The kinetics of the enzymatic O-methylation of catechols and catecholamines

Abstract

The enzymaticO-methylation of twelve catecholamines and two catechols was studied. From initial reaction rates K_m and V_max were determined. Under the reaction conditions applied the enzymaticO-methylation appears to be first order in the concentration of catechol(amine). For all twelve catecholamines and the two catechols the K_m and V_max based on initial reaction rates are equal to those from the ln(c/c_o) plots (= first-order kinetics). This fact and the variation of k_obs (reaction rate constant in the first-order kinetics) with the starting concentration comply with the formulae derived for these kinetics. The variation of k_obs is caused by product inhibition. From the formulae it follows that K_i (inhibition constant) = K_m; this is confirmed by the experimental data. The polarity of the compounds seems to contribute to the reaction rate at low substrate concentration; the linear relationship between this value and log D may support an apolar region in the binding part of catechol-O-methyltransferase, without denying other contributing factors.