DIABETES-ASSOCIATED IMPAIRMENT OF HEPATIC INSULIN-RECEPTOR TYROSINE KINASE-ACTIVITY - A STUDY OF MECHANISMS

Insulin receptor tyrosine kinase activity solubilized from liver of control and streptozotocin diabetic rats was studied using histone H2b and poly-Glu-Tyr (4:1) as phosphoacceptors. Both substrates inhibited autophosphorylation and exogenous kinase activity when added before, but not after, receptor activation with ATP. When H2b was added before ATP, insulin stimulated exogenous kinase activity of diabetic-derived receptors was significantly higher (approximately-50%) than control values at low H2b concentrations, but significantly lower (approximately-50%) than control values at high H2b concentrations, suggesting a decrease in the apparent K(m) and maximal velocity of the diabetic receptor tyrosine kinase toward H2b. When receptors were allowed to maximally autophosphorylate before the addition of H2b, the maximal H2b kinase activity of diabetic-derived receptors was only approximately-25% lower than that of controls. These effects were not attributable to altered ATP kinetics. Insulin receptor kinase activity toward the substrate poly-Glu-Tyr (4:1) was unaltered by insulinopenic diabetes. Insulin receptor alpha-beta dimers were not detectable in either control or diabetic-derived preparations. We conclude that the impairment of hepatic insulin receptor kinase activity associated with insulinopenic diabetes reflects a decreased ability to maximally activate, which is enhanced when the receptor is activated in the presence of some substrates, e.g. H2b. Impaired signalling by the diabetic-derived receptor appears to be dependent on the type of substrate and its concentration.