A coupled spectrophotometric enzyme assay for methyltransferases

Adenosine deaminase (EC 3.5.4.4), purified from Aspergillus oryzae, is active in deaminating S-adenosylhomocysteine and its related thioethers, whereas the related sulfonium compound, S-adenosylmethionine, is not deaminated. By taking advantage of the different reactivity of the two compounds, a coupled optical enzyme assay for methyl transfer reactions has been developed. The amount of Ado-Hcy formed is calculated from the decrease in optical density at 265 nm, after addition of an excess of adenosine deaminase. The validity of the method has been tested with three purified enzymes, i.e., homocysteine methyltransferase, histamine methylase, and acetylserotonin methyltransferase. Some kinetic constants of these enzymes have been obtained. The procedure is highly accurate, reproducible, and relatively simple compared to the conventional radio-chemical methods currently in use.