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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 Mrvalue of about 40000-50000. It catalysed the formation of stoicheiometric amounts of formaldehyde, sulphide and H2O2 from MM and O2. It had a Km of 5-10 μm for MM and was strongly inhibited by substrate concentrations above 14 μm, the Ki for this inhibition being 42 μm. Ethyl mercaptan and sulphide also served as substrates for the enzyme (Km 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 (Ki 90 μm). Alcohol oxidase (EC 1.1.3.13) purified from Hansenula polymorpha was also found to oxidize MM (Km 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.

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