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Purification and properties of methyl formate synthase, a mitochondrial alcohol dehydrogenase, participating in formaldehyde oxidation in methylotrophic yeasts

Methyl formate synthase, which catalyzes methyl formate formation during the growth of methylotrophic yeasts, was purified to homogeneity from methanol-grown Candida boidinii and Pichia methanolica cells. Both purified enzymes were tetrameric, with identical subunits with molecular masses of 42 to 45 kDa, containing two atoms of zinc per subunit. The enzymes catalyze NAD(+)-linked dehydrogenation of the hydroxyl group of the hemiacetal adduct [CH2(OH)OCH3] of methanol and formaldehyde, leading to the formation of a stoichiometric amount of methyl formate. Although neither methanol nor formaldehyde alone acted as a substrate for the enzymes, they showed simple NAD(+)-linked alcohol dehydrogenase activity toward aliphatic long-chain alcohols such as octanol, showing that they belong to the class III alcohol dehydrogenase family. The methyl formate synthase activity of C. boidinii was found in the mitochondrial fraction in subcellular fractionation experiments, suggesting that methyl formate synthase is a homolog of Saccharomyces cerevisiae Adh3p. These results indicate that formaldehyde could be oxidized in a glutathione-independent manner by methyl formate synthase in methylotrophic yeasts. The significance of methyl formate synthase in both formaldehyde resistance and energy metabolism is also discussed.