The occurrence of insulin-degrading activity in the components of human blood has been studied. Leukocytes have been found to contain such activity. On the basis of activation by reduced glutathione, complete inhibition by N-ethyl-maleimide, kinetic studies, and reaction with antibody to purified human liver glutathione-insulin transhydrogenase (GIT), it is concluded that the insulin-degrading activity present in leukocytes is, in fact, GIT. The data indicate that erythrocytes contain a very small amount of insulin-degrading activity; this activity is probably associated with the cell membrane. Preliminary studies in normal human volunteers indicate that there are apparent differences between individuals in the leukocyte GIT content. No evidence was found for any consistent changes in leukocyte GIT content during the course of a glucose tolerance test.

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