Degradation of porcine relaxin by glutathione-insulin transhydrogenase and a neutral peptidase

Degradation of relaxin family peptides by insulin-degrading enzyme

Insulin-degrading enzyme (IDE) is a ubiquitously expressed metalloproteinase responsible for the intracellular degradation of insulin. IDE also interacts with other members of the insulin superfamily, including relaxin, but no studies have been reported regarding the interaction of other relaxin-like peptides with IDE. In this study, we determined that relaxin, relaxin-3, and InsL3 all competitively inhibited the degradation of insulin by IDE to different degrees, and all inhibited covalent cross-linking of insulin to IDE. Each of the peptides was degraded by IDE to various degrees (insulin > relaxin > InsL3 = relaxin-3). In summary, relaxin, InsL3, and relaxin-3 all bound to IDE, competed for the binding and degradation of insulin, and were all substrates for the proteolytic activity of IDE. Therefore, it is possible that in addition to insulin, IDE may be important for the cellular proteolysis of relaxin, InsL3, and relaxin-3.

Identification of an insulin fragment produced by an insulin degrading enzyme, neutral thiopeptidase

Previous studies have shown that neutral thiopeptidase (E.C.3.4.22.11, insulinase) degrades (processes) insulin with a high affinity (Km = 30 X 10(-9) M). In the current studies, insulin was subjected to digestion with a highly purified rat liver neutral thiopeptidase and the peptides generated were separated by HPLC using a C8 column. With the use of structural analysis (which included the determination of amino terminal residues and amino acid composition), the major product was identified as a peptide containing portions of both chains of insulin, A1 to A13 and B1 to B9 having two disulfide bonds, an interchain disulfide bond and presumably the intra-A chain disulfide bond as well. Examination of insulin-like biological activity using a primary cultured hepatocyte test system showed that the fragment promoted neither short-term (alpha-aminoisobutyric acid uptake) nor long-term (glycogen synthesis) bioactivities of 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.