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

The enzymatic O-methylation of twelve catecholamines and two catechols was studied. From initial reaction rates Km and Vmax were determined. Under the reaction conditions applied the enzymatic O-methylation appears to be first order in the concentration of catechol(amine). For all twelve catecholamines and the two catechols the Km and Vmax based on initial reaction rates are equal to those from the ln(c/co) plots (= first-order kinetics). This fact and the variation of kobs (reaction rate constant in the first-order kinetics) with the starting concentration comply with the formulae derived for these kinetics. The variation of kobs is caused by product inhibition. From the formulae it follows that Ki (inhibition constant) = Km; 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.