1 Sinai Hospital, Baltimore, Maryland 21215
ONE of the first demonstrations of human diversity at the molecular level was the observation that sickle cell hemoglobin moves more slowly in an electrophoretic field than does normal hemoglobin. In recent years, a qualitatively similar type of molecular diversity has been recognized in which multiple forms of an enzyme with the same substrate specificity may exist in a single tissue or even in the same cell. The terms "isozyme" or "isoenzyme" have been applied to this form of enzyme heterogeneity. Studies of this phenomenon have provided new insight into the nature of enzymes and enzyme action, and have also suggested both a basis for biochemical heterogeneity as well as some mechanisms for metabolic regulation and biochemical evolution in the tissues of complex multicellular organisms. In addition, these studies have shown that isozymes may have important clinical applications.