Sugar-Nucleotide Precursors of Arabinopyranosyl, Arabinofuranosyl, and Xylopyranosyl Residues in Spinach Polysaccharides

Cultured spinach (Spinacia oleracea L. cv Monstrous Viroflay) cells incorporated exogenous l-[(3)H]arabinose sequentially into beta-l-arabinopyranose-1-phosphate, uridine diphospho-beta-l-arabinopyranose, uridine diphospho-alpha-d-xylopyranose and (in some experiments) alpha-d-xylopyranose-1-phosphate. The amount of (3)H in each of these compounds reached a plateau after a few minutes, and could be rapidly chased with nonradioactive l-arabinose, demonstrating rapid turnover. After a few minutes' lag, incorporation of (3)H into the arabinofuranosyl, arabinopyranosyl, and xylopyranosyl residues of polysaccharides was linear with respect to time. The kinetics of labeling were compatible with UDP-beta-l-arabinopyranose and UDP-alpha-d-xylopyranose being the immediate precursors of arabians (both the pyranose and the furanose residues) and xylans, respectively. No other radioactive nucleotides were formed; in particular, UDP-arabinofuranose was absent. There was no evidence for conversion of arabinopyranose to arabinofuranose within the polysaccharides, suggesting that this conversion occurs during polymer synthesis. The glycolipids detected showed too slow a turnover to be intermediates of pentosan synthesis.