Intrinsic kinase activity of the insulin receptor. The intact (alpha 2 beta 2) insulin receptor from rat liver contains a kinase domain with greater intrinsic activity than the intact insulin receptor from human placenta.

We are interested in developing methods to rigorously characterize the intrinsic enzymatic activity of the insulin receptor. We have previously shown that the intact, kinase active form of the receptor can be separated from inactive forms isolated from human placenta. Therefore, the determination of kinase activity, when normalized to the number of receptors based on binding, is not complicated by the presence of insulin receptor forms which bind insulin normally, but are kinase inactive. We now have extended this separation technique to insulin receptor preparations from rat liver. Thus, the determination and comparison of the intrinsic kinase activity of insulin receptor from human placenta and rat liver was performed. When normalized to the same number of insulin receptors which are autophosphorylated to the same degree, the rat liver insulin receptor catalyzes the transfer of phosphate from ATP to three different substrates, on average, 2.8-fold quicker than receptor from human placenta. This probably represents an inherent difference in the intrinsic kinase activity (Vmax), since the values for KM of the substrates are essentially identical, for insulin receptors from both sources. Intrinsic kinase differences may reflect different biological roles and/or differential regulation by exogenous factors. We are now examining this hypothesis in light of reports that demonstrate regulation of intrinsic kinase activity of the insulin receptor in certain physiological and pathological states.