Biological activity of proinsulin and related polypeptides in the fat tissue.

A number of insulin-like and proinsulin-like polypeptides were employed in the study of the structure-activity relationship of proinsulin. Glucose incorporation into CO2 and lipids, and the antilipolytic activity, were used as parameters to measure the biological activity of these polypeptides. The split proinsulin and the long A chain proinsulin, with 2 connecting amino acid residues, Arg-Arg, missing, have little more activity over that of native proinsulin. The long B chain proinsulin, with the absence of 2 connecting amino acid residues, Lys-Arg, has a 3to 4-fold increase in activity compared to proinsulin. Further removal of amino acid residues from proinsulin molecule beyond Lys-Arg level can progressively increase proinsulin activity. These studies suggest that the decreased biological activity of proinsulin may be due essentially to the blocking of the A chain of insulin part in proinsulin molecule rather than the blocking of B chain. Insulin, with 1 or 2 arginine residues attached to COOHterminal of B chain, loses 60% biological activity when compared to insulin molecule. The effect of positively charged groups may alter the insulin conformation or decrease the binding capacity between the insulin derivatives and the insulin receptor site, or both. C-peptide, a by-product in the proinsulin conversion to insulin, shows no insulin-like activity in homologous and heterologous fat tissues. No antagonistic or potentiating effect of C-peptide for insulin or proinsulin can be demonstrated in homologous and heterologous fat tissues. The feedback control of C-peptide on insulin or proinsulin action is not likely to exist in the adipose tissue.