Kinetics of inhibition by tyrphostins of the tyrosine kinase activity of the epidermal growth factor receptor and analysis by a new computer program

The kinetics of inhibition of the epidermal growth factor (EGF) receptor (EGFR) tyrosine kinase (TK) activity by erbstatin, tyrphostins, and lavendustin derivatives were studied in a system that employs poly(Glu₆Ala₃Tyr) (GAT) and ATP as substrates, after preactivation with EGF. All data were analyzed for computer best-fit curves by a program that was written for this purpose and is available upon request to those interested. The inhibition kinetics followed a sequential, Bi-Bi, rapid equilibrium, random mechanism, the mechanism of the EGFR-TK. Erbstatin and a few tyrphostins that contain a 3,4-dihydroxy-(cis)-cinnamonitrile [1-(3',4'- dihydroxyphenyl)-2-nitriloethene] group were found to be pure competitive inhibitors with respect to both substrates of the kinase reaction, i.e., GAT and ATP. Two tyrphostins, each containing an additional dihydroxyphenyl group in the α-position, were found to be pure competitive inhibitors with respect to GAT and noncompetitive (or mixed-competitive) inhibitors with respect to ATP. A lavendustin derivative with a 2,5-dihydroxyphenyl ring and a lavendustin derivative with a 3,4-dihydroyphenyl ring were also found to be competitive inhibitors with respect to both ATP and GAT. Various possible modes of binding at the EGFR-TK active center for the tyrphostins studied are proposed and the significance of the present findings, as well as the interpretation of computer analyses of kinetic data, is discussed.