INHIBITORY EFFECT OF FLUORIDE ON INSULIN-RECEPTOR AUTOPHOSPHORYLATIONAND TYROSINE KINASE-ACTIVITY

Fluoride is a nucleophilic reagent which has been reported to inhibit a variety of different enzymes such as esterases, asymmetrical hydrola ses and phosphatases. In this report, we demonstrate that fluoride inh ibits tyrosine kinase activity of insulin receptors partially purified from rat skeletal muscle and human placenta. Fluoride inhibited in a similar dose-dependent manner both beta-subunit autophosphorylation an d tyrosine kinase activity for exogenous substrates. This inhibitory e ffect of fluoride was not due to the formation of complexes with alumi nium and took place in the absence of modifications of insulin-binding properties of the insulin receptor. Fluoride did not compete with the binding site for ATP or Mn2+. Fluoride also inhibited the autophospho rylation and tyrosine kinase activity of receptors for insulin-like gr owth factor I from human placenta. Addition of fluoride to the pre-pho sphorylated insulin receptor produced a slow (time range of minutes) i nhibition of receptor kinase activity. Furthermore, fluoride inhibited tyrosine kinase activity in the absence of changes in the phosphoryla tion of pre-phosphorylated insulin receptors, and the sensitivity to f luoride was similar to the sensitivity of the unphosphorylated insulin receptor. The effect of fluoride on tyrosine kinase activity was mark edly decreased when insulin receptors were preincubated with the copol ymer of glutamate/tyrosine. Prior exposure of receptors to free tyrosi ne or phosphotyrosine also prevented the inhibitory effect of fluoride ..However, the protective effect of tyrosine or phosphotyrosine was ma ximal at low concentrations, suggesting the interaction of these compo unds with the receptor itself rather than with fluoride. These data su ggest: (i) that fluoride interacts directly and slowly with the insuli n receptor, which causes inhibition of its phosphotransferase activity ; (ii) that the binding site of fluoride is not structurally modified by receptor phosphorylation; and (iii) based on the fact that fluoride inhibits phosphotransferase activity in the absence of alterations in the binding of ATP, Mn2+ or insulin, we speculate that fluoride bindi ng might affect the transfer of phosphate from ATP to the tyrosine res idues of the beta-subunit of the insulin receptor and to the tyrosine residues of exogenous substrates.