METABOLISM OF URIDINE 5'-DIPHOSPHATE-GLUCOSE IN GOLGI VESICLES FROM PEA STEMS

Uridine 5'-diphosphate-glucose (UDP-Glc) is transported into the lumen of the Golgi cisternae, where is used for polysaccharide biosynthesis . When Golgi vesicles were incubated with UDP-[H-3]Glc, [H-3]Glc was r apidly transferred to endogenous accepters and UDP-GIc was undetectabl e in Golgi vesicles. This result indicated that a uridine-containing n ucleotide was rapidly formed in the Golgi vesicles. Since little is kn own about the fate of the nucleotide derived from UDP-Glc, we analyzed the metabolism of the nucleotide moiety of UDP-Glc by incubating Golg i vesicles with [alpha-P-32]UDP-Glc, [beta-P-32]UDP-Glc, and [H-3]UDP- Glc and identifying the resulting products. After incubation of Golgi vesicles with these radiolabeled substrates we could detect only uridi ne 5'-monophosphate (UMP) and inorganic phosphate (Pi). UDP could not be detected, suggesting a rapid hydrolysis of UDP by the Golgi UDPase. The by-products of UDP hydrolysis, UMP and Pi, did not accumulate in the lumen, indicating that they were able to exit the Golgi lumen. The exit of UMP was stimulated by UDP-Glc, suggesting the presence of a p utative UDP-Glc/UMP antiporter in the Golgi membrane. However, the exi t of Pi was not stimulated by UDP-Glc, suggesting that the exit of Pi occurs via an independent membrane transporter.