Sorbitol dehydrogenase as a part of Aspergillus niger D-galactose catabolism

D-galactose is a hexose sugar found in hemicellulose. There are probably three different pathways for D-galactose catabolism in fungi, the Leloir, an oxidative and an oxidoreductive. The Leloir pathway has phosphorylated intermediates and galactokinase as the first enzyme. The oxidative pathway has no phosphorylated intermediates and D-galactose is oxidised to D-galactonate in the first reaction. In subsequent reactions D-galactonate is converted to pyruvate and glycerol. In addition to these two pathways an oxidoreductive pathway for D-galactose catabolism, that partly employs the enzymes of the L-arabinose pathway, was suggested to exist. In this work we studied the oxidoreductive D-galactose pathway where D-galactose is first reduced to
D-galactitol by a reductase that is also active with L-arabinose. It is then oxidised to L-xylo-3-hexulose by the L-arabitol dehydrogenase. It was suggested that L-xylo-3-hexulose is then reduced to D-sorbitol which is finally oxidised to D-fructose by a sorbitol dehydrogenase. We identified a gene coding
for a sorbitol dehydrogenase that is strongly induced on sorbitol and we evaluated the role of the enzyme in D-galactose catabolism.