Functions as thioether S-methyltransferase and is active with a variety of thioethers and the corresponding selenium and tellurium compounds, including 3-methylthiopropionaldehyde, dimethyl selenide, dimethyl telluride, 2-methylthioethylamine, 2-methylthioethanol, methyl-n-propyl sulfide and diethyl sulfide. Plays an important role in the detoxification of selenium compounds (By similarity). Catalyzes the N-methylation of tryptamine and structurally related compounds. a tertiary amine + S-adenosyl-L-methionine = a methylated tertiary amine + H(+) + S-adenosyl-L-homocysteine a secondary amine + S-adenosyl-L-methionine = a methylated secondary amine + H(+) + S-adenosyl-L-homocysteine a primary amine + S-adenosyl-L-methionine = a methylated primary amine + H(+) + S-adenosyl-L-homocysteine dimethyl sulfide + S-adenosyl-L-methionine = S-adenosyl-L-homocysteine + trimethylsulfonium Monomer. Widely expressed. The highest levels were in thyroid, adrenal gland, adult and fetal lung. Intermediate levels in heart, placenta, skeletal muscle, testis, small intestine, pancreas, stomach, spinal cord, lymph node and trachea. Very low levels in adult and fetal kidney and liver, in adult spleen, thymus, ovary, colon and bone marrow. Not expressed in peripheral blood leukocytes and brain. Belongs to the class I-like SAM-binding methyltransferase superfamily. NNMT/PNMT/TEMT family.