A radioimmunoassay for isolated A-chain is described. The immunospecificity of A-chain is shown to vary depending on the variation in structure about the cysteine residues. The presence of one or two intrachain disulfide bonds alters the immunoreactivity greatly in a system in which A-chain with all four cysteine residues carboxymethylated is the homologous antigen. Species specificity is also suggested by differences in the immunoreactivity of the cleaved products of bovine and porcine insulin. Further attempts to assay A-chain in human plasma by immunological methods must take these two factors into consideration.

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