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 9c,12t- 18:2 and 9t, 12c- 18:2 into long chain polyunsaturated fatty acids (PUFA) containing 20 carbons (geometrical isomers of eicosadienoic and eicosatetraenoic acids). In the present work, some in vitro studies were carried out in order to have precise information on the conversion of these two isomers. In a first set of experiments, studies were focused on the in vitro Delta 6 desaturation, the first regulatory step of the biosynthesis of n-6 long chain PUFA, from 9c,12c- 18:2. Rat liver microsomes were prepared and incubated under desaturation conditions with [1-C-14]-9c,12c-18:2 in presence of unlabelled 9c,12t-, 9t,12c- or 9t, 12t-18:2. The data show that each trans isomer induced a decrease of the Delta 6 desaturation of the [1-C-14]-9c,12c- 18:2, but the 9c,12t- 18:2 was the most potent inhibitor (up to 63%). Rat liver microsomes were also 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 desaturation conditions. The results indicated that 18:2 Delta 9c,12t is a much better substrate for desaturase 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 homologue, at low substrate concentration only. In a second set of experiments, in vitro elongation studies of each mono-trans 18:2 isomer and 9c,12c- 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 product formed from [1-C-14]-9c,12t-18:2 was lower than was produced from the 9c,12c-18:2. Thus, the desaturation enzymes presented a higher affinity for the 9c,12t-18:2 whereas the elongation enzyme presented a higher affinity for the 9t,12c-18:2.