Lipid digestion in turbot (Scopthalmus maximus) 11: Lipolysis in vitro of ¹⁴C-labelled triacylglycerol, cholesterol ester and phosphatidylcholine by digesta from different segments of the digestive tract

Preparations of digesta from the stomach, foregut, hindgut and rectum of turbot (Scophthalmus maximus) were assayed for the ability to hydrolyse glycerol tri[1-¹⁴C]oleate (¹⁴C-TAG), 1,2-di[1-¹⁴C]palmitoyl L-3-phosphatidylcholine (¹⁴C-PC) and cholesterol [1-¹⁴]oleate (¹⁴C-CE) over 1,2,3 and 17h. In the assay of foregut digesta with ¹⁴C-TAG substrate, 37.8% of the total radioactivity was found in the FFA class after the first hour of incubation. This value increased to a maximum of 68.5% of the available label after 17h incubation. Over the same time the proportion of radioactivity in diacylglycerols (DAG, 31.6%-7.4%) decreased while that in the monoacylglycerols increased (MAG, 14.0%-22.3%). In assays of digesta from the hindgut and rectum, after 1 h of incubation, the proportion of radioactivity recovered in FFA represented 64.9% and 74.8%, respectively, whereas the proportions in both DAG and MAG decreased with incubation time. Similarly to ¹⁴C-TAG, the highest rate of lipolytic hydrolysis of ¹⁴C-CE occurred in digesta from the posterior digestive tract where the proportions of radioactivity recovered in FFA of the hindgut (50.0%) and rectum (81.9%) preparations were substantially higher than those of the stomach (3.5%) and the foregut (14.4%) after 1h. With ¹⁴C-PC as substrate the levels of radiolabelled FFA in both the foregut and the hindgut (2.4% and 7.6%, respectively) were markedly lower than the 37.5% in the rectum. The results suggest that the posterior digestive tract is very active in non-specific and phospholipid lipolysis and a region where the major part of lipid digestion takes place.