COMPOSITION AND STRUCTURE OF TRIACYLGLYCEROLS IN BROWN ADIPOSE-TISSUEOF RATS FED FISH-OIL

This study examines the incorporation of highly unsaturated n-3 fatty acids (HUFA) into triacylglycerols (TAG) of brown adipose tissue (EAT) , and their effect on the positional distribution of saturated (SFA) a nd of unsaturated (UFA) 16- or 18-carbon fatty acids. To this end, rat s were fed a fish oil diet for up to four weeks. The stereospecific an alysis of TAG was based on generation of sn-1,2- and sn-2,3-acylglycer ols by Grignard degradation, followed by synthesis of phosphatidic aci d and specific hydrolysis with phospholipase A(2). From the end of the first week of fish oil feeding, a steady-state in the fatty acid comp osition of TAG in BAT was reached. HUFA concentration increased 30-fol d, mainly at the expense of n-9 UFA and of SFA. The amount of SFA decr eased selectively at position 3, where these fatty acids were progress ively replaced by n-3 HUFA. By contrast, the amount of UFA decreased a t all positions, and their positional distribution was not affected. A bout 60% of HUFA was incorporated at position 3. Nearly twice as much 22:6n-3 was incorporated into TAG than had been previously observed in white adipose tissue (WAT) [Leray, C., Raclot, T., and Groscolas, R. (1993) Lipids 28, 279-284]. At the steady-state, the distribution of H UFA was characterized by high proportions of 22:6n-3 and 20:5n-3 in po sition 3. Moreover, in each position of TAG, a steady level was reache d rapidly (within 1 wk). It is concluded that, during fish-oil feeding , fatty acids in TAG of BAT show characteristic time-course changes th at lead to a characteristic composition and a tissue specific position al distribution. This suggests that adipose tissue has its own specifi city in controlling the build-up of TAG stores, which is likely to be regulated by the specificity of acylating enzymes as well as molecular rearrangements.