Three forms of disulfide-linked insulin receptor complexes are labeled by covalent cross-linking to receptor- bound 125I-insulin in native adipocyte or liver membranes. These receptors of Mr 350,000, Mr 320,000, and Mr 290,000 are composed of α(Mr 125,000), β(Mr 90,000), and β1(Mr 49,000) subunits in stoichiometries of (αβ)2, (αβ)(αβ)1 and (αβ1)2, respectively. In adipocyte membranes, these receptor structures can undergo a first step of reduction by dithiothreitol, dissociating into Mr 210,000 (αβ) and Mr 160,000 (αβ1) partially reduced receptor fragments. Complete dissociation of such fragments into the free α,β , and β1 receptor subunits is achieved at high reductant concentrations. In liver plasma membranes the partially reduced receptor species of Mr 210,000 and Mr 160,000 are observed even when electrophoresis is performed under nonreducing conditions. This observation indicates that native liver plasma membranes contain multiple redox states of the high affinity insulin receptor.

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