A sensitive radioimmunoassay has been developed for the detection of the insulin chains in the plasma of normal and diabetic human subjects. The S-sulfonated derivatives of bovine insulin A and B chains were purified, iodinated with high specific activity and used for immunoassay by the double antibody technic. Twelve noninsulin-requiring diabetic and nine normal, nonobese subjects were studied before and during a four-hour oral glucose tolerance test by determination of plasma immunoreactive insulin, A chain and B chain. The results of our data indicate that: detectable levels of both insulin A and B chain do exist in human plasma; A chain is present in higher concentration than B chain in both normal and diabetic plasma; and greater than normal amounts of A chain can be detected in diabetic subjects and the concentration increases during a glucose tolerance lest in these patients.

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