A large fractionation of ¹³C/¹²C ratios for palmitate metabolism in plants

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Isotopic compositions in fatty acids have widely been used in the study for tracing sources and delivery of organic materials in the earth surface and for reconstructing environments in the geological timescale. Changes in the isotopic compositions associated with the lipid degradation of organisms, however, have been poorly understood. In this study, we determined changes in the carbon and hydrogen isotopic compositions (i.e., the δ¹³C and δD values, respectively) of palmitate (i.e., C16:0 carboxylic acid esters), a major alkanoate in eukarya and bacteria, during the germination of plant seeds under no photosynthetic activity. During the germination, abundance of palmitate is decreased by 41 and 94 % for 5 and 14 days, which causes a considerable change in the δ¹³C value of palmitate by 3.0 and 17.2‰, respectively. In contrast, non-substantial difference is found in the δD values of palmitate for this germination. These enrichments indicate that isotopic fractionation occurs on a carbon but not hydrogen atom of palmitate in an initial reaction of the degradation, as reaction of carbonyl carbon in palmitate with enzyme (lipase) can be proposed for a candidate mechanism. If such reaction mainly occurs on palmitate components in triacylglycerol for the germination of plant seeds, the enrichment in ¹³C estimated is more than 250‰ for the carbonyl carbon in palmitate based on a mass balance calculation, and the isotope fractionation factor (α) accounts for 0.9086 in this reaction based on the Rayleigh fractionation model.