isoenzyme

Isoenzymes, or isozymes, are multiple forms of an enzyme that catalyze the same chemical reaction but differ in their amino acid sequences and often in their kinetic properties, regulatory requirements, tissue distribution, or subcellular localization. They typically arise from different genes within a gene family and are expressed in a tissue-specific manner, enabling metabolic regulation to suit the needs of different cell types. In some cases, isoenzymes can also result from alternative splicing or post-translational modification of a single gene product.

Common examples include lactate dehydrogenase (LDH), which consists of five tissue-related isoenzymes formed by different combinations of heart- and muscle-type subunits; creatine kinase (CK) with CK-MM, CK-MB, and CK-BB isoforms; and alkaline phosphatase, which has liver, bone, intestinal, and placental isoenzymes. Other enzymes such as amylase and certain transferases also exhibit isoenzyme variation.

Clinically, isoenzyme patterns in serum or tissues help diagnose and monitor disease, because different tissues release particular isoforms under stress or injury. For example, CK-MB elevation supports myocardial injury, and LDH isoenzyme patterns can assist in distinguishing cardiac from hepatic origins in certain contexts. Alkaline phosphatase isoenzyme analysis can aid in differentiating bone versus liver disease. Separation and quantification are commonly achieved by electrophoresis, chromatography, or immunoassays.

In summary, isoenzymes reflect evolutionary diversification of enzyme function and provide mechanisms for tissue-specific regulation, as well as valuable diagnostic tools in medicine.