A chemical model of catechol-O-methyltransferase. Methylation of 3,4-dihydroxybenzaldehyde in aqueous solution

The reaction of 3,4-dihydroxybenzaldehyde (LH2) and dimethylsulfate (DMS) to form the m- and p-O-methylated products (vanillin and isovanillin, respectively) in aqueous 2-(N-morpholino)ethane sulfonate buffer was studied kinetically. The products were determined by means of high-performance liquid chromatography. The O-methylation occurred principally at the p-hydroxyl group in the absence of divalent metal ions. In the presence of Cu(II), the m-methylation was promoted and became predominant. Zn(II) showed a similar but less pronounced effect. The effects were explained in terms of the complex formation of LH2. The second order rate constants for the m- and p-methylation of the species, LH2, CuL and CuL2-2 by DMS were calculated. The values and their ratio for the m-/p-reactions increased in the order of LH2 less than CuL less than CuL2-2. The reaction may serve as a chemical model for catechol-O-methyltransferase, which requires divalent metals and catalyzes the m-methylation.