Catalyzes the oxidation of beta-D-galacturonate and beta-D-glucuronate to galactarate and D-glucarate, respectively. Cannot use NADP(+) instead of NAD(+) as cosubstrate. beta-D-galacturonate + NAD(+) = D-galactaro-1,5-lactone + H(+) + NADH beta-D-glucuronate + NAD(+) = D-glucaro-1,5-lactone + H(+) + NADH kcat is 74 sec(-1) and 24 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 is below 20% of the maximum after exposure at 37 degrees Celsius for 30 minutes. Carbohydrate acid metabolism; D-galacturonate degradation via prokaryotic oxidative pathway. Homohexamer. Belongs to the NAD(P)-dependent epimerase/dehydratase family.