KINETICS OF INHIBITION BY TYRPHOSTINS OF THE TYROSINE KINASE-ACTIVITYOF THE EPIDERMAL GROWTH-FACTOR RECEPTOR AND ANALYSIS BY A NEW COMPUTER

The kinetics of inhibition of the epidermal growth factor (EGF) recept or (EGFR) tyrosine kinase (TK) activity by erbstatin, tyrphostins, and lavendustin derivatives were studied in a system that employs poly(Gl u(6)Ala(3)Tyr) (GAT) and ATP as substrates, after preactivation with E GF. All data were analyzed for computer best-fit curves by a program t hat was written for this purpose and is available upon request to thos e interested. The inhibition kinetics followed a sequential, Bi-Bi, ra pid equilibrium, random mechanism, the mechanism of the EGFR-TK. Erbst atin and a few tyrphostins that contain a 3,4-dihydroxy-(cis)-cinnamon itrile [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 alpha-position, were found to be pure competitive inhibitors with respect to GAT and noncompetiti ve (or mixed-competitive) inhibitors with respect to ATP. A lavendusti n derivative with a 2,5-dihydroxyphenyl ring and a lavendustin derivat ive with a 3,4-dihydroyphenyl ring were also found to be competitive i nhibitors with respect to both ATP and GAT. Various possible modes of binding at the EGFR-TK active center for the tyrphostins studied are p roposed and the significance of the present findings, as well as the i nterpretation of computer analyses of kinetic data, is discussed.