Disulfide reduction converts the insulin receptor of human placenta to a low affinity form.

A B S T R A C T Treatment of human placenta membranes with dithiothietol (DTT) followed by N-ethylmaleimide results in a 60% reduction in insulin bind- ing.Treatment with N-ethylmaleimide alone has little effect.The decrease in insulin binding that results from DTT treatment is due to a decrease in affinity for insulin, with little change in total receptor number.DTT has similar effects on receptor solubilized from placenta membranes with Triton X-100, indicating that its effects are not attributable to changes in the arrange- ment of receptors in the membrane.In contrast to placenta membranes, treatment of liver membranes with DTT does not decrease insulin binding.These results suggest that reduction of a critical disulfide bond in insulin receptors from human placenta converts the receptor to a low affinity form.METHODS Human placenta membranes (5) and rat liver membranes (6) were prepared as described previously, with the modification that 10 utg/ml phenyl methyl sulfonyl fluoride and 5 mM EDTA were included in the homogenization buffer to prevent proteolysis.Placenta membranes (10 mg protein/ml of 50 mM Tris HCI, pH 7.7) were solubilized with 2% Triton X-100, and after centrifugation at 200,000 g for 1 h, the supernate was