Abstract: Disclosed are novel methyltransferase assay methods, comprising: including, in a reaction mixture for a methyltransferase activity, a purified or recombinant adenosine nucleosidase activity that catalyses release of an adenine or adenine derivative moiety from a transmethylation product, and a purified or recombinant adenine deaminase activity that catalyses deamination of the released moiety to hypoxanthine or respective derivative and ammonia, wherein the methyltransferase activity is rate-limiting; and determining the methyltransferase activity by spectrophotometric or chromatographic monitoring of the coupled deamination reaction products, or of subsequent enzymatic or chemical reactions coupled thereto.