With isoelectric focusing, we examined heterogeneity of autoantibodies to insulin receptors in serums of two patients with insulin-resistant diabetes and one patient with hypoglycemia. Immunoglobulins were prepared by ammonium sulfate precipitation and ion-exchange chromatography with DEAE-Sepharose and subjected to isoelectric focusing for separation into 30 fractions. The fractions were tested for their ability to inhibit 125I-labeled insulin binding to human placental membranes, immunoprecipitate solubilized insulin receptor cross-linked with 125I-insulin, and mimic or inhibit the action of insulin in rat adipocytes. The results varied among the three patients. In the first patient, inhibition of 125I-insulin-binding activity (IBA) and insulin-receptor-precipitating activity (IPA) were distributed almost identically, but the distribution of insulinlike bioactivity (ILBA) was somewhat different. In the second patient, some fractions exhibited potent IBA without IPA, and these fractions inhibited the action of insulin in rat adipocytes. In the third patient, all of the isoelectric fractions showed IBA without IPA and were insulin antagonists. These observations indicate that some patients have antibodies with pure insulin-antagonist properties and provide further evidence that autoantibodies to insulin receptors are polyclonal and recognize different antigenic sites on insulin-receptor molecules. The findings also suggest that the ability of antibodies to elicit ILBA is linked to the ability to immunoprecipitate 125I-insulin-cross-linked and solubilized receptors, whereas antibodies that only inhibit insulin binding behave as insulin antagonists.

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