Biosynthesis of Glycosaminoglycans: Uridine Diphosphate Glucose 4′–Epimerase from Cornea and Epiphysial–Plate Cartilage

Abstract

UDP–glucose 4′–epimerase (EC 5. 1. 3. 2.) was extracted from newborn–pig epiphysial–plate cartilage and whole bovine cornea. The formation of radioactive UDP–galactose from UDP[U–¹⁴C]glucose was demonstrated by radioautography after separation of the sugar nucleotides by paper chromatography or t. l. c. The pH optimum and the K_m values for UDP–glucose, UDP–galactose and NAD⁺ were determined in both tissues. UDP–galactose and UDP–glucuronic acid formation after incubation with different UDP–glucose concentrations was followed; the same experiment was carried out using different UDP–galactose concentrations and following the formation of UDP–glucose and UDP–glucuronic acid. At equilibrium, the ratio UDP–glucose/UDP–galactose reaches a value of about 3. 5. The results obtained seem to indicate that UDP–glucose 4′–epimerase activity is strongly dependent on that of UDP–glucose dehydrogenase. The physiological meaning of UDP–glucose 4′–epimerase in glycosaminoglycan biosynthesis in the two tissues under study is discussed on the basis of the K_m values of UDP–glucose 4′–epimerase and UDP–glucose dehydrogenase and on the basis of the rate of UDP–glucose and UDP–galactose utilization.