Stereochemical course of the transmethylation catalyzed by catechol O-methyltransferase

The steric course of the methyl group transfer catalyzed by catechol O-methyltransferase was studied using S-adenosylmethionine (AdoMet) carrying a methyl group made chiral by labeling with 1H, 2H, and 3H in an asymmetrical arrangement. Incubation of the two diastereomers of this substrate with catechol O-methyl-transferase purified from rat liver and epinephrine or protocatechuic acid as acceptor gave the corresponding methylated catechols. These were degraded to convert the methoxy group in a series of stereochemically unambiguous reactions into the methyl group of acetate, which was then analyzed for its configuration. The results indicate that the transfer of the methyl group from AdoMet to either acceptor occurs in an inversion mode. The catechol O-methyltransferase reaction thus involves a direct transfer of the methyl group from the sulfur of AdoMet to the oxygen of the catechol in an SN2 process, without a methylated enzyme intermediate.