Evidence for a non-phosphorylated route of galactose breakdown in cell-free extracts of Aspergillus niger

Aspergillus niger could utilize D-galactose as sole source of carbon. Cell-free extracts of D-galactose-grown mycelia were able to catalyze the oxidation of D-galactose to D-galactonic acid-γ-lactone (GalA-γ-lact) in the presence of NAD, followed by the appearance of 2-keto-3-deoxy-D-galactonate (KDGal), pyruvate and glyceraldehyde. From 10 μmoles only 6.6 μmoles of GalA-γ-lact were disappeared after 60 min of reaction indicating the presence of GalA-γ-lactonase. Identification of GalA-γ-lact was achieved by ascending paper chromatography. KDGal, pyruvate and glyceraldehyde were also chromatographically identified in the reaction mixture containing D-galactonate which suggests that D-galactonate is degraded into pyruvate and glyceraldehyde via the intermediate formation of KDGal. Such reactions are supposed to be catalyzed by an inducible D-galactonate dehydratase and a constitutive KDGal aldolase. The amount of KDGal, pyruvate and glyceraldehyde were found to be almost equivalent and the equilibrium of the reaction being toward the formation of KDGal. The apparent equilibrium constant (Keq) was calculated and found to be 0.5 × 10−3 M. Results also proved the reversibility of the reaction catalyzed by KDGal aldolase of A. niger. In the light of the findings obtained from the degradation of D-galactose by cell-free extracts of A. niger grown on D-galactose and D-galactonate a nonphosphorolytic pathway was suggested to be operative for the degradation of D-galactose in extracts of A. niger.