Isozymes: Definition, Occurrence and Characteristics!
Definition of Isozymes:
The enzymes that occur in a number of different forms and differ from each other chemically, immunologically and electrophoretically are called “Isoenzymes” or “isozymes”.
Occurrence of Isozymes:
Isozymes are present in the serum and tissues of mammals, amphibians, birds, insects, plants and unicellular organisms.
Examples:
Isozymes of numerous dehydrogenases, and several oxidases, transaminases, phosphatases, transphosphorylases, proteolytic enzymes, aldolases.
Characteristics of Isozymes:
1. They catalyze the same reaction but they can be distinguished by physical methods such as electrophoresis or by immunological methods.
2. The difference between some isozymes are due to differences in the quarternary structure of the enzymes, e.g., lactate dehydrogenase exists in five isozymic forms.
3. The isozymic forms of lactate dehydrogenase are tetramers, each is made up from two types of units H and M. The molecular weight of active lactate dehydrogenase is 1,30,000. Only the tetrameric molecule possesses catalytic activity.
The subunits are expressed in the following 5 ways:
4. Splitting and reconstitution of lactate dehydrogenase –I1 or lactate dehydrogenase- 15 produces on new isozymes. Therefore, each consists of a single subunit.
But when a mixture of purified lactate dehydrogenase – I1 and lactate dehydrogenase – I5 is subjected to splitting and re- constitution, lactate dehydrogenase – I2, – I3 and – I4 are also produced.
The approximate proportions of the isozymes result if the relationships are:
Synthesis of H and M subunits are controlled by distinct genetic loci.
5. Lactate dehydrogenase (LDH) catalyzes the transfer of two electrons and one hydrogen ion from lactate to NAD:
6. Medical discovery in 1957 had shown that the relative proportions of several lactate dehydrogenase isozymes of human serum were changed significantly in some pathologic conditions.
Method of Assay:
1. Serum sample is subjected to electrophoresis at pH 8.6 using starch, agar medium.
2. The isozymes have different charges at this pH and migrate to 5 regions of the electrophoretogram.
3. Isozymes are then localized by means of their ability to catalyse reduction of a colourless dye to a coloured form.
Diagnostic Importance of Isozymes:
1. In certain solid tumors, there is an increase in serum LD1 and LD2. These isoenzymes are also present in the blood of patients with acute leukemia.
2. LD5 is usually the predominant isozyme in the tumors.
3. Serum isozyme levels are elevated in acute leukemia.
4. Alkaline phosphatase isozymes can distinguish liver lesions from bone lesions in metastatic carcinoma.
5. The normal serum is 0 enzyme levels of LDH are:
