Binding and autophosphorylating activity of human insulin analogs

Insulin receptor binding and autophosphorylating activities of a number of synthetic analogs of human insulin have been examined using highly purified insulin receptor from human placenta. In general, autophosphorylation correlates well with the ability of the analogs to stimulate glucose oxidation and to inhibit lipolysis in adipocytes although their biological activities varied over a wide range. These findings support the hypothesis that autophosphorylation is an obligatory step in the pathways leading to glucose oxidation and inhibition of lipolysis. The relative biological potencies of the analogs in the autophosphorylation assay also correlated well with their receptor-binding affinities except for the peptides [endo-TyrB16a]insulin, in which an additional Tyr has been inserted between TyrB16 and LeuB17 and [ProA2]insulin. The relative receptor binding affinity of [endo-TyrB16a]insulin is significantly greater than its biological activity in the adipocyte or receptor autophosphorylation assays. The converse is true for [ProA2]insulin. These results demonstrate that the amino-acid residues involved in binding and receptor activation may not be identical.