Incorporation into lipid classes of products from microsomal desaturation of isomeric trans-octadecenoic acids.

The microsomal desaturation of positional isomers of trans-octadecenoic acids is effected by the delta 9-desaturase and, with concomitant geometric isomerization, cis,trans- and cis,cis-octadecadienoic acids of unusual structure are formed. Incorporation of the substrates and their products into lipids varied from 50.5% for incubations with 14-18:1 to 81.0% for 6-81:1. A detailed study of the composition of each of the major lipid classes, i.e., phospholipids, triacylglycerol and cholesteryl esters, as well as the composition of the free fatty acid fraction, revealed a complex picture. Generally, the c,c-18:2 products were enriched in the phospholipids fraction, whereas the c,t-18:2 appeared preferentially in cholesteryl esters. The 18:1 substrates themselves did not show marked preferences for any of the lipid classes. Phospholipase A2 action on phosphatidylcholine and phosphatidylethanolamine demonstrated enrichment of the c,c- and the c,t-18:2 products in the 2-position, whereas the 18:1 substrates were preferentially inserted into the 1-positions. The c,c- and c,t-18:2 formed by desaturation of t11-18:1 varied from this pattern, probably due to their conjugated double bond structures. Linoleic acid, c9,c12-18:2, formed during desaturation of t12-18:1, surprisingly showed enrichment in the 1-position of phosphatidylcholine. Incubation experiments with t5- and t6-isomers using liver microsomes from rats fed a corn-oil-supplemented diet showed conversion and incorporation rates similar to the rates obtained with microsomes from EFA-deficient rats. The fatty acid composition of lipid classes and the distribution of products and substrate between the 1- and 2-positions of phosphatidylcholine also agreed with results obtained using microsomes from EFA-deficient rats.