Studies on the nature of antigenicity of A and B chains of bovine insulin

Characterization and application of monoclonal antibodies directed to separate epitopes of glutathione-insulin transhydrogenase

Five monoclonal antibodies specific for glutathione-insulin transhydrogenase were characterized. None of the monoclonal antibodies cross-reacted with another insulin-degrading enzyme, neutral thiopeptidase. The isotype of four antibodies was IgG1 and of the fifth IgG2b. Affinity studies, competitive binding studies and immunoblot analysis of CNBr and trypsin cleavage products of glutathione-insulin transhydrogenase demonstrated that the four IgG1 antibodies were directed to an epitope of the enzyme which was distinct from the epitope recognized by the IgG2b antibody. Inhibition studies indicated that each monoclonal antibody, when added singly to glutathione-insulin transhydrogenase, was unable to inhibit the insulin-degrading activity of the enzyme. However, when monoclonal antibodies directed against separate epitopes of glutathione-insulin transhydrogenase were presented together (i.e., the IgG2b with any one of the four IgG1 antibodies), a loss in enzymatic activity was noted. Immunoblot analysis of rat organ extracts with the IgG1 antibodies demonstrated one immunoreactive protein band of Mr 56,000 in all tissues examined (liver, fat, pancreas and kidney) except the spleen, which demonstrated two immunoreactive protein bands of Mr 56,000 and 51,000. The same immunoblots, when probed with the IgG2b antibody, demonstrated the same immunoreactive protein banding pattern as above plus an additional immunoreactive protein band of Mr 67,000 in all tissues. Studies with spleen extracts from steptozotocin-induced diabetic rats demonstrated that there was a loss of the 51,000 immunoreactive band in diabetes. This 51,000 protein was restored upon insulin treatment of the diabetic rats and nullified upon concomitant administration of cycloheximide or actinomycin D with insulin. Immunoblots of human liver, adipose and skeletal muscle extracts indicated that each monoclonal antibody cross-reacted with the human form of the enzyme which had a molecular weight of Mr 63,000; a second minor immunoreactive band of 67,000 was detected with the IgG2b antibody. The physiological significance of additional molecular forms of the enzyme (i.e., 67,000 and 51,000) remains to be determined.

Stabilization of insulin receptor subunit structure by glutathione-insulin transhydrogenase

A partially purified insulin receptor preparation from rat liver was incubated at 37 degrees C with and without the protein-disulfide interchange enzyme, glutathione-insulin transhydrogenase (thiol: protein-disulfide oxidoreductase/isomerase, EC 1.8.4.2/5.3.4.1). Insulin-binding activity was then assessed by crosslinking receptor-125I-insulin complexes and subjecting them to electrophoresis on SDS-polyacrylamide gels in the absence and presence of reductant followed by autoradiography. Prior incubation of the receptor at 37 degrees C in the absence of the enzyme markedly decreased the subsequent binding of 125I-insulin to the holoreceptor (Mr 350 000) and to its subunits (Mr 180 000 and 130 000), while addition of the enzyme to the preincubation medium served to substantially prevent this decrease. The loss in binding at 37 degrees C was not restored by subsequent addition of the enzyme, nor was the loss prevented by any of the several known inhibitors of proteolysis. The apparent stabilization of receptor by transhydrogenase, as evidenced by the increase in binding above control levels, was proportional to both the enzyme concentration and the duration of incubation. These effects seem to be specific for transhydrogenase, since several other disulfide-containing proteins were found to be ineffective. These data suggest that the stabilization of the subunit structure of the insulin receptor at physiological temperatures may take place via a disulfide interchange reaction catalyzed by glutathione-insulin transhydrogenase.

Inhibition of glutathione-insulin transhydrogenase by metal ions and activation by histidine and other chelating agents

The catalytic activity of purified glutathione-insulin transhydrogenase (thiol:protein-disulfide oxidoreductase/isomerase, EC 1.8.4.2) from bovine pancreas is markedly stimulated by histidine and other chelating agents. The activation produced was highest with EDTA, followed by EGTA, 8-hydroxyquinoline and 1,10-phenanthroline. Of the many amino acids tested, histidine was the only one that activated the enzyme; the structurally related compounds, 3-methylhistidine and imidazole also stimulated the enzyme, but 1-methylhistidine and histamine were without effect. The activation of EDTA was negated by metal ions, most effectively by Se2+, Hg2+, Cu2+ and Zn2+, and less effectively by Ca2+ and Ni2+. Likewise, activation by histidine was negated by Zn2+ but not by Ca2+ or Mg2+. Thus, activation of glutathione-insulin transhydrogenase is apparently achieved in part by the chelation of inhibitory metal ion(s). These findings are consistent with a regulatory scheme for glutathione-insulin transhydrogenase in which (a) the enzyme is inhibited by selenium and heavy metal ions normally present in tissues and (b) this inhibition can be relieved by the addition of histidine or chelating agents.

