New, Simple Insulin-Receptor Assay With Universal Application to Solubilized Insulin Receptors and Receptors in Broken and Intact Cells

A new, simple insulin-receptor-binding assay has been devised. The assay is based on the separation of free and receptor-bound ¹²⁵I-labeled insulin in 80% ethanol. It was found that the insulin-receptor complex was fully stable at this ethanol concentration, regardless of the source of the receptor employed. The assay has been evaluated with solubilized insulin receptors and membrane-bound receptors from human placenta and porcine liver as well as intact cells with the IM-9 cell line. The assay is simple, rapid, and has large capacity. Comparisons of the ethanol-based assay to the conventionally employed assays with polyethylene glycol or microfuge centrifugation for the separation of free and bound ¹²⁵I-insulin revealed large discrepancies between the assays. The ethanol-based assay always appeared to provide a better separation. Microfuge centrifugation of placental membranes precipitated ∼3% of the ethanol-precipitable insulin-receptor complex, while polyethylene glycol precipitation of solubilized insulin receptors varied between 40 and 80% of the ethanol precipitability, depending on the receptor concentration employed.