Catalyzes the oxidation of D-galacturonate and D-glucuronate to galactarate and D-glucarate, respectively. In fact, in water solution the substrate D-galacturonate is predominantly in pyranosic form whose beta anomer is converted by the enzyme to D-galactaro-1,5-lactone; in solution, this reaction product rearranges to the more stable D-galactaro-1,4-lactone. Makes part of the oxidative degradation pathway of D-galacturonate, which allows A.tumefaciens to utilize D-galacturonate as a sole carbon source. Cannot use NADP(+) instead of NAD(+) as cosubstrate. Is not active on D-galactose, D-glucose, D-galactonate and D-gluconate. beta-D-galacturonate + NAD(+) = D-galactaro-1,5-lactone + H(+) + NADH beta-D-glucuronate + NAD(+) = D-glucaro-1,5-lactone + H(+) + NADH kcat is 190 sec(-1) and 92 sec(-1) with D-glucuronate and D-galacturonate as substrate, respectively. Optimum pH is 8. Activity increases with increasing temperatures between 4 and 42 degrees Celsius. Activity remains near 80% of the maximum after exposure at 37 degrees Celsius for 30 minutes. Carbohydrate acid metabolism; D-galacturonate degradation via prokaryotic oxidative pathway. Homohexamer. By D-galacturonate. D-galactaro-1,4-lactone was previously believed to be the product of the reaction of D-galacturonate oxidation. Belongs to the NAD(P)-dependent epimerase/dehydratase family.