Re-evaluation of the 13C isotope fractionation associated with lipids biosynthesis by position-specific isotope analysis of plant fatty acids

Carbon-13 position-specific isotope analysis of fatty acids from vegetable oils was performed using Nuclear Magnetic Resonance in the present study. The measured 13C patterns are not totally in accordance with the conventional view of the relative 13C-depletion of acetogenic lipids and their alternation of 13C-enriched and 13C-depleted carbon positions. The results presented here provide a new evaluation of the isotopic fractionation associated with fatty acids biosynthesis. Whereas it is commonly acknowledged that the pyruvate dehydrogenase (PDH) is responsible for the 13C distribution within fatty acids, data from the present work demonstrate that the conversion of acetyl-CoA to malonyl-CoA catalyzed by acetyl-CoA carboxylase (ACC) needs to be considered while explaining the measured non-stochastic 13C pattern within fatty acids. These data combined with steady-state calculations give a new description of metabolic steps responsible for the typical 13C intramolecular distribution of acetogenic lipids. In addition, the non-stochastic pattern measured in these plant fatty acids is similar to those previously detected within long-chain n-alkanes suggesting a preservation through geological time and demonstrating the interest of position-specific isotope analysis for studying the evolution of metabolic pathways.