Download Enzymes

By:
H.Baniamerian
Kermanshah university of medical science
Enzymes
Enzymes are known as markers
of cellular damage, and their
measurement is an important
function of clinical laboratories.
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, AND
SKELETAL 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”
Aminotransferases
The 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 aminotransferase
are 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
the onset of chest pain. after
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 + 2,4DNPH
Pyruvate hydrazone(brown complex)
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.