EFFECT OF CATIONS ON THE TYROSINE KINASE-ACTIVITY OF THE INSULIN-RECEPTOR - INHIBITION BY FLUORIDE IS MAGNESIUM-DEPENDENT

We have recently reported that fluoride interacts directly with the in sulin receptor, which causes inhibition of its phosphotransferase acti vity. The inhibitory effect of fluoride on phosphotransferase activity is not due to the formation of complexes with aluminium and occurs in the absence of alterations to the binding of ATP or insulin. In this report we substantiate that the tyrosine kinase activity of insulin re ceptors partially purified from rat skeletal muscle shows a strict req uirement of Mg2+ ions (Ka near 11 mM). This effect of Mg2+ was inhibit ed in a competitive manner by Mn2+, which is compatible with competiti on of both divalent ions for binding sites. The inhibition of tyrosine kinase activity caused by fluoride was dependent on the concentration of Mg2+ in the medium and no inhibitory effect was detected at low co ncentrations of Mg2+. Moreover, the addition of increasing concentrati ons of Mn2+ in the presence of a constant high concentration of Mg2+, led to a gradual decrease in the inhibitory effect of fluoride. These results indicate that the Mg-insulin receptor complex is the major flu oride-susceptible form. Based on the characteristics of the inhibition of tyrosine kinase shown by fluoride it might be proposed that its ac tion is exerted by the formation of multi-ionic MgF complexes analogou s to Pi, which bind to the insulin receptor kinase.