Effects of metalloendoprotease inhibitors on insulin binding, internalization and processing in adipocytes

Papain-specific activating esters in aqueous dipeptide synthesis

Enzymatic peptide synthesis has the potential to be a viable alternative for chemical peptide synthesis. Because of the increasing commercial interest in peptides, new and improved enzymatic synthesis methods are desirable. In recently developed enzymatic strategies such as substrate mimetic approaches and enzyme-specific activation, use of the guanidinophenyl ester (OGp) group has been shown to suffer from some drawbacks. OGp esters are sensitive to spontaneous chemical hydrolysis and the group is expensive to synthesize and therefore not suitable for large-scale applications. On the basis of earlier computational studies, we hypothesized that OGp might be replaceable by simpler ester groups to make the enzyme-specific activation approach to peptide bond formation more accessible. To this end, a set of potential activating esters (Z-Gly-Act) was designed, synthesized, and evaluated. Both the benzyl (OBn) and the dimethylaminophenyl (ODmap) esters gave promising results. For these esters, the scope of a model dipeptide synthesis reaction under aqueous conditions was investigated by varying the amino acid donor. The results were compared with those obtained from a previous study of Z-X(AA) -OGp esters. Computational docking analysis of the set of esters was performed in order to provide insight into the differences in the reactivities of all the potential activating esters. Finally, selected ODmap- and OBn-activated amino acids were applied in the synthesis of two biologically active dipeptides on preparative scales.

Insulin degradation: progress and potential

Insulin degradation is a regulated process that plays a role in controlling insulin action by removing and inactivating the hormone. Abnormalities in insulin clearance and degradation are present in various pathological conditions including type 2 diabetes and obesity and may be important in producing clinical problems. The uptake, processing, and degradation of insulin by cells is a complex process with multiple intracellular pathways. Most evidence supports IDE as the primary degradative mechanism, but other systems (PDI, lysosomes, and other enzymes) undoubtedly contribute to insulin metabolism. Recent studies support a multifunctional role for IDE, as an intracellular binding, regulatory, and degradative protein. IDE increases proteasome and steroid hormone receptor activity, and this activation is reversed by insulin. This raises the possibility of a direct intracellular interaction of insulin with IDE that could modulate protein and fat metabolism. The recent findings would place intracellular insulin-IDE interaction into the insulin signal transduction pathway for mediating the intermediate effects of insulin on fat and protein turnover.

The characterization of endosomal insulin degradation intermediates and their sequence of production

Insulin degradation within isolated rat liver endosomes was studied in vitro with the aid of three 125I-insulin isomers specifically labelled at tyrosine (A14, B16 and B26). Chloroquine and 1,10-phenanthroline were used to minimize insulin proteolysis during endosome preparation, whereas the manipulation of endosomal processing of insulin in vitro by Co2+ ions (to activate) and 1,10-phenanthroline (to inhibit) permitted the study of degradation intermediates and their time-dependent production. Structural and kinetic analysis of intermediates isolated from both intra- and extra-endosomal compartments allowed the determination of major cleavage sites and the probable sequence of proteolytic events. It was found that 125I-tyrosine is the ultimate labelled degradation product of all iodo-insulin isomers, suggesting that endosomal proteases are able to degrade insulin to the level of its constituent amino acids. 125I-tyrosine was also the only radiolabelled product able to cross the endosomal membrane. Intra-endosomal insulin degradation proceeds via two inter-related cleavage routes after metalloendoprotease cleavage of the B-chain. One pathway results from an initial cleavage in the centre region of the B-chain (B7-19), probably at B14-15, whereas the major route results from a cleavage at B24-25. B24-25 cleavage removes the B-chain C-terminal hexapeptide (B25-30), which is subsequently cleaved by an aminopeptidase activity to produce first the pentapeptide B26-30 and then 125I-tyrosine. The isolation of intact radiolabelled A-chain from the degradation of 125I-[A14]-insulin suggests that further degradation of proteolytic intermediates containing cleaved B-chain proceeds via interchain disulphide reduction. The A-chain is then processed by several cleavages, one of which occurs at A13-14.

Inhibitory effect of amiloride on glucose transport in isolated rat adipocytes

The effect of amiloride on 3-O-methylglucose (3-O-MG) uptake was studied in isolated rat adipocytes to define to what extent amiloride inhibited the process of insulin action or glucose transport. Amiloride (1 mM), which did not change the intracellular water space of adipocytes, inhibited by 43.3% the insulin-stimulated uptake of 3-O-MG, while it did not appear to inhibit the basal uptake. To distinguish the inhibitory effect on glucose transport activity from that on the process of insulin action, the effect of amiloride was evaluated in the transport system using adipocytes deprived of ATP, in which glucose transporters were considered immobile. Amiloride (1 mM) inhibited this transport by 32.8% in an insulin-stimulated state, which was obtained using adipocytes that had been treated with 20 nM insulin and exposed to 2 mM KCN, whereas it did not inhibit the transport system at the basal state. In the inhibitory effect, 76% was thus attributable to the inhibition of glucose transport activity recruited by insulin and 24% to the inhibition of the action of 20 nM insulin itself. These results indicate that amiloride can not be used as a specific inhibitor of the insulin action itself.