Isolation and Characterization of a New Quinoprotein Dehydrogenase, L-Sorbose/L-Sorbosone Dehydrogenase

Gluconobacter oxydans DSM 4025 effectively oxidizes L-sorbose to 2-keto-L-gulonic acid (2 KGA), an industrial precursor of vitamin C. From this microorganism, we purified the enzyme involved in this oxidation reaction. The enzyme is a unique quinoprotein dehydrogenase catalyzing not only the conversion of L-sorbose to L-sorbosone, but also that of L-sorbosone to 2 KGA. The molecular weight of the enzyme was about 135,000, consisting of two subunits with molecular weights of 64,500 and 62,500. As its prosthetic group, non-covalently bound PQQ was found. The dye-linked spectrophotometric enzyme assay showed that the optimum enzyme activity occurred in the pH range about 7.0-9.0, and the enzyme activity was inhibited by EDTA or EGTA. The enzyme showed extremely broad substrate specificity for primary and secondary alcohols, aldehydes, aldoses, ketoses, and other sugar alcohols, but not for methanol or formaldehyde. The cytochrome c obtained from the soluble fraction of this strain was found to act as a physiological electron acceptor of the enzyme.