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Some lipid digestion pathways in fish deviate from those in mammals, and many differences may also be species dependent. This report describes a pathway for monoacylglycerol (MAG) and lysophospholipid absorption by intestinal enterocytes in brown trout that may be of significance in salmonids. When culturing primary cells in a medium containing 1- and 2-MAG, we observed a massive hydrolysis of unesterified fatty acids. The hydrolysis activity was retained in the medium even after the removal of the cells. To further characterize these activities, both extracellular and isolated membrane proteins were tested for lipase activity toward triacylglycerol (TAG), diacylglycerol (DAG), MAG, phosphatidylcholine (PtdCho), and lysoPtdCho. In both cases, the main hydrolyzing activity was toward MAG followed by lysoPtdCho with very little activity toward DAG, TAG, or PtdCho. The extracellular and membrane proteins were partially purified by fast protein liquid chromatography and identified by proteomics (liquid chromatography-tandem mass spectrometry) focusing on lipase/hydrolase enzymes. In the membrane protein fraction, the data suggested that MAG was produced as an intermediate in the hydrolysis of lysoPtdCho by either lysophospholipase C or lysophospholipase D activity. Both abhydrolase-domain-containing protein 6 and abhydrolase-domain-containing protein 12 were identified in the membrane protein and they could be responsible for the hydrolysis of MAG. In the culture medium, low-peptide matches were found for ABHD6 and phospholipases and further studies are needed. In summary, trout enterocytes are capable of hydrolyzing MAG and lysoPtdCho. The enzymes are both extracellular and membrane bound. The pathways may be of significance during lipid absorption in fish lacking a 1,3 specific pancreatic lipase.

sn-2-Monoacylglycerol (2-MAG) and sn-1(3)-monoacylglycerol [1(3)-MAG] are important but yet little studied intermediates in lipid metabolism. The current study compared the metabolic fate of 2-MAG and 1(3)-MAG in isolated caecal enterocytes and hepatocytes of brown trout (Salmo trutta). 1(3)-Oleoyl [9,10-3H(N)]-glycerol and 2-Oleoyl [9,10-3H(N)]-glycerol were prepared by pancreatic lipase digestion of triolein [9,10-3H(N)]. The 1(3)-MAG and 2-MAG were efficiently absorbed by enterocytes and hepatocytes at similar rates. The 2-MAG was quickly resynthesized into TAG through the monoacylglycerol acyltransferase (EC: 2.3.1.22, MGAT) pathway in both tissues, whereas 1(3)-MAG was processed into TAG and phospholipids at a much slower rate, suggesting 2-MAG was the preferred substrates for MGAT. Further analysis showed that 1(3)-MAG was synthesized into 1,3-DAG, but there were no accumulation of 1,3-DAG in either enterocytes or hepatocytes, which contrasts that of mammalian studies. Some of the 1(3)-MAG may be acylated to 1,2(2,3)-DAG and then utilized for TAG synthesis. Alternatively, 1(3)-MAG can be hydrolyzed to free fatty acid and glycerol, and re-synthesized into TAG through the glycerol-3-phosphate (Gro-3-P) pathway. The overall data suggested that the limiting step of the intracellular 1(3)-MAG metabolism is the conversion of 1(3)-MAG itself.