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 insulin receptor, which causes inhibition of its phosphotransferase activity. 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 receptors partially purified from rat skeletal muscle shows a strict requirement of Mg2+ ions (Ka near 11 mM). This effect of Mg2+ was inhibited in a competitive manner by Mn2+, which is compatible with competition 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 concentrations of Mg2+. Moreover, the addition of increasing concentrations 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 fluoride-susceptible form. Based on the characteristics of the inhibition of tyrosine kinase shown by fluoride it might be proposed that its action is exerted by the formation of multi-ionic MgF complexes analogous to Pi, which bind to the insulin receptor kinase.