The effect of bulk hydrogen ion concentration upon the apparent kinetic parameters of purified pig liver catechol O-methyltransferase.

In order to investigate the pH dependence of catechol O-methyltransferase (S-adenosyl-L-methionine:catechol O-methyltransferase, EC 2.1.1.6), kinetic parameters have been determined for the highly purified enzyme from pig liver over the pH range 6.75-8.20 using the substrates S-adenosylmethionine (AdoMet) and 3,4-dihydroxyphenylacetic acid (DOPAC). The Km for AdoMet was found to be invariant with pH while the Km for DOPAC decreased sharply with increasing pH. The group responsible for the latter has a pK of approx. 7.1. The logarithmic (Dixon) plot of Km against pH for both substrates and that of Vmax/Km against pH for DOPAC mirror the kinetic behaviour revealed by linear plots. However, for other parameters, linear graphs indicate peaks too narrow to be explicable by a simple kinetic mechanism, whereas logarithmic plots of these parameters produce graphs apparently not reflecting this behaviour. We conclude that these results are not the products of random error or artefactual data analysis but are too complex to be explicable by a simple model of kinetic behaviour. Possible explanations (adherence of catechol O-methyltransferase to a higher-order mechanism or a dual mode of substrate binding) are advanced.