LC‐PUFA Biosynthesis in Rainbow Trout is Substrate Limited: Use of the Whole Body Fatty Acid Balance Method and Different 18:3n‐3/18:2n‐6 Ratios

Abstract

Five experimental diets with constant total C(18) PUFA and varying 18:3n-3/18:2n-6 ratios were fed to rainbow trout over an entire production cycle. The whole-body fatty acid balance method demonstrated a clear trend of progressively reduced fatty acid bioconversion activity along the n-3 and n-6 pathways, up to the production of 20:5n-3 and 20:4n-6, respectively. This suggests that the pathway exhibits a "funnel like" progression of activity rather than the existence of a single rate limiting step. The production of 22:5n-3 and 22:6n-3 was more active than that of 20:5n-3. However, despite this trend in reduced apparent in vivo net enzyme activity, the efficiency of the various bioconversion steps (measured as % of bioconverted substrate) confirmed an opposing trend. A 3.2-fold higher Δ-6 desaturase affinity towards 18:3n-3 over 18:2n-6 and an 8-fold greater Δ-5 desaturase affinity towards 20:4n-3 over 20:3n-6 were recorded. The main results of the study were that (1) rainbow trout are quite efficient at bioconverting 18:3n-3 to 22:6n-3, and (2) the LC-PUFA biosynthetic pathway is substrate limited. Fillet n-3 LC-PUFA concentrations increased with the increasing dietary supply of 18:3n-3. Despite an almost identical dietary supply of n-3 LC-PUFA, originating from the fish meal fraction of the diets, the fillets of trout fed the diet richest in 18:3n-3 were 2-fold higher in n-3 LC-PUFA than fish fed low 18:3n-3 diets. Nevertheless, fillets of trout fed a fish oil control diet contained more than double the amount of n-3 LC-PUFA compared to fish fed the diets richest in 18:3n-3.

Δ-6 Desaturase Substrate Competition: Dietary Linoleic Acid (18∶2n-6) Has Only Trivial Effects on α-Linolenic Acid (18∶3n-3) Bioconversion in the Teleost Rainbow Trout

“…In the present study, we have recorded significant effects of the dietary treatment on the apparent in vivo bioconversion of MUFA and n-6 PUFA, and this is consistent with a previous observation that these metabolic pathways are substrate limited in teleosts [31]. Thus, fish fed the LLA diet (receiving more dietary 18∶1n-9) recorded an increased bioconversion of this fatty acid, whilst fish fed the HLA diet (richer in 18∶2n-6) recorded increased activities in the various bioconversion steps of n-6 PUFA.…”

“…Thus, finding possible remedial strategies towards minimising the use of this commodity, while at the same time maintaining optimal nutritional quality of the final product, is a highly relevant and timely objective [9]. We have recently shown the following in trout: i) they are capable of efficiently bioconverting ALA up to EPA and DHA [18]; ii) the elimination of dietary n-3 LC-PUFA (enzyme products) up-regulates the transcription rate of Δ-6 desaturase mRNA; however, the total apparent in vivo enzyme activity does not correlate with Δ-6 desaturase mRNA expression [30]; iii) that the LC-PUFA biosynthetic pathway is substrate limited [31]; and iv) that the provision of increased dietary stearidonic acid (18∶4n-3), over ALA, has only minimal benefit in terms of total n-3 LC-PUFA biosynthesis, suggesting that Δ-6 desaturase cannot be considered as the rate-limiting step in this pathway [32].…”

Fish oil–based finishing diets strongly increase long‐chain polyunsaturated fatty acid concentrations in farm‐raised common carp ( C yprinus carpio L.)

“…80% of n‐3 PUFA) when compared with a FO supplement throughout the grow‐out period. Additional studies showed that in omnivorous species such as common carp ( Cyprinus carpio L.) and also piscivorous rainbow trout the steady supplementation of C 18 PUFA‐rich diets subsequently increases C 20–22 PUFA concentrations in muscle tissues (Thanuthong, Francis, Senadheera, Jones & Turchini ; Mráz, Máchová, Kozák & Pickova ). Based on these findings, this study investigates how LO (rich in C 18 PUFA) and FO (rich in C 20–22 PUFA) compound feeds supplemented as finishing diet for a very short period (30 days) after VO‐based grow‐out (180 days) affected total lipids and PUFA profiles of omnivorous common carp cultivated in pond production systems.…”

Preliminary Validation of a High Docosahexaenoic Acid (DHA) and -Linolenic Acid (ALA) Dietary Oil Blend: Tissue Fatty Acid Composition and Liver Proteome Response in Atlantic Salmon (Salmo salar) Smolts

“…The fairly high concentration of LA in the TOFX diet (7.5% total fatty acids) could also evoke competition between LA and ALA as substrates for the delta-6-desaturase enzyme, as proposed by composition-based studies in teleost [61]. However, this possibility is unlikely for salmonids, where more specific FAMB studies have shown that bioconversion of ALA is favored over LA due to the greater affinity of delta-6-desaturase for the n-3 pathway [12, 62]. In addition to bioconversion, an alternative pathway for ALA to improve tissue deposition of n-3 LC PUFA is through beta-oxidation.…”