THIOL S-METHYLTRANSFERASE FROM RAT-LIVER

The thiol S-methyltransferase from rat liver has been solubilized and prepared in homogeneous form. The enzyme exists in a monomer of Mr 28,000 although enzyme activity is highly unstable with a half-life of 4 days under the best conditions of storage. The reaction requires S-adenosylmethionine as methyl donor but, as is the case with many enzymes active in detoxification, a large variety of lipophilic compounds can serve as acceptors. Acceptor activity is limited to thiols. The naturally occurring hydrophilic thiols, glutathione and cysteine, act neither as substrates nor as inhibitors. The course of the reaction is biphasic with an initial rapid formation of product that is followed by a slower linear rate. The suggestion is offered that this behavior reflects the slow dissociation of an enzyme-product complex composed of enzyme and S-adenosyl-homocysteine. © 1979.