Phenylethanolamine-N-methyltransferase:alcohol and the mechanism of action

Mixed-solvent systems of methanol and other alcohols and water were used to study the properties of bovine phenylethanolamine-N-methyltransferase. The presence of methanol decreased the binding affinity of the enzyme for its amine substrate but did not alter the maximum velocity. The change in binding was accompanied by an alkaline shift in the pK of an ionizable group in the active site. The well-known property of enzyme inhibition by substrate was also alleviated. Increasing the pH of the medium, in the presence or absence of methanol, increased the maximum velocity but did not alter substrate inhibition. It is proposed that substrate inhibition is due in part to the ionic state of a single unidentified ionizable group in the active site of the enzyme and to a slow release of product. Evidence that an essential, pH-dependent sulfhydryl modulates product release is presented. The properties of phenylethanolamine-N-methyl-transferase are quite responsive to changes in pH, ionic strength, and water content so that the enzyme may well be regulated at the microenvironmental level.