Phenylalanine and cysteine comprise common miss-sense variants (i.e., single nucleotide polymorphisms [SNPs]) at amino acid position 254 of the human indole(ethyl)amine-N-methyltransferase (hINMT). The phenylalanine variant, which occurs in linkage disequilibrium with two 3’ UTR SNPs, has been reported to associate with elevated urine levels of trimethylselenonium (TMSe), the Se-methylated product of volatile dimethylselenide. hINMT allozymes expressing either cysteine (254C) or phenylalanine (254F) at position 254 were compared for enzyme activity (i.e., K m and V max ) towards the INMT substrates tryptamine, dimethylsulfide (DMS) and dimethylselenide (DMSe) in vitro . The SNP 254C had a higher V max for DMS and tryptamine in the presence of reducing agent than in its absence. Conversely, V max for 254F was insensitive to the presence or absence of reducing agent for these substrates. SNP 254F showed a lower K m for tryptamine in the absence of reducing agent than 254C. No statistically significant difference in V max or K m was observed between 254C and 254F allozymes in the presence of reducing agent for DMSe, The K m values for DMSe methylation were about 10-fold (254C) or 6-fold (254F) more favorable than for tryptamine methylation with reducing agent present. These findings indicated that: 1) That phenylalanine at position 254 renders hINMT methylation of substrates DMS and tryptamine insensitive to a non reducing environment. 2) That human INMT harbors significant thioether-S-methyltransferase (TEMT) activity with a higher affinity for DMSe than tryptamine, 3) The reduction of a 44C/254C disulfide bond in hINMT that increases V max is proposed.

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