Methyl Mercaptan Oxidase, a Key Enzyme in the Metabolism of Methylated Sulphur Compounds by Hyphomicrobium EG

SUMMARY: Methyl mercaptan (MM)-oxidase was purified tenfold to near homogeneity from Hyphomicrobium EG grown on dimethyl sulphoxide. The enzyme was a monomer with an Mr value of about 40000-50000. It catalysed the formation of stoicheiometric amounts of formaldehyde, sulphide and H2O2 from MM and O2. It had a K m of 5-10 μm for MM and was strongly inhibited by substrate concentrations above 14 μm, the K i for this inhibition being 42 μm. Ethyl mercaptan and sulphide also served as substrates for the enzyme (K m 18 and 60 μm respectively), whereas methanol was not oxidized. Upon oxidation of these compounds H2O2 was formed. Although sulphide was a substrate for the enzyme, it also acted as a non-competitive inhibitor of MM oxidation (K i 90 μm). Alcohol oxidase (EC 1.1.3.13) purified from Hansenula polymorpha was also found to oxidize MM (K m 110 μm) although at a low rate. The products formed were the same as for MM oxidation by the bacterial enzyme. In contrast to MM-oxidase however, alcohol oxidase was inactive with sulphide and was not inhibited by methyl mercaptan concentrations up to 500 μm.