Familial hyperproinsulinemia. Two cohorts secreting indistinguishable type II intermediates of proinsulin conversion

Familial hyperproinsulinemia, a hereditary syndrome in which individuals secrete high amounts of 9,000-mol wt proinsulin-like material, has been identified in two unrelated cohorts. Separate analysis of the material from each of the two cohorts had suggested that the proinsulin-like peptide was a conversion intermediate in which the C-peptide remained attached to the insulin B-chain in one case, whereas it was a conversion intermediate in which the C-peptide remained attached to the insulin A-chain in the other. To reinvestigate this apparent discrepancy, we have now used chemical, biochemical, immunochemical, and physical techniques to compare in parallel the structures of the immunoaffinity chromatography-purified, proinsulin-like peptides isolated from the serum of members of both families. Our results show that affected individuals in both cohorts secrete two-chained intermediates of proinsulin conversion in which the COOH-terminus of the C-peptide is extended by the insulin A-chain and from which the insulin B-chain is released by oxidative sulfitolysis. Analysis of the conversion intermediates by reverse-phase high-performance liquid chromatography using two different buffer systems showed that the proinsulin-related peptides from both families elute at a single position very near that of the normal intermediate des-Arg31, Arg32-proinsulin. Further, treatment of these peptides with acetic anhydride prevented trypsin-catalyzed cleavage of the C-peptide from the insulin A-chain, a result demonstrating the presence of Lys64 and the absence of Arg65 in both abnormal forms. We conclude that individuals from both cohorts with familial hyperproinsulinemia secret very similar or identical intermediates of proinsulin conversion in which the C-peptide remains attached to the insulin A chain and in which Arg65 has been replaced by another amino acid residue.

Binding, degradation, and bioactivity of insulin in primary cultures of rat hepatocytes

Hepatocytes from fasted rats, previously maintained as a monolayer in a serum-free glucagon-containing culture medium are demonstrated to provide a useful model system for the study of receptor-mediated mechanisms of insulin action. The cultured liver cells show glucagon- and insulin-responsive biological effects. These cultures show the long-term effects of insulin on the syntheses of protein ([3H]leucine incorporation), glycogen, and lipids (conversion of [3H]glucose) in a dose-dependent manner in the physiological range of insulin concentrations. The order of the effects of different analogues of insulin on each of the bioactivities studied at 37 degrees C is the same as their order to compete at 25 degrees C for binding to insulin-specific receptors. The characterization of radioactive products of 125I-insulin using gel filtration and anti-insulin A chain antibody has shown a transient accumulation of insulin A chain, indicating that the sequential pathway of insulin degradation is operative in the anchored, cultured hepatocytes. Whereas the synthesis of proteins proceeds linearly after insulin inoculation, the syntheses of glycogen and lipids first occur after a lag period of about 10 and 12 h, respectively. None of the three 125I-labeled fragments or products of 125I-insulin released in the culture mediums showed any biological activity (glycogenesis) in cultured hepatocytes although a high-molecular-weight 125I-product isolated from cells could not be tested because of its insolubility. The possibility that the insulin effects might have been mediated via a nonradioactive fragment of insulin or another chemical agent remains open.

Degradation of proinsulin and isolated C-peptide by rat kidney neutral metallo-endopeptidase

Previous studies have shown that a neutral metallo-endopeptidase purified from rat kidney degrades the B chain of insulin, glucagon, ACTH and, at a markedly slower rate, the A chain of insulin. In contrast the enzyme does not attack native insulin, oxytocin, vasopressin, ribonuclease, albumin or denatured hemoglobin. The current studies demonstrate that the neutral peptidase also degrades the isolated C-peptide of proinsulin and cleaves certain peptide bonds in and near the C-peptide moiety of native proinsulin. Time courses of the formation of fluorescamine-reactive material during digestion of proinsulin and isolated C-peptide with the peptidase were identical. However, structural analysis of the peptidase-digested proinsulin showed that the enzyme does not convert proinsulin to insulin but that the peptidase cleaves one bond, Tyr26-Thr27, in the B chain moiety and five bonds in the C-peptide moiety, producing four split proinsulins. One of the split proinsulins is des-octacosa-peptide (27-54) porcine proinsulin or des-tetracosapeptide (27-50) bovine proinsulin. Each is a derivative of the insulin molecule having an extension of nine residues (ten residues in the case of the derivative from bovine proinsulin) at the N-terminus of A chain and lacking four residues at the C-terminus of B chain. This two chain derivative retains full immunoreactivity with insulin antibodies and exhibits 2.4-times more biological activity (promotion of glycogenesis in primary cultured hepatocytes) than proinsulin and about two-thirds the activity of insulin.

Insulin degradation. XV. Use of different assay methods for the study of mechanism of action of glutathione-insulin transhydrogenase

Insulin degradation by glutathione-insulin transhydrogenase has been studied using three different assay procedures: the measurement of the change in insulin immunoreactivity; the formation of 5% trichloroacetic acid-soluble radioactivity from 125 I-labeled insulin and the formation of GSSG via coupling to the oxidation of NADPH with the use of glutathione reductase. The extent of reaction as measured by each assay was different, and the ratios between the assays were not constant with time. Kinetic experiments with the NADPH-coupled assay and the trichloroacetic acid assay yielded similar results: Line-weaver-Burke plots with insulin as variable and GSH as fixed substrate gave a set of straight, intersecting lines, and such plots with GSH as variable and insulin as fixed substrate were parabolic. Apparent Km values for insulin at 1 mM GSH were found to be quite similar by three assay techniques; however, the V values per unit of enzyme protein varied considerably with different procedures. The results are interpreted as indicating that immunoreactivity is lost after reduction of only one of the disulfide bonds of insulin whereas the two interchain disulfide linkages must be broken to produce the trichloroacetic acid-soluble A chain. The results of the NADPH-coupled assay suggest that all three disulfide bonds of insulin are possible substrates for the enzyme. The trichloroacetic acid precipitation assay seems to be the most practicable technique for general use because of the greater ease in performing large number of samples, precision and sensitivity.

Sequential degradation of insulin by rat liver homogenates

Insulin was incubated with rat liver homogenate in the presence of glutathione. The products formed were examined by chromatography on a Sephadex G-75 column, with 50% acetic acid as eluent. The results show that insulin is degraded by rat liver homogenates in sequential order: first, a splitting of insulin into A and B chains by glutathione-insulin transhydrogenase, followed by proteolysis of the resulting polypeptides to small molecular weight components.