Isozymes: Definition, Occurrence and Characteristics | Enzymes

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: