Insulin degradation. XXVIII. Immunocytochemical localization of glutathione-insulin transhydrogenase in the pancreas, kidney and liver of normal and streptozotocin-diabetic rats and of lean and obese (ob/ob) mice

Diabetic changes in the redox status of the microsomal protein folding machinery

Changes in assisted protein folding are largely unexplored in diabetes. In the present studies, we have identified a reductive shift in the redox status of rat liver microsomes after 4 weeks of streptozotocin-induced diabetes. This change was reflected by a significant increase in the total- and protein-sulfhydryl content, as well as in the free sulfhydryl groups of the major protein disulfide isomerases (PDIs), the 58 kDa PDI and the 57 kDa ERp57 but not other chaperones. A parallel decrease of the protein-disulfide oxidoreductase activity was detected in the microsomal fraction of diabetic livers. The oxidant of PDI, Ero1-Lalpha showed a more oxidized status in diabetic rats. Our results reveal major changes in the redox status of the endoplasmic reticulum and its redox chaperones in diabetic rats, which may contribute to the defective protein secretion of the diabetic liver.

Glutathione metabolism in the pancreas compared with that in the liver, kidney, and small intestine

The pancreas plays a major role, along with the kidney, liver, small intestine, and several other organs, in glutathione (GSH) metabolism, as evidenced by the large concentration of GSH in the pancreas, its rapid turnover rate, and the presence, at significant levels, of various enzymes involved in GSH metabolism. The pancreas appears to obtain much of the cysteine that is required for both GSH and protein synthesis by hydrolyzing plasma GSH to its constituent amino acids and then transporting cysteine into the cells. GSH hydrolysis is accomplished by the ectoenzymes gamma-glutamyl transferase (GGTase) and aminopeptidase N, both of which are present in the pancreas. Only the kidney has a greater GGTase activity. Although pancreatic GSH synthesis has not been directly demonstrated, pancreatic secretory protein synthesis is substantial, and these proteins contain significant amounts of cysteine as disulfides. The pancreas also contains significant levels of protein disulfide isomerase, glutathione peroxidase, and NADPH:GSH oxidoreductase. Protein disulfide isomerase, using oxidized glutathione generated by glutathione peroxidase, is important in the formation of disulfide bonds in secretory proteins in the pancreas. No other organ has a higher specific activity of protein disulfide isomerase. By analogy with kidney and liver, the pancreas presumably exhibits a rapid apical secretion of GSH. The purpose of this apical secretion is unknown in the kidney. In the liver, it is important in bile secretion. The large GGTase activity of apical plasma membranes in the pancreas is likely to be instrumental in the hydrolysis, and subsequent recovery of the constituent amino acids of apically secreted GSH, as occurs in the kidney and liver.

Insulin-degrading neutral cysteine proteinase activity of adipose tissue and liver of nondiabetic, streptozotocin-diabetic, and insulin-treated diabetic rats

The activity of the insulin-degrading enzyme neutral cysteine proteinase (EC 3.4.22.11, insulinase) was studied in adipose tissue and in liver of nondiabetic, streptozotocin-diabetic, and insulin-treated diabetic rats. Proteinase activity was found to be significantly decreased during diabetes and was restored to near normal levels in both tissues following insulin treatment. The insulin-mediated increase of proteinase activity in both tissues was partially or completely blocked by actinomycin D (an inhibitor of RNA synthesis) and by cyclohexamide (an inhibitor of protein synthesis). Kinetic analysis showed that the changes in proteinase activity of both liver and adipose tissues were accompanied by a change in Vmax (i.e., maximal enzyme activity) without a change in Km (i.e., substrate affinity). These data indicate that insulin functions as an inducer for neutral cysteine proteinase in both tissues. These alterations in the proteinase activity paralleled the alterations in the activity of a second insulin-degrading enzyme, glutathione-insulin transhydrogenase in adipose tissue (this paper) and in liver (previously published papers) under the same physiological conditions.

The renal metabolism of insulin

The kidney plays a pivotal role in the clearance and degradation of circulating insulin and is also an important site of insulin action. The kidney clears insulin via two distinct routes. The first route entails glomerular filtration and subsequent luminal reabsorption of insulin by proximal tubular cells by means of endocytosis. The second involves diffusion of insulin from peritubular capillaries and subsequent binding of insulin to the contraluminal membranes of tubular cells, especially those lining the distal half of the nephron. Insulin delivered to the latter sites stimulates several important processes, including reabsorption of sodium, phosphate, and glucose. In contrast, insulin delivered to proximal tubular cells is degraded to oligopeptides and amino-acids by one of two poorly delineated enzymatic pathways. One pathway probably involves the sequential action of insulin protease and either GIT or non-specific proteases; the other probably involves the sequential action of GIT and lysosomal proteases. The products of insulin degradation are reabsorbed into the peritubular capillaries, apparently via simple diffusion. Impairment of the renal clearance of insulin prolongs the half-life of circulating insulin by a number of mechanisms and often results in a decrease in the insulin requirement of diabetic patients. Much needs to be learned about these metabolic events at the subcellular level and how they are affected by disease states. Owing to the heterogeneity of cell types within the kidney and to their anatomical and functional polarity, investigation of these areas will be challenging indeed.

Insulin degradation in hepatic cirrhosis

Hepatic insulinase activity was studied in cirrhosis in an attempt to explain the hyperinsulinemia known to occur in this disease. Liver tissue was obtained during laparotomy in seven patients with cirrhosis of the liver and five patients without liver disease. Insulin degradation was significantly decreased in the cirrhotic liver at each time interval measured (p less than 0.05). Insulinase inhibitor activity was measured in the plasma of 12 patients with cirrhosis of the liver and six controls. There was no significant difference between the two groups. Decreased insulinase activity in cirrhosis may account, in part, for the hyperinsulinemia seen in this disease.