Insulin-degrading activity in mononuclear and polymorphonuclear circulating leukocytes of nondiabetic and diabetic...

Insulin-degrading activity in mononuclear (MN) and polymorphonuclear (PMN) fractions of circulating leukocytes obtained from 7 nondiabetic and 13 insulin-dependent diabetic subjects was studied. Insulin-degrading activity in both MN and PMN fractions was activated by reduced glutathione and was inhibited completely by N-ethylmaleimide. Both fractions had Michaelis-Menten constant (Km) (insulin) values within the range of values reported for purified glutathione-insulin transhydrogenase (GIT). In double immunodiffusion tests with antibody to human liver GIT, the MN fraction showed immunoprecipitin bands continuous with those of purified liver enzyme, but the PMN fraction showed little or no reaction with the antibody. These data indicate that both leukocyte fractions contain thiol-dependent insulin-degrading activity; however, only in the MN fraction was the degrading activity immunologically similar to that of liver GIT. Kinetic studies showed that the insulin-degrading activity of MN and PMN cells from diabetic patients had a 3.6- and 14.5-fold, respectively, higher maximal capacity (Vmax) than the insulin-degrading activity of these cells from nondiabetic subjects, without any change in the half-saturation constant for the substrate (Km for insulin). These results demonstrate that diabetes and/or insulin therapy result in increased leukocyte glutathione-dependent insulin-degrading activity.