Metabolism of labeled isomeric octadecenoates by the laying hen

Discrimination between octadecenoic acid isomers by the laying hen has been studied using tritium (³H), carbon-14 (¹⁴C) and deuterium (d) labeled oleate and elaidate esters. Hydrogen isotopes were positioned at the double bond, whereas¹⁴C was located in the 1-, carboxyl carbon. The egg acted as a biological trap, providing an automatic daily biopsy with which to study the metabolism of the fed isomers. Monitoring the incorporation of isomers was facilitated by dual label feeding experiments, and³H/¹⁴C, d₂/d_o and d₂/d₁ ratios were determined on the isomeric mixtures fed, on the total egg lipids extracted and on the isolated neutral lipid and phospholipid fractions. Comparison of isotopic ratios of the fed mixture and of the lipid fractions provided an evaluation of discrimination by the hen during the transport of isomeric octadecenoates into the egg lipids. Radioactive and stabl isotope ratios determined for the neutral lipid indicated a preferential incorporation of thecis isomer. Stable isotope ratios determined for the phospholipid showed that thetrans isomer is preferentially incorporated. The³H/¹⁴C ratios for the phospholipid recovered in each experiment increased greatly whichever isomer was labeled with³H, indicating an elimination of the 1-¹⁴C-label. Gas liquid radiochromatographic separation of the methyl esters from the neutral lipids and phospholipids showed that the isotopic labels were present almost exclusively in the octadecenoic acid constituent. Presented at the AOCS Meeting, Houston, May 1971. No. Mktg. and. Nutr. Res. Div., ARS, USDA.