Characterization of [LeuB-24]- and [LeuB-25]-insulin analogues. Receptor binding and biological activity

Human [LeuB-24]- and [LeuB-25]-insulins were semi-synthesized from porcine insulin by an enzyme-assisted coupling method. The receptor-binding ability of [LeuB-24]- and [LeuB-25]-insulins was 30--48% and 2--5% respectively of that of human insulin. There was no significant difference in degradation between human insulin and these analogues on incubation with isolated adipocytes. The decreased affinity of these analogues was due to an increased dissociation rate rather than a change in the association rate of their binding to human cultured lymphocytes. The negative co-operative effect of [LeuB-24]- and [LeuB-25]-insulin was decreased to 50 and 1% respectively of that of human insulin at a concentration of 100 ng/ml. The ability of [LeuB-24]- and [LeuB-25]-insulin to stimulate 2-deoxyglucose uptake in isolated rat adipocytes was 35 and 4% respectively of that of human insulin. These analogues did not have an antagonistic effect on the biological activity of human insulin. The immunoreactivity of [LeuB-25]insulin was similar to that of porcine or human insulin, whereas [LeuB-24]insulin demonstrated decreased binding to anti-(porcine insulin) antibodies. These findings suggest that B-chain phenylalanine-25 residue is more crucial for receptor binding and negative co-operativity, whereas the B-chain phenylalanine-24 residue may play a more important role in binding to anti-insulin antibody.