A radioreceptor assay for insulin: direct measurement of human serum insulin.

A simple radioreceptor assay was established with sensitivity and accuracy sufficient to detect concentrations of insulin in human peripheral sera. The radioreceptor assay was carried out at 4 degrees C using rabbit erythrocyte membranes and 125I-insulin. In the absence of native insulin, approximately 31% of the total 125I-insulin concentration was bound to erythrocyte membranes. Fourty-six per cent of this binding was displaced by 312 microunits/ml of native insulin. The precision expressed as coefficient of variation was 3 to 21% of the mean for the within-assay, and 1 to 21% for the between-assay, respectively. The recovery of standard porcine insulin added to the serum was 105% on the average. Insulin values in 24 samples of human sera measured by the present assay agreed well with immunoreactive insulin values.