A given enzyme may exist in slightly different forms, called isoenzymes, in different cells. Isoenzymes catalyze the same chemical reactions but have enough structural differences that they can be distinguished by standard laboratory techniques. This is useful in the diagnosis of disease. When organs are diseased, some of their cells break down and release specific isoenzymes that can be detected in the blood. Normally, these isoenzymes would not be present in the blood or would have very low concentrations. If their blood levels are elevated, it can help pinpoint what cells in the body have been damaged.
For example, creatine kinase (CK) occurs in different forms in different cells. An elevated serum level of CK-1 indicates a breakdown of skeletal muscle and is one of the signs of muscular dystrophy. An elevated CK-2 level indicates heart disease, because this isoenzyme comes only from cardiac muscle. There are five isoenzymes of lactate dehydrogenase (LDH). High serum levels of LDH-1 may indicate a tumor of the ovaries or testes, while LDH-5 may indicate liver disease or muscular dystrophy. Different isoenzymes of phosphatase in the blood may indicate bone or prostate disease.
Antibodies and other proteins attack and neutralize organisms that invade the body. Clotting proteins protect the body against blood loss.
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