Differences in organizational structure of insulin receptor on rat adipocyte and liver plasma membranes: role of disulfide bonds.

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Abstract
Binding of 125I-labeled insulin to rat liver and adipocyte plasma membranes was investigated after treatment of the membranes with agents that modify disulfide bonds or sulfhydryl groups. Dithiothreitol, a disulfide-reducing agent, produced a bimodal response in adipocyte plasma membranes with dose-dependent increases in binding occurring over the range of 0-1 mM dithiothreitol; 5 mM dithiothreitol produced decreased binding. Insulin binding reached its maximal increase at 1 mM and was 3 times control values. Scatchard analysis of the 1 mM dithiothreitol effect revealed a straight line plot indicative of 1 class of sites with a Ka of 1.0 .times. 108 M-1 which is intermediate between the 2 Ka obtained from the curvilinear Scatchard plot of control membranes. There was a 20-fold increase in the number of intermediate-affinity receptors compared to high-affinity receptors. The increased 125I-labeled insulin binding after dithiothreitol treatment was reversed by oxidized glutathione in a dose-dependent manner. Interposition of treatment with N-ethylmaleimide, an alkylating agent, prevented oxidized glutathione from reversing the dithiothreitol effect. Reduced glutathione produced the same effect as dithiothreitol. Liver plasma membranes treated with up to 1 mM dithiothreitol exhibited a maximum increase in insulin binding of 20% compared to control. Dithiothreitol at 5 mM decreased insulin binding below that of control membranes. The dithiothreitol effect on insulin binding to adipocyte plasma membranes is due to disruption of disulfide bonds, and the structural organization of the insulin receptor on the plasma membranes is different for liver and for adipose tissue. Apparently the insulin receptors on the plasma membrane of adipocytes possess at least 2 functionally distinct subclasses of disulfide bond but liver insulin receptors do not.