Quantitation of the class I disulfides of the insulin receptor.

The disulfide structure of the insulin receptor was probed using dithiothreitol and [3H]-N-ethylmaleimide to reduce purified human placental receptor and label the cysteine residues. After protein quantitation, the amount of radioactivity in both the alpha and beta subunits was counted for each of the three species generated: native dimer, reduced receptor which remained covalently dimeric, and reduced monomer. It was found that the native receptor contained only one sulfhydryl group, on the beta subunit; the reduced dimer contained one new sulfhydryl on each alpha subunit, and the alpha subunit of the monomer contained two sulfhydryls. This suggests a simple model for the receptor class I disulfides.