Effect of N-methylation of selected peptide bonds on the biological activity of insulin. [2-N-methylisoleucine-A]insulin and [3-N-methylvaline-A]insulin.

Hydrogen bonding involving peptide bonds of the backbone of the insulin molecule may play an important role in insulin-receptor interaction. Our previous work suggested that the A2-A8 helical segment of the hormone molecule participates in this interaction. To investigate the possible involvement of peptide bonds of this segment in insulin-receptor interaction the [2-N-methylisoleucine-A]insulin and [3-N-methylvaline-A]insulin ([MeIle2-A]- and [MeVal3-A]insulins) were synthesized. The circular dichroic spectra of the analogues were obtained and their properties were examined in several biological assays. The circular dichroic spectra suggested that the analogues remained monomeric at concentrations at which insulin is predominantly dimeric, and that their A2-A8 helical segments are distorted. The in vitro biological activity and the receptor binding affinity of these analogues were compared with that of natural insulin. Both analogues are weak full agonists. [MeIle2-A]insulin displayed a potency of 5.4 +/- 0.3% in stimulating lipogenesis and 4.6 +/- 2.3% in receptor binding affinity in rat fat cells and rat liver plasma membranes respectively. [MeVal3-A]insulin displayed a potency of 2.1 +/- 0.2% in lipogenesis and 1.0 +/- 0.3% in receptor binding assays. In radioimmunoassays [MeIle2-A]- and [MeVal3-A]insulins exhibited potencies of 13% and 11% respectively relative to the natural hormone. The substantially decreased biological activity and receptor binding affinity of these analogues may be attributed partly to the change of conformation and partly to the loss of hydrogen bonding capacity of the A2-A8 segment brought about by N-methylation of the A1-A2 or A2-A3 peptide bonds.