To determine whether antibodies to exogenous insulin bind and inactivate endogenous insulin in vivo, technics were devised to isolate and measure directly antibody bound endogenous insulin in the presence of residual exogenous hormone in human and rabbit (leporine) serum. Insulin-antibody fractions in serum were isolated by ultra centrifugation or preferential salt precipitation, insulin was extracted by acid alcohol, and exogenous (bovine) and endogenous (human or leporine insulins) were differentially determined by immunoassay.

Insulin in the antibody fraction of normal serum is undetectable (< 10μU. per ml.). In two resistant diabetic patients all measurable insulin proved to be bound to antibody. Exogenous insulin, which was 4 to 5 mU. per ml. seventeen hours after the last insulin treatment, remained elevated for three to sixteen days but disappeared after ten to twenty days. The subjects proved to be capable of secreting endogenous insulin that was almost completely bound to antibody in the serum. High concentrations of bound endogenous insulin (1 to 8 mU. per ml.) persisted for two to five weeks and levels were still elevated after fifty-four days. In immunized rabbits, only bound endogenous insulin was detectable three weeks after bovine insulin was discontinued. Levels remained high (1 to 2 mU. per ml.) for five weeks. Blood sugar was normal at all times, indicating that most of the bound insulin was biologically inactive.

In man and rabbits endogenous bound insulin decreased with declining antibody titers, hence did not serve as an antigenic stimulus sufficient to maintain antibody production.

Purified insulins, both human and leporine, competitively displaced bovine insulin-I-131 from the antibody sites of human and leporine serum respectively, though they were less effective than purified bovine insulin.

It is concluded that the administration of crystalline insulin can produce circulating autoantibodies capable of binding and inactivating large amounts of endogenous hormone in vivo.

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