Tissue deposition of n-3 FA pathway intermediates in the synthesis of DHA in rainbow trout (Oncorhynchus mykiss).

The tissue distribution of newly synthesized 22:6n-3 and intermediate PUFA was examined in rainbow trout to further our understanding of the metabolism of this EFA in fish. Rainbow trout were fed a pulse of deuterated linolenic acid (D5-17,17,18,18,18-18:3n-3), and the tissue distribution of deuterated anabolites was determined at intervals up to 35 d post-dose by GC-negative chemical ionization MS of the pentafluorobenzyl derivatives. D5-22:6n-3 was the major deuterated FA in liver and cecal mucosa 2 and 5 d post-dose. All the n-3 FA pathway intermediates were found in liver, cecal mucosa, and blood including D5-24:5n-3 and D5-24:6n-3. Brain and eyes also contained the full suite of intermediate deuterated FA, but with a different profile from liver when analyzed over a longer time course up to 35 d. D5-20:5n-3 was the major component in brain up to 7 d, after which D5-22:6n-3 became predominant, but D5-22:5n-3 constituted ca. 20% of FA throughout the time period. The pattern in eyes was similar but less pronounced. In visceral adipose tissue there was a much greater accumulation of the initial substrate, D5-18:3n-3, with D5-18:4n-3 and D5-22:6n-3 the predominant deuterated FA at all time points. There was a similar though less pronounced trend in eye socket adipose tissue. The C24 PUFA were not detected in visceral fat and barely detected in eye socket fat. The results show that the kinetics of accumulation and depletion of the various n-3 PUFA differ between tissues. The presence of pathway intermediate FA provides evidence that liver and ceca possess the full metabolic pathway for synthesis of 22:6n-3, whereas brain and eyes are less active, with an accumulation of pentaene intermediate FA, and adipose tissue is inactive.