We report a 31-yr-old nondiabetic male patient with acanthosis nigricans whose hyperinsulinemia and insulin resistance could not be explained by anti-receptor antibodies or by an intrinsic defect of insulin binding to his cells. An acid-alcohol extract of the patient's serum contained a factor that inhibited insulin-stimulated glucose transport in rat adipocytes. Low levels of the factor could be detected in 9 of 13 unselected patients with non-insulin-dependent diabetes. The factor was heat stable and resistant to treatment with acid, base, and various lytic enzymes. It eluted from a Bio-Gel β-2 column with an apparent molecular weight of 300. The factor also inhibited stimulation of glucose transport in adipocytes by the insulin mimickers hydrogen peroxide and sodium vanadate. In vitro incubation of rat soleus muscles in the presence of the factor resulted in inhibition of insulin-stimulated glucose transport. The factor enhanced 125I-labeled insulin binding in both adipocytes and muscle. A preparation of insulin receptors obtained from muscles incubated with serum factor showed increased binding of 125I-insulin to the α-subunit of the insulin receptor. Autophosphorylation of the β-subunit and phosphorylation of exogenous substrate were increased in the receptor preparation obtained from muscles that had been incubated with serum factor. However, the increase in kinase activity was approximately the same as the increase in binding activity. No difference in kinase activity was observed when assayed under conditions in which 125I-insulin binding activity had been equalized. These results suggest that insulin resistance in this patient was due to a circulating factor of low molecular weight that uncoupled insulin stimulation of glucose transport from receptor binding and phosphorylation. The factor appears to increase the binding activity of the α-subunit of the insulin receptor without affecting the kinase activity of the β-subunit.

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