THE DELEY-DOUDOROFF PATHWAY OF GALACTOSE METABOLISM IN AZOTOBACTER-VINELANDII

Azotobacter vinelandii cell extracts reduced NAD(+) and oxidized D-gal actose to galactonate that subsequently was converted to 2-keto-3-deox y-galactonate. Further metabolism of 2-keto-3-deoxy-galactonate requir ed the presence of ATP and resulted in the formation of pyruvate and g lyceraldehyde 3-P. Radiorespirometry indicated a preferential release of CO2 at the first carbon position of the D-galactose molecule. This suggested that Azotobacter vinelandii metabolizes D-galactose via the DeLey-Doudoroff pathway. The first enzyme of this pathway, D-galactose dehydrogenase, was partially characterized. It has a molecular weight of about 74,000 Da and an isoetectric point of 6.15. The pH optimum o f the galactose dehydrogenase was about 9. The apparent K(m)s for NAD( +) and D-galactose were 0.125 and 0.56 mM, respectively. Besides D-gal actose, the active fraction of this galactose dehydrogenase also oxidi zed L-arabinose effectively. The electron acceptor for D-galactose or L-arabinose oxidation, NAD(+), could not be replaced by NADP(+). These substrate specificities were different from those reported in Pseudom onas saccharophila, Pseudomonas fluorescens, and Rhizobium meliloti.