Colorimetric assay for guaiacol O-methyltransferase

A relatively simple colorimetric assay was developed for guaiacol O-methyltransferase. Monomethylated phenolic acid substrates are enzymatically methylated to their corresponding dimethoxyl compounds; S-adenosylmethionine serves as the methyl donor. Enzymatic activity is measured by colorimetric assay of the monomethylated (nonreacted) substrate by coupling of the phenolic acid with a diazotized sulfanilic acid. The methoxyl and hydroxyl groups may be in the 3,4- or 4,3-positions on the substrate molecules; however, a side chain (COOH) or a substituted side chain is necessary for enzyme action. The radioactive demonstration of guaiacol O-methyltransferase as a separate entity from catechol O-methyl-transferase was confirmed.