Mediates the reduction of D-galacturonate to L-galactonate, the first step in D-galacturonate catabolic process. Also has activity with D-glucuronate and DL-glyceraldehyde. No activity is observed with D-glucose, D-fructose, D-xylose, D-galactose, L-arabinose or D-mannose. Activity is seen only with NADPH and not with NADH. L-galactonate + NADP(+) = aldehydo-D-galacturonate + H(+) + NADPH Carbohydrate acid metabolism. Belongs to the aldo/keto reductase family.