The alternative D-galactose degrading pathway of Aspergillus nidulans proceeds via L-sorbose

The catabolism of D-galactose in yeast depends on the enzymes of the Leloir pathway. In contrast, Aspergillus nidulans mutants in galactokinase (galE) can still grow on D-galactose in the presence of ammonium-but not nitrate-ions as nitrogen source. A. nidulans galE mutants transiently accumulate high (400 mM) intracellular concentrations of galactitol, indicating that the alternative D-galactose degrading pathway may proceed via this intermediate. The enzyme degrading galactitol was identified as L-arabitol dehydrogenase, because an A. nidulans loss-of-function mutant in this enzyme (araA1) did not show NAD(+)-dependent galactitol dehydrogenase activity, still accumulated galactitol but was unable to catabolize it thereafter, and a double galE/araA1 mutant was unable to grow on D-galactose or galactitol. The product of galactitol oxidation was identified as L-sorbose, which is a substrate for hexokinase, as evidenced by a loss of L-sorbose phosphorylating activity in an A. nidulans hexokinase (frA1) mutant. L-Sorbose catabolism involves a hexokinase step, indicated by the inability of the frA1 mutant to grow on galactitol or L-sorbose, and by the fact that a galE/frA1 double mutant of A. nidulans was unable to grow on D-galactose. The results therefore provide evidence for an alternative pathway of D-galactose catabolism in A. nidulans that involves reduction of the D-galactose to galactitol and NAD(+)-dependent oxidation of galactitol by L-arabitol dehydrogenase to L-sorbose.