BIOSYNTHESIS OF CHYLOMICRON TRIACYLGLYCEROLS BY RATS FED GLYCERYL OR ALKYL ESTERS OF MENHADEN OIL FATTY-ACIDS

We have previously shown great similarity in the distribution of fatty acids in the sn-1 and sn-3 positions of the chylomicron triacylglycer ols (TG) from rats fed menhaden oil or its ethyl esters, and have prop osed that the acylglycerol products of the phosphatidic acid (PA) path way (ester feeding) are hydrolyzed to 5-monoacylglycerols (2-MG) prior to reconversion to TG via the 2-MG pathway (oil feeding) and secretio n as chylomicrons. As the composition of the sn-2-position would also be retained if the TG were hydrolyzed only to the X-1,2-diacylglycerol (DG) stage before resynthesis, we have now retested the hypothesis by determining the molecular association and reverse isomer content of t he sn-1,2- and sn-2,3-DG derived from the chylomicron TG and the PA re sulting from the two feedings. The new data demonstrate a better than 90% homology among the molecular species of the PA from the oil and es ter feeding, along with the characteristic association of the saturate d acids with sn-1- and the unsaturated acids with sn-2-position. Due t o increased proportion of unsaturated acids in the sn-1-position of th e TG, there was only a 15-20% homology between the PA and the sn-1,2-D G moieties of the chylomicron TG from the oil and ester feeding. A lac k of homology was also observed between the PA and free sn-1,2-DG, as well as between the free sn-1,2-DG and the sn-1,2-DG moieties of the c hylomicron TG. On the basis of molecular association and the sn-1-/sn- 3- reverse isomer content of the chylomicron TG a better than 90% homo logy was recognized between the chylomicron TG resulting from the oil and ester feeding. It is therefore concluded that hydrolysis to 2-MG f ollowed by reesterification via the 2-MG pathway constitutes the most plausible mechanism for the transfer to chylomicrons of the TG arising from alkyl ester feeding.