IN-VITRO DESATURATION OR ELONGATION OF MONOTRANS ISOMERS OF LINOLEIC-ACID BY RAT-LIVER MICROSOMES

Several nutritional studies have shown the in vivo conversion of the 9 c,12t- 18:2 and 9t, 12c- 18:2 into long chain polyunsaturated fatty ac ids (PUFA) containing 20 carbons (geometrical isomers of eicosadienoic and eicosatetraenoic acids). In the present work, some in vitro studi es were carried out in order to have precise information on the conver sion of these two isomers. In a first set of experiments, studies were focused on the in vitro Delta 6 desaturation, the first regulatory st ep of the biosynthesis of n-6 long chain PUFA, from 9c,12c- 18:2. Rat liver microsomes were prepared and incubated under desaturation condit ions with [1-C-14]-9c,12c-18:2 in presence of unlabelled 9c,12t-, 9t,1 2c- or 9t, 12t-18:2. The data show that each trans isomer induced a de crease of the Delta 6 desaturation of the [1-C-14]-9c,12c- 18:2, but t he 9c,12t- 18:2 was the most potent inhibitor (up to 63%). Rat liver m icrosomes were also incubated with [1-C-14]-9c,12c-18:2, [1-C-14]-9c,1 2t-18:2 or [1-C-14]-9t,12c-18:2 under desaturation conditions. The res ults indicated that 18:2 Delta 9c,12t is a much better substrate for d esaturase than 9t,12c-18:2. Moreover, the conversion levels of [1-C-14 ]-9c,12t-18:2 was similar to what was observed for its all cis homolog ue, at low substrate concentration only. In a second set of experiment s, in vitro elongation studies of each mono-trans 18:2 isomer and 9c,1 2c- 18:2 were carried out. For that purpose, rat liver microsomes were incubated with [1-C-14]-9c,12c-18:2, [1-C-14]-9c,12t-18:2 or [1-C-14] -9t,12c-18:2 under elongation conditions. The data show that [1-C-14]- 9t,12c-18:2 is better elongated than 9c,12c-18:2 while the amount of p roduct formed from [1-C-14]-9c,12t-18:2 was lower than was produced fr om the 9c,12c-18:2.Thus, the desaturation enzymes presented a higher a ffinity for the 9c,12t-18:2 whereas the elongation enzyme presented a higher affinity for the 9t,12c-18:2.