (CHE) .

The aminotransferases and ALP are widely

used.

They have long been mistakenly called, as a

group, "liver function tests”

AminotransferasesThe aminotransferases are a group of enzymes

that catalyze the interconversion of amino acids to 2-oxo-acids by transfer of amino groups .

Aspartate and alanine aminotransferaseare examples of aminotransferases of clinical

interest .

Aspartate aminotransferase (EC 2.6.1.1) also is known as Aspartate transaminase, also is L-aspartate: 2-oxoglutarate aminotransferase,

AST .

It was known formerly as glutamate oxaloacetate transaminase (GOT) .

Alanine aminotransferase (EC 2.6.1.2) also is

known as alanine tramaminase,

L-alanine:2-oxoglutarate aminotransferase, ALT.

It was known formerly as glutamate pyruvate transaminase (GPT).

Pyridoxal-5' -phosphate (P-5' -P) and its

amino analogue, pyridoxamine-5'-phosphate, function as coenzymes in the amino-transfer

reactions .

The P-5'-P is bound to the apoenzyme and

serves as a true prosthetic group .

The P-5'-P bound to the apoenzyme accepts the amino group from the first substrate, aspartate or alanine, to form enzyme-bound pyridoxamine-5'-phosphate and the first reaction product: oxaloacetate or pyruvate, respectively.

Clinical Significance

1( Liver disease

2( Myocardial disease

LIVER DISEASES

Generally increased↑ levels of ALT and AST in liver diseases associated with some of hepatic necrosis such as cirrhosis , carcinoma,viral and toxic hepatitis and obstructive jaundice.

Characteristically ALT is generally higher than AST in acute viral or toxic hepatitis.

Elevated ALT levels also fiund in trauma,muscle disease,hypoxia,MI,and haemolytic disease.

With viral hepatitis and other forms of liver disease associated with hepatic necrosis serum AST and ALT levels are elevated↑, even before the clinical signs and symptoms of disease (such as jaundice) appear .

Levels for both enzymes may reach values

as high as 100 times the upper limit of the reference interval, although twentyfold to fiftyfold elevations are encountered most frequently.

Peak values of transaminase activity occur between days 7 and 12;

activities then gradually decrease↓, reaching normal levels by the third to fifth week if recovery

is uneventful .Alcoholic hepatitis has more modest elevations.In this

patient P-5`-P concentration is low thus ALT/AST Activity is only 2-3 fold of upper limit.

In cases of infectious hepatitis and other inflammatory conditions affecting the liver, ALT is higher than AST,

and the ALT/AST (De Ritis) ratio, which normally is less than 1, becomes greater than unity.

The relatively similar elevations of AST and ALT in

hepatitis cases have been attributed to the release of only the cytoplasmic isoenzyme of AST into the circulation from

reversibly damaged parenchymal cells .

Although serum levels of both AST and ALT become elevated whenever disease processes affect liver cell integrity, ALT is

the more liver-specific enzyme.

Myocardial disease

:myocardial infarction (MI)After a myocardial infarction (MI), increased ↑

AST activity appears in serum, as might be expected from the relatively high AST concentration in heart muscle. On average, serum levels do not become abnormal, however, until 6 to 8 hours after

the onset of chest pain. Abnormal AST levels are observed in more

than 97% of cases of myocardial infarction when correctly timed blood specimens are analyzed.

Peak values of AST activity are reached after 18 to 24 hours, and the activity values fall to within

the reference interval by the fourth or fifth day, provided no new infarct has occurred.

ALT levels are within normal limits or only marginally increased in cases of uncomplicated myocardial infarction

because the concentration of ALT activity

in heart muscle is only a fraction of that of AST activity.

Pulmonary emboli can raise AST levels to

two to three times normal, and slight to moderate elevations (two to five times normal).

Methods for the measurement of Transaminase activity :Enzymatic:colorimetric

Historically, various photometric

substrates (2,4-dinitrophenylhydrazine and various dyes) were coupled to the transaminase reactions.

Pyruvate hydrazone(brown complex) pyruvate + 2,4DNPH

Reference intervals (NORMAL RANGE)

The AST reference interval for adults is 8 to 38 U/L at 37°C.

The ALT reference interval for adults is 5 to 35 U/L at 37°C.

Values in men are slightly higher than those in women.