Radioimmunoassays for the measurement of the A chain and B chain of insulin in the presence of insulin are described. The assays are sensitive to 0.3 ng./ml. each of A chain and B chain. The results of the immunoassays of human plasma indicate that both A and B chains are present in human plasma. The criteria for this evidence were (a) quantitative recovery of added A and B chains from plasma and (b) parallel relationships between the standard curves of A and B chains and results upon dilution of plasma. The mean concentrations of A chain and B chain in plasma from nondiabetic subjects were 3.7 ± 1.0 and 3.4 ± 1.1 ng./ml., respectively. The concentrations of A chain were markedly higher in diabetic and high risk potential diabetic subjects. It appears possible that an elevated level of A chain may be used as a diagnostic criterion for prediabetes. There was no apparent effect of oral glucose on the concentration of A and B chains in the plasma of nondiabetic subjects as that observed on the concentration of insulin.
Studies on the immunospecificity of the cysteine residues of the two chains showed that thiol, S-sulfonated, oxidized and S-carboxymethylated chains reacted similarly; this indicates that S-sulfonated cysteine residues are probably not involved in the antigen-binding reaction.