IMMUNOASSAY OF ENDOGENOUS PLASMA INSULIN IN MAN

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Abstract
An immunoassay for plasma insulin in man is presented, based on the reaction of human insulin, competing with beef insulin-Il31, with insulin-binding antibodies in the sera of guinea pigs immunized with beef insulin. The method is sensitive to less than 1 [mu]V of insulin, permitting measurement of insulin concentrations in 10 to 20 [mu]l of plasma. Human insulin added in vitro to plasma is recovered quantitatively, and measured endogenous insulin concentrations decrease proportionately on dilution of plasma over the range 1:2 to 1:100. Endogenous plasma insulin is destroyed by incubation with cysteine and endogenous insulin adsorption by cellulose columns is quantitatively similar to the adsorption of added beef insulin-I131. Repeat determinations of insulin concentrations on the same plasma samples (stored frozen in the interim) one or more months apart, with different lots of insulin-1131, were generally in good agreement. Data are presented for insulin concentrations in nondiabetic and diabetic human subjects. Calculation of the average normal daily insulin secretion rate, on the basis of the data presented," yields an estimate of about 55 U of insulin per day.