Characterization of affinity-purified insulin receptor/kinase. Effects of dithiothreitol on receptor/kinase function

The insulin receptor/kinase was purified to near homogeneity from human placenta. The purified kinase exhibited a specific activity of 300 nmol/min/mg of protein at 30° C using the synthetic peptide, Arg-Arg-Leu-Ile-Glu-Asp-Ala-Glu-Tyr-Ala-Ala-Arg -Gly, as substrate in the presence of insulin. Treatment of the receptor/kinase with dithiothreitol (DTT) reduced insulin binding by 40-50% and also inhibited tyrosine kinase activity. Phosphorylation and activation of the receptor/kinase did not prevent the DTT-induced loss of binding but completely protected it from the deleterious effects of reducing agent on enzymic activity. Analyses of the structure of the receptor/kinase following phosphorylation and treatment with DTT indicated that the class I disulfide bonds were reduced under the conditions employed, but the tetrameric structure of the receptor/kinase was essentially unaltered. These findings indicate that intact class I disulfides are required for insulin binding but are not necessary for maintenance of the preactivated kinase. DTT was also found to enhance the autoactivation of the insulin receptor/kinase and to promote the reversal of the autophosphorylation reaction. Thus disulfide bonds appear to have multiple roles in the function of the insulin receptor/kinase.