Metabolic Studies on Intermediates in the myo-Inositol Oxidation Pathway in Lilium longiflorum Pollen

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Abstract
Myo-Inositol-linked glucogenesis in germinated lily (L. longiflorum Thumb., cv. Ace) pollen was investigated by studying the effects of added L-arabinose or D-xylose on metabolism of myo-[2-3H]inositol and by determining the distribution of radioisotope in pentosyl and hexosyl residues of polysaccharides from pollen labeled with myo-[2-14C]inositol, myo-[2-3H]inositol, L-[5-14C]arabinose and D-[5R,5S-3H]xylose. myo-[2-14C]arabinose produced labeled glucose with similar patterns of distribution of 14C, 35% in C1 and 55% in C6. Arabinosyl units were labeled exclusively in C5. Incorporation of 3H into arabinosyl and xylosyl units in pollen labeled with myo-[2-3H]inositol was repressed when unlabeled L-arabinose was included in the germination medium and a related 3H exchange with water was stimulated. Results are consistent with a process of glucogenesis in which the myo-inositol oxidation pathway furnishes UDP-D-xylose as a key intermediate for conversion to hexose via free D-xylose and the pentose phosphate pathway. Additional evidence for this process was obtained from pollen labeled with D-[5R,5S-3H]xylose or myo-[2-3H]inositol which produces D-[5R-3H]xylose. Glucosyl units from polysaccharides in the former had 11% of the 3H in C1 and 78% in C6 while glucosyl units in the latter had only 4% in C1 and 78% in C6. Stereochemical considerations involving selective exchange with water of prochiral-R 3H in C1 of fructose-6-phosphate during conversion to glucose provide explanation for observed differences in the metabolism of these 5-labeled xyloses. Incorporation of 3H from myo-[2-3H]inositol into arabinosyl and xylosyl units of pollen polysaccharides was unaffected by the presence of unlabeled D-xylose in the medium. Exchange of 3H with water was greatly affected, decreasing from a value of 21% exchange in the absence of unlabeled D-xylose to 5% in the presence of 6.7 mM D-xylose. D-Xylose was rapidly utilized for glucogenesis by germinated pollen tubes. This observation supports the view that free D-xylose is an important intermediate following breakdown of UDP-D-xylose during myo-inositol-linked glucogenesis.