Assay of Plasma Insulin in Human Subjects by Immunological Methods

WE have previously reported on the immuno-assay of beef insulin and certain other animal insulins, employing antiserums from human subjects treated with commercial mixtures of beef and pork insulin1. The insulin-binding antibodies present in these anti-serums do not form precipitable complexes with insulin, but with the use of insulin labelled with iodine-131 the complexes are readily demonstrable by paper chromatography and electrophoresis2. Beef, pork, sheep and horse insulins can be assayed quantitatively by measurement of the degree of competitive inhibition of binding of any insulin labelled with iodine-1311–3. As might have been anticipated, however, human insulin competes too weakly in systems employing human antiserum to be measurable at concentrations which obtain in vivo. Furthermore, the lack of availability of significant quantities of pure human insulin precludes its use as an antigen for animal immunization. However, in the present work, it has been found that human insulin cross-reacts strongly with insulin-binding antibodies in guinea pigs immunized with crystalline beef insulin, and that guinea pig anti-beef insulin serum has characteristics suitable for the detection and measurement of human insulin at concentrations which exist in the plasma of normal fasting subjects.