Evidence implicating a novel thiol methyltransferase in the detoxificationof glucosinolate hydrolysis products in Brassica oleracea L.

The physiological relevance of a novel thiol methyltransferase from cabbage , and its possible role in sulphur metabolism have been investigated. The e nzyme was absent from the chloroplast, the site of sulphate reduction, and was localized in the cytosol. Potential substrates were initially screened on the basis of their ability to inhibit the methylation of iodide, a previ ously known substrate for the enzyme. Thiocyanate, 4,4'-thiobisbenzenethiol , thiophenol, and thiosalicylic acid were identified as possible substrates . Methylation of these thiols by the purified enzyme using [Methyl-H-3]S-ad enosyl-L-methionine confirmed their nature as substrates. The purified enzy me strongly preferred thiocyanate as a methyl acceptor. The enzyme had K-m values of 11, 51, 250 and 746 mmol m(-3) for thiocyanate, 4,4'-thiobisbenze nethiol, thiophenol and thiosalicylic acid, respectively. The identity of m ethylthiocyanate as the product of thiocyanate methylation by the purified enzyme was confirmed by mass spectrometry. The enzyme was strictly associat ed with glucosinolate-containing plants. Thiol substrates of the enzyme are known products of glucosinolate hydrolysis. Our observations indicate that this enzyme could be involved in the detoxification of reactive thiols pro duced upon glucosinolate degradation in these plants.