Enzymes as Markers for Disease

Enzymes as Markers for Disease

Some enzymes are found only in speciÞc tissues or in a limited number of such tissues. The enzyme lactate dehydrogenase (LDH) has two different types of subunitsÑone found primarily in heart muscle (H), and another found in skeletal muscle (M). The two different subunits differ slightly in amino acid composition; consequently, they can be separated electrophoretically or chromatographically on the basis of charge. Because LDH is a tetramer of four subunits, and because the H and M subunits can combine in all possible combinations, LDH can exist in Þve different forms, called isozymes, depending on the source. An increase of any form of LDH in the blood indicates some kind of tissue damage. A heart attack used to be diagnosed by an increase of LDH from heart muscle. Similarly, there are different forms of creatine kinase (CK), an enzyme that occurs in the brain, heart, and skeletal muscle. Appearance of the brain type can indicate a stroke or a brain tumor, whereas the heart type indicates a heart attack. After a heart attack, CK shows up more rapidly in the blood than LDH. Monitoring the presence of both enzymes extends the possibility of diagnosis, which is useful, because a very mild heart attack might be difÞcult to diagnose. An elevated level of the isozyme from heart muscle in blood is a deÞnite indication of damage to the heart tissue.

A particularly useful enzyme to assay is acetylcholinesterase (ACE), which is important in controlling certain nerve impulses. Many pesticides interfere with this enzyme, so farm workers are often tested to be sure that they have not received inappropriate exposure to these important agricultural toxins. In fact, more than 20 enzymes are typically used in the clinical lab to diagnose disease. There are highly specific markers for enzymes active in the pancreas, red blood cells, liver, heart, brain, and prostate gland, and many of the endocrine glands. Because these enzymes are relatively easy to assay, even using automated techniques, they are part of the ÒstandardÓ blood test your doctor is likely to request.