Insulin degradation. XVIII. On the regulation of glutathione-insulin transhydrogenase in the hyperglycemic obese (ob/ob) mouse.

The occurrence of insulin-degrading activity in the liver of the obese hyperglycemic mouse (ob/ob) and its litter mate has been studied. The trichloroacetic acid-soluble product formed from insulin upon incubation with liver homogenate was identified as the A chain of insulin. In Ouchterlony double-diffusion experiments with antibody to purified rat liver glutathione-insulin transhydrogenase, mouse liver homogenate and the microsomal fraction each gave a single precipitation band of identity with the purified rat liver enzyme. These results indicate that the insulin-degrading activity present in the mouse liver is, in fact, glutathione-insulin transhydrogenase. Subcellular distribution studies of glutathione-insulin transhydrogenase and marker enzymes indicate that the transhydrogenase is located primarily in the microsomal fraction of mouse liver homogenate. The ob/ob mouse, which is a genetic mutant characterized by obesity, hyperinsulinism and resistance to the hypoglycemic action of insulin, contains hepatic glutathione-insulin transhydrogenase activity (per mg microsomal protein) markedly higher (40--60%) than its lean litter mates. However, a major portion of the increased hepatic enzyme in the ob/ob mouse occurs in a latent state; the increased amount of enzyme either is unavailable or is nonfunctional, although the ob/ob mouse still contains more of the functional form than the lean mouse. Thus, the results are consistent with the suggestion that the hepatic glutathione-insulin transhydrogenase is probably under a feedback control by circulating insulin.