The Formation of Uridine Diphosphate-Glucuronic Acid in Plants

UDP-glucuronic acid pyrophosphorylase was purified approximately 80-fold from barley seedlings by ammonium sulfate fractionation and column chromatography on DEAE-cellulose and Sephadex G-100. The partially purified enzyme can be assayed radiochemically in either the forward or reverse direction and has Km values of 0.33 mm and 0.5 mm for d-glucuronic acid 1-phosphate and UDP-d-glucuronic acid, respectively. Its pH optimum lies between pH 8 and 9. The equilibrium constant of the reaction is 2.9 measured in the direction of glucuronic acid 1-phosphate formation. Mg2+ best fulfills the requirement for divalent cations. However, this dependency is related to substrate concentration, and, on the basis of kinetic experiments, an Mg2+:UTP ratio of 2:1 in the forward direction and an Mg2+ to pyrophosphate ratio of about 1:1 in the reverse direction appear to be required for maximum activity. Excess Mg2+ inhibits the reaction. The total activity of the enzyme increased 15-fold during the first 6 days of germination and is present in amounts potentially sufficient to meet the full needs of the young plant for UDP-glucuronic acid in cell wall polysaccharide biosynthesis. By contrast, levels of UDP-glucose dehydrogenase were very low. This suggests that the pathway myo-inositol → glucuronic acid → glucuronic acid 1-phosphate → UDP-glucuronic acid may be the predominant route whereby UDP-glucuronic acid is formed in the young barley seedling.