Microsomal Thiol S-Methyltransferase Activity in Rat Salivary Glands.

In the present study, the dithiothreitol methylation activity in submandibular gland microsomeswas characterized. The reaction did not require a divalent cation, and was sulfhydryl group-specific and Sadenosyl-L-homocysteine susceptible. The activity preferred alkyl thiol compounds such as 1, 4-butanedithiol and 2-mercaptoethanol to biological thiol compounds such as L-cysteine and glutathione. These findings suggest that methylation is performed by the action of thiol S-methyltransferase. Thiol S-methyltransferase in the submandibular gland microsomes showed high activity for dithiothreitol and formed dimethyldithiothreitol in addition to monomethyldithiothreitol. The activity was also enhanced by the addition of non-ionic detergent. These findings suggest that the membrane structure embedding thiol S-methyltransferase affects the conformation of the substrate binding-site and/or catalytic site in the enzyme protein. We speculated that the microsomal thiol S-methyltransferase found in salivary glands detoxifies the extracellular thiol compounds and/or the intracellular hydrogen sulfide to protect the secretory functions from the toxic thiol compounds. In addition, the enzymes in the submandibular gland may regulate the thiol-activatable Tkininogenase-T-kinin system via metabolism of thiol compounds.