The genus Gluconobacter is known to carry out limited oxidations using the
NAD(P)-independent membrane-bound dehydrogenases in which the products are
released back to the medium. Reports of further limited oxidations of these primary
oxidation products by Gluconobacter in single step or sequential oxidations by secondary
dehydrogenases are also published. The objective of this project was to evaluate the
nature of one primary (sorbitol) dehydrogenase and one secondary (sorbose)
dehydrogenase because of their importance in Vitamin C production. My hypotheses
were that sorbitol (the primary) dehydrogenase is constitutive, while sorbose (the
secondary) dehydrogenase is inducible. Six Gluconobacter strains from three different
species grew on plates containing 50/0 sorbose, indicating their ability to oxidize sorbose
thus possessing a secondary dehydrogenase. When four strains were tested for their ability to carry out the sequential oxidation of sorbitol and then sorbose on media
containing growth-limiting sorbitol concentrations, three strains showed possible biphasic
growth. However, thin layer chromatography of culture media did not support sequential
sorbitol and sorbose oxidation. F erricyanide assays for sorbitol and sorbose
dehydrogenases from membrane fractions isolated from cells grown on glycerol, sorbitol,
or sorbose showed that sorbitol dehydrogenase activity in all four strains (three species)
tested was always present (constitutive) and its specific activity was always enhanced
by growth on sorbose. Membrane fractions showed no or very low constitutive sorbose
dehydrogenase activity and no evidence that this secondary dehydrogenase was induced.
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