Biosynthesis of diacylglycerols by rat intestinal mucosa in vivo

Abstract

The biosynthesis of diacylglycerols was studied in rat intestinal mucosa during in vivo absorption of a low molecular weight fraction fraction of butter oil and of the corresponding medium and long chain fatty acids. The experimental fat solutions were given by stomach tube to the animals after a 24-h fast and mucosal scraping were collected 3 h later. The lipids were isolated and the acylclycerols determined by combined thin-layer chromatography gas-liquid chromatography techniques and stereospecific analyses. Free fatty acid feeding led mainly to sn-1,2-diacyl-glycerols, which contained exogenous and endogenous fatty acids. During triacylglycerol feeding, both sn-1,2-and sn-2,3-diacylglycerols were recovered in significant amounts from the intestinal mucosa. The composition of the sn-2,3-diacylglycerols corresponded to that with exogenous fatty acids but the sn-1,2-diacylglycerols clearly contained both exogenous and endogenous fatty acids. In all cases it was possible to isolate endogenous sn-1,2-diacylglycerols made up largely of species with linoleic and arachidonic acids in the 2 position and palmitic and stearic acids in the 1 position, which apparently were not converted to triacylglycerols. The in vivo reacylation of 2-monoacylglycerols via both sn-1,2- and sn-2,3-diacylglycerols is in agreement with similar findings in vitro with everted sacs of rat intestinal mucosa.

Lipid absorption and metabolism.

“…During triacylglycerol feeding both sn-1,2-Environmental Health Perspectives and sn-2,3-diacylglycerols were recovered in significant amounts from the intestinal mucosa. The composition of the sn-2,3-diacylglycerols corresponded to that with the exogenous fatty acids, while the sn-1,2-diacylglycerols clearly contained both exogenous and endogenous fatty acids (15). In addition to these sn-1,2-and sn-2,3-diacylglycerols containing the original fatty acids in the sn-2-position there were sn-1,2-diacylglycerols, labeled from free fatty acids, which did not contain the acid present in the 2-monoacylglycerol supplied in the diet, and which therefore must have originated via the phosphatidic acid pathway.…”

“…Fasting intestinal mucosa contains exclusively sn-1,2-diacylglycerols (15). This finding can be rationalized on the basis of the known products of the phosphatidic acid pathway and/or a backreaction from the CDP-choline pathway of phosphatidyicholine biosynthesis (16).…”

“…In other instances the contribution of the phosphatidic acid pathway may be minimal due to an excess of monoacylglycerol because of a loss of short chain fatty acids to the portal route. The endogenous diacylglycerol pool which apparently did not participate in triacylglycerol synthesis was estimated at 0.5 + 0.3 mg/g tissue (15).…”

“…The latter possibility is likely in view of the high content of the polyunsaturated fatty acids in the diacylglycerols of the fasting mucosa. The diacylglycerols originating from free fatty acid feeding are also of the sn-1,2-type, and clearly the products of phosphatidic acid synthesis (15).…”

“…It is not known to what extent this conclusion may have been influenced by any difficulty of substrate accessibility in such in vitro experiments. Nevertheless, a segregation of diacylglycerol pools or at least the acyl CoA pools is also suggested by analysis of the molecular species of diacylglycerols recovered from the intestinal mucosa during a-ctive fat absorption (15,25). It has been demonstrated (15) that an sn-1,2-diacylglycerol pool exists which is characterized by high proportions of 18:2 and 20:4 acids in the sn-2-position and which does not participate in triacylglycerol formation, although it could have contributed to the biosynthesis of phosphatidylcholine.…”