Hydrogen and Oxygen Isotopic Fractionation During Heterotrophic Cellulose Synthesis

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Abstract
Hydrogen and oxygen isotopic fractionation relative to medium water for two different carbohydrate metabolic pathways leading to cellulose synthesis were measured. This was accomplished by analysing stable hydrogen and oxygen isotope ratios of water and cellulose for seedlings. The seedlings had been germinated and heterotrophically grown in closed vessels from species having starch (Triticum aestivum L. and Hordeum vulgare L.) and lipids (Ricinus communisL. and Arachis hypogaea L.) as the primary substrate. Isotopic fractionation factors occurring during enzyme-mediated exchange of carbon-bound hydrogen with water or the addition of carbon-bound hydrogens from water during the synthesis of cellulose from either starch or lipids were similar (ranging from +144 to +166%). About 34% and 67% of carbon-bound hydrogens were derived from water during the synthesis of cellulose from starch and lipid, respectively. Thus, the greater deuterium enrichment in cellulose from oil seed species associated with gluconeogenesis was caused by a greater proportion of water-derived carbon-bound hydrogens and not because of differences in fractionation factors. The proportion of carbon-bound hydrogens derived from water during these metabolic pathways was similar to that of oxygen derived from water. These results may explain the variability in D/H ratios of cellulose nitrate from terrestrial and aquatic plants.

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