Biosynthetic control of the natural abundance of carbon 13 at specific positions within fatty acids in Saccharomyces cerevisiae. Isotopic fractionation in lipid synthesis as evidence for peroxisomal regulation.

Measurements of the natural abundance of 13C at C-1, C-9, and C-10 in fatty acids synthesized by Saccharomyces cerevisiae grown aerobically at 30 degrees C show that alkyl chain positions derived from the carboxyl group of the acetate precursor must be enriched in 13C by 2.5 +/- 0.6 parts per thousand while those derived from the methyl group in acetate must be depleted in 13C by an equal amount. Selective depletions of 13C observed at the C-9 and C-10 positions of palmitoleate and oleate require that (i) the carbon kinetic isotope effect associated with the action of desaturase at C-9 must be between 1.2 and 1.6% in vivo, (ii) at C-10 the effect must be between 0.9 and 1.3%, and (iii) less than 20% of the C18 carbon skeletons synthesized are preserved within the cell, the remainder apparently being degraded. It is shown that the novo synthesis (i.e. by fatty acid synthetase) is responsible for the production of more than 95% of the supply of 18-carbon acyl groups, the remainder being provided by all other elongation pathways. In an ancillary study designed to test the accuracy and generality of these results, it was observed that still larger specific depletions occurred at olefinic carbon position in fatty acids from soybeans, thus suggesting that the degradation of substantial quantities of C18 carbon skeletons may be a widespread feature of fatty acid metabolism in eukaryotes. It is suggested that the required degradation is associated with the action of peroxisomes.