Hydrolysis of Acyl‐Homogeneous and Fish Oil Triacylglycerols Using Desalted Midgut Extract from Atlantic Salmon, Salmo salar

Abstract

Despite several studies aimed at evaluating the positional and fatty acid specificity of fish triacylglycerol (TAG) digestive lipases, there is still much uncertainty regarding these issues. The aim of the present study was therefore to address these questions in Atlantic salmon (Salmo salar L.). Crude luminal midgut extracts were collected from fed salmon and the hydrolysis studied for various substrates including triolein (Tri-18:1), trilinolein (Tri-18:2), trilinolenin (Tri-18:3), trieicosapentaenoin (Tri-20:5), tridocosahexaenoin (Tri-22:6) and natural fish oil TAG. Using Tri-18:1, in a time-curve model showed an initial high degree of sn-1 or sn-3 specificity where sn-1,2(2,3)-diacylglycerol (1,2(2,3)-DAG) and free fatty acid (FFA) were the main hydrolytic products up to 15 min. Lack of initial sn-2 specificity was confirmed by negligible sn-1,3-diacylglycerol (1,3-DAG) being produced. During the further hydrolysis of DAG, all positions appeared susceptible to attack causing a concomitantly small increase in sn-1(3)-monoacylglycerol (1(3)-MAG) and 2-MAG, but not at the level expected for an exclusively sn-1,3-specific lipase. Oleic acid (18:1n-9) and eicosapentaenoic acid (20:5n-3) were preferred substrates for hydrolysis using both fish oil and acyl-homogeneous TAGs with FFA as the main product of lipolysis. Hydrolysis of the natural fish oil TAG appeared slower yet produced proportionally more MAG and DAG after 5 min, and similar specificities, as for synthetic TAG substrates, were exhibited with 18:1n-9 and 20:5n-3 accumulating in the FFA fraction after 30 min. Notably, 16:0 was particularly conserved in MAG. As TAG resynthesis of absorbed lipid in salmon enterocytes proceeds preferably with 2-MAG as templates, the absorption of 2-MAG, produced during initial stages of TAG hydrolysis, would need to occur rapidly to be effectively utilised via the MAG pathway.

Fatty acid digestibility in gilthead sea bream fed diets containing native, re-esterified or acid vegetable oils

“…The lowest fatty acid digestibility of acid oils could be related to a feedback inhibition on the lipase activity caused by FFA, as suggested by Bogevik et al . () for Atlantic salmon. Thus, the richness in FFA of acid oils in our study could reduce the activity of lipase, resulting in a lower hydrolysis of the TAG and the partial acylglycerols present in these oils, which would have a negative effect on its digestibility.…”

“…Thus, this leads to the possibility that it may possess certain sn‐1,3 hydrolytic activity (Bogevik et al . ; Bakke et al . ).…”

Enzymatic Interesterification of Heterotrophic Microalgal Oil with Rapeseed Oil to Decrease the Levels of Tripalmitin

“…Nevertheless, a significantly higher content of TAG isomers with unsaturated fatty acids was observed after interesterification with Lipozyme RM IM. TAG‐hydrolysis in Atlantic salmon favors formation of 1,2/2,3‐DAG prior to 1,3‐DAG . Hydrolysis is thus more likely to start at sn ‐1/3, with faster hydrolysis of unsaturated compared to saturated fatty acids in fish .…”

“…Fish lipases hydrolyze sn‐2 fatty acids (with a bile salt‐dependent lipase) that could increase the content of FFA 16:0 when digesting 16:0 rich TAG containing ingredients compared to mammals that mainly secrete sn ‐1,3 position specific pancreatic lipase, which would produce sn ‐2 16:0 monoacylglycerols (MAG) that are more readily absorbed . Hydrolysis of saturated fatty acids occurs slower compared to unsaturated fatty acids . Tripalmitin, i.e., TAG with 16:0 in all three positions, a common lipid in solid fat and most likely present in HM lipids, will therefore cause slower hydrolysis and absorption of these lipids compared to sources with a more varied content of TAG isomers.…”

“…This could affect the final product stability and nutritional value, as FFA can form calcium soaps in the intestine that negatively affect nutrient absorption [9] and may cause constipation. [16] Fish lipases hydrolyze sn-2 fatty acids (with a bile saltdependent lipase) that could increase the content of FFA 16:0 [17] when digesting 16:0 rich TAG containing ingredients compared to mammals that mainly secrete sn-1,3 position specific pancreatic lipase, which would produce sn-2 16:0 monoacylglycerols (MAG) that are more readily absorbed. [18] Hydrolysis of saturated fatty acids occurs slower compared to unsaturated fatty acids.…”