| Abstract: | Insulin receptor kinase activity was measured in partially purified receptor preparations from livers of rats fed a standard diet or subjected to either prolonged fasting or a high carbohydrate (CHO) diet, conditions known to decrease (fasting) and increase (CHO) insulin action. Basal and insulin-stimulated phosphorylation of the beta subunit of the insulin receptor was comparable in all groups with a half-maximal effect at approximately 2.0 ng/ml free insulin and a 10-12-fold maximal effect. The kinase activity of insulin receptors from the three groups was further examined using the synthetic polypeptide Glu 4:Tyr 1. Basal and insulin-stimulated rates of Glu 4:Tyr 1 phosphorylation were highest in the CHO-fed and lowest in the fasted group. The magnitude of these differences was the same in the absence or presence of insulin; thus, the alterations in receptor kinase activity in fasting and CHO feeding were entirely expressed in the basal rate of peptide phosphorylation. Antireceptor antibody immunoprecipitated 70-80% of the basal Glu 4:Tyr 1 kinase activity in each group; the remaining 20-30% showed minor group differences when normalized for the amount of protein present in the receptor preparations. These results indicate that the group differences in basal kinase were intrinsic to the insulin receptor. Insulin increased the Vmax of Glu 4:Tyr 1 phosphorylation by approximately 30 fmol of phosphorus/fmol of binding activity/30 min in all three groups; however, the absolute Vmax was highest in the CHO-fed and lowest in the fasted group. The Km of Glu 4:Tyr 1 phosphorylation was unaffected by insulin and was comparable (approximately 0.25 mg/ml) in the three groups. These findings indicate that fasting and CHO feeding produce changes in receptor kinase activity which are regulated by mechanisms independent of insulin and that the alterations show substrate specificity so that differences are detected with one substrate (Glu 4:Tyr 1) but not another (the beta subunit). |
