2. About…
• Isozymes also known as isoenzymes (or more generally
as M
ultiple forms of enzymes) are the physically distinct
forms of enzyme but catalyses the same chemical reaction.
•
Enzymes that differ in structurally (amino acid sequence),
electrophoretically, immunologically and site of occurrence.
• Different isoenzymes are often organ-specific and their
determination may improve the specificity of enzyme tests.
•
The heterogeneity of some isoenzymes is due to different
protein subunits which are coded for by separate genes.
3. Reasons For Isoenzyme……
•
Synthesized from different genes.
(Eg: malate dehydrogenase in cytosol versus in
mitochondria)
•
Oligomeric forms of more than one type of subunits.
(Eg: lactate dehydrogenase)
• Different carbohydrate content.
(Eg: alkaline phosphatase)
4. Examples……
Creatine P
hosphokinase(CP ):
K
• CPK-1 (also called CPK-BB) is found mostly in the brain and
lungs.
• CPK-2 (also called CPK-MB) is found mostly in the heart.
• CPK-3 (also called CPK-MM) is found mostly in skeletal
muscle.
H
exokinase:
• Hexokinase I/A.
• Hexokinase II/B.
• Hexokinase III/C .
• Hexokinase IV, also referred to as glucokinase.
5. Alkaline phosphatase (AP AL , SAP :
, P
)
• Liver-ALP (L-ALP).
• Corticosteroid-ALP (C-ALP, only in dogs).
• Bone-ALP (B-ALP).
• Intestinal-ALP (I-ALP).
Aspartate
transaminase (AST ASAT AAT
/
/
)
or serum
glutamic oxaloacetic transaminase(SGOT
):
• GOT1/cAST, the cytosolic isoenzyme derives mainly from red
blood cells and heart.
• GOT2/mAST,
the mitochondrial isoenzyme is present
predominantly in liver.
6.
7. Lactate Dehydrogenase(LDH)
• "Lactate dehydrogenase (LDH) is an enzyme present in a wide
variety of organisms, including plants and animals".
• Its Enzyme Comission number is EC 1.1.1.27 where;
– EC 1 = oxidoreductase.
– EC 1.1 = acting on the CH-OH group of the donor.
– EC 1.1.1 = With NAD or NADP as acceptor.
– EC 1.1.1.27 = L-lactate dehydrogenase.
• Thus, it is an oxidoreductase enzyme that reversible reaction
of lactate(LA) to pyruvate(PA) accompanied by the
interconversion of NADH and NAD+.
8. • In serum five physically distinct isoenzymes exist and are
known as LDH-1, LDH-2, LDH-3, LDH-4, LDH-5.
• All these enzymes catalyze the same reaction of oxidation of
LA to PA.
• A sixth, typical isoenzyme LDH has been found in male genital
tissues, called LDHx.
• Each Isozymes protein is made up of four polypeptide subunits,
thus each is a “ tetramer” .
• Each tetramer consists of four sub-units or monomers each with
mass of 36 kDa giving the tetramer the mass of 144 kDa.
• Each monomer consists of a peptide chain of 334 amino acids
with its own active center.
• Each subunit maybe one of two types termed H and M.
• The different isoenzymes contain H and M in different
proportions.
9.
10.
11. Identification…….
• The different form can be separated by electrophoresis.
• The difference in electrophoretic mobilities due to different electric
charges on the isoenzymes due to difference in content of acidic and
basic amino acids.
• The H gene is more strongly negatively charged than M gene due to
higher number of acidic residues.
Example:
– LDH-1 has the highest negative charge and hence, moves fastest
during electrophoresis. It contains a higher proportions of Asp and
glutamate than the other forms.
– LDH-5 is the slowest moving fraction.
12. • Rate of chemical reactions is catalyzed, the different
isoenzymes may catalyzes the same reaction at different rates.
Example:
– Rate of oxidation of Hydroxy Butyrate is greater by LDH-1 and LDH2, when compared with rate of oxidation of LDH-4 and LDH-5.
• The isoenzymes may have different physical properties also.
Example:
– LDH-4 and LDH-5 are easily destroyed by heat, whereas LDH-1 and
LDH-2 are not, if heated up to about 60 oc (“Heat-resistant”).
• Myocardial LDH (LDH-1) is found to be more heat stable
than that of Hepatic LDH.
• Hepatic LDH (LDH-5) is inhibited by urea.
13. • The isoenzymes have different pH optima and Km values.
• The isozymes have different Km values for pyruvate and
therefore are better suited for different environments inside
the body.
• A pure tetramer of M subunit i.e. M4, has lower Km for
pyruvate and is concentrated in skeleton muscles which are
anaerobic.
• Therefore M4 promotes glycolysis by catalyzing the
production of lactate from pyruvate quickly and efficiently.
• On the other hand pure tetramer of H subunit i.e. H4, has
greater Km for pyruvate and is more concentrated in heart
muscles which is aerobic.
14. Clinical Significance……
• Normal serum LDH level is 60 – 250 IU/L.
• After damage to either of tissues viz; myocardium or liver,
total serum LDH is increased and it may be useful to know the
origin of the enzyme increase.
• In normal serum, LDH-2 is predominent isoenzyme and the
slowest peak of LDH-5 is rarely seen.
• After myocardial infarction, the faster isoenzymes LDH-1 and
LDH-2 predominate.
• In acute viral hepatitis, the slowest isoenzymes LDH-4 and
LDH-5 predominate.
15. • Total serum LDH is frequently elevated in neoplastic
diseases.
• In malignancies, isoenzyme pattern shifts towards slower
migrating zone, there is increase usually LDH-3, LDH-4 and
LDH-5.
• An increase in LDH-5 is seen in Breast carcinoma,
malignancies of CNS, prostatic carcinoma.
• In leukemia, rise more in LDH-2 and LDH-3.
• Malignant tumors of testes and ovary shows rise in LDH-2,
LDH-3 and LDH-4.
