We studied the structural and functional characteristics of insulin receptors from guinea pig liver and brain. Binding to crude membrane preparations of liver and brain was time, temperature, and pH dependent. Maximal specific binding to liver crude membrane preparations was 16.4 ± 0.5%, and that to brain crude membrane preparations was 10.4 ± 1.8%. Specificity studies demonstrated typical affinities for insulin receptors with chicken insulin > porcine insulin>human proinsulin>desoctapeptide insulin in both liver and brain. Antiinsulin receptor antiserum inhibited binding of [125I]insulin to both liver and brain crude membrane preparations. Electrophoresis performed under reducing conditions after affinity cross-linking of liver and brain insulin receptors with [125I]insulin revealed labeled proteins (α-subunit) with apparent mol wt of 136,000 in liver and 121,000 in brain. Treatment of liver and brain receptors with both endoglycosidase H and endoglycosidase F increased the electrophoretic mobility of the a-subunit. [125I]Insulin crosslinked receptors from both liver and brain adsorbed to and eluted from wheat germ agglutinin columns in a similar manner, as demonstrated by binding and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In solubilized lectin-purified receptor preparations from liver and brain, insulin stimulated the phosphorylation of the β-subunit and exogenous substrates. When the Δ-kinase activity, as measured by exogenous substrate phosphorylation, of the brain and liver preparations was normalized to the maximal bound to free ratio of the preparations, the Δ-kinase activity was about 4-fold greater in brain than in liver. These studies suggest that the differences between brain and liver insulin receptor α-subunits previously demonstrated in rats are also present in guinea pigs.
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