Subunit structure of the purified human placental insulin receptor: Intramolecular subunit dissociation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis

Insulin receptors purified from human placental membranes by gel-filtration and insulin-agarose affinity chromatography were found to be composed of eight different high molecular weight complexes as identified by nonreducing sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The subunit stoichiometry of these different high molecular weight forms of the insulin receptor were determined by comparisons of silver-stained gel profiles with the autoradiograms of ¹²⁵I-insulin specifically cross-linked to the α subunit and γ-³²P]ATP specifically autophosphorylated β subunit gel profiles. Two-dimensional SDS-polyacrylamide gel electrophoresis in the absence and presence of reductant confirmed the subunit stoichiometries as α₂β₂, α₂ββ₁, α₂(β₁)₂, α₂β, α₂β₁, α₂, αβ, and β, where α is the M(r) = 130,000 subunit, β is the M(r) = 95,000 subunit, and β₁ is the M(r) = 45,000 subunit. Treatment of the insulin receptor preparations with oxidized glutathione or N-ethylmaleimide prior to SDS-polyacrylamide gel electrophoresis increased the relative amount of the α₂β₂ complex concomitant with a total disappearance of the α₂β, α₂β₁, α₂, and free β forms. The effects of oxidized glutathione were found to be completely reversible upon extensive washing of the treated insulin receptors. In contrast, the effects of N-ethylmaleimide were totally irreversible by washing, consistent with known sulfhydryl alkylating properties of this reagent. The formation of these lower molecular weight insulin receptor subunit complexes was further demonstrated to be due to SDS/heat-dependent intramolecular sulfhydryl-disulfide exchange occurring within the α₂β₂ complex. These studies demonstrate that the largest disulfide-linked complex (α₂β₂) is the predominant insulin receptor form purified from the human placenta with the other complexes being generated by proteolysis and by internal subunit dissociation.