Hicrosomal S-methylation activity for thiol compounds in rat submandibular gland

We report the formation of an unknown methylated compound, in addition to the methylated phospholipids catalyzed by phospholipid W-methyltransferases, during the incubation of submandibular gland microsomes with Sadenosyl-L-{ CH3-4H]methionine in the presence of dithiothreitol (DTT). This compound was formed in microsomal-protein- and time-dependent manners, but not without DTT. When DTT which has two thiol and two hydroxy groups was replaced by 1,4-butanedithiol or 2,3-butanediol, only the former thiol compound was reactive, implying the £methylat ion of DTT by thiol S-methyltransferase activity. Since mono- and dimethylated DTTs were formed during a 60min incubation, the latter may be produced by the successive methylation of a monomethylated form. The thiol Sinethyltransferase activity was unaffected by Triton X-100, Lubrol PX and several cholates. Although EDTA partially inhibited the activity, Ca2 + and Hg2+ had no effect. i,Cysteine and glutathione were poor substrates as they were for thiol S-methyltransferase in the liver and intestine. The physiological function of the enzyme in the submandibular gland is unclear at present. Since several thiols have high nucleophilic reactivity on biomolecules, the enzyme may play a role in the detoxication of bioreactive thiols like that in the liver and intestine.