The rate at near saturation of methylation of 3,4-dihydroxyacetophenone by S-adenosyl-L-methionine, catalyzed by rat-liver catechol O-methyltransferase at pH 7.58 and 37.00 ± 0.05 °C, is increased by deuteration of the methyl group ( FchJ Hcd3 = 0.83 ± 0.05) and decreased by introduction of l3C to the methyl group (V\i/V\i = 1.09 ± 0.05). This shows the rate-determining step to be transfer of the methyl group, with an S^l-like transition state in which the methyl is located "symmetrically" and "tightly" between leaving group and nucleophile.

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