Monoacylglycerols absorption

The products of triacylglycerol digestion, mainly monoacylglycerol and long-chain fatty acids, interact with bile salts to form micelles, which comprise bile salts/ monoacylglycerols/phospholipids and fatty acids. The micelle aids the absorption of monoacylglycerol and fatty... 

Correct answer = A. Pancreatic lipase hydrolyzes dietary triacylglycerol primarily to 2-monoacylglycerol plus two fatty acids. These products of hydrolysis can be absorbed by the intestinal mucosal cells. Bile salts do not inhibit release of fatty acids from triacylglycerol, but rather are necessary for the proper solubilization and hydrolysis of dietary triacylglycerol in the small intestine. Short- and medium-chain length fatty acids enter the portal circulation after absorption from the small intestine. Synthesis of apolipoproteins, especially apo B-48, is essential for the assembly and secretion of chylomicrons. 

Figure 26-5. Principle of the 13C-mixed triglyceride breath test. Absorption of 13C-mixed triglycerides requires prior hydrolysis by pancreatic lipase (1), which leads to production of free fatty acids (stearic acid) and monoacylglycerol [2-(l-13C)octanoylglycerol]. These metabolites are incorporated into micelles, absorbed, and transported to the liver (2). Further degradation by hepatic enzymes and P-oxidation results in formation of 13C02, which is absorbed into the bloodstream, transported to the lung, and exhaled (3). Thus, exhalation of 13C02 reflects intestinal lipolysis and is a marker of pancreatic exocrine function.

Figure 22.4. Action of Pancreatic Lipases. Lipases secreted by the pancreas convert triacylglycerols into fatty acids and monoacylglycerol for absorption into the intestine.

The digestion of triacylglycerols in adult nonruminant mammals has been described as initiated in the mouth by hngual lipase released in the sahva at the base of the tongue (52). Up to 6% of the fatty acids are hydrolyzed and initiate emulsion formation in the stomach. The digesta (called chyme at this location) is released from the stomach slowly into the duodenum to ensure complete mixing with the bile salts and emulsification. Lipolysis occurs by association of pancreatic lipase and co-lipase at the surface of the bile salt-stabihzed emulsion. Amphipathic molecules (fatty acids, sn-2 monoacylglycerols, and lysolecithins) are produced and associate with the bile salts to form water-soluble micelles from which absorption occurs. 

A lipid system of biosomes composed of phosphatidylcholine from soybean and medium-chain monoacylglycerol and low-molecular-weight heparin (LMWH), used in the treatment and prevention of thromboses, was reported (51). Heparin is known as having poor oral absorption. The system is applicable for oral and parentral administration as well as for enhancing dermal, rectal, and nasal absorption of other dmgs. 

The CB used was a standard factory product originating from the Ivory Coast. Its composition, determined by gas chromatography and HPLC for TAG analysis and by atomic absorption for phospholipid content, was as follows TAG 97% diacyl-glycerols 1.1% monoacylglycerols 0.2% free fatty acids 1.3% phosphatides 0.15% and others 0.25% (4). The TAG composition which is given in detail elsewhere (4) is for main TAG, POP 17.3%, POS 37.3%, and SOS 27.3% saturated TAG represented 2.3%. 

FAs liberated from food during absorption are metabolized more easily if they are short or medium chain, i.e., C10 or below. The sn-2 monoacylglycerols can be absorbed directly. Therefore, essential or desired FAs are most efficiently utilized from the sn-2 position in acylglycerols. In accordance with this, TAGs with short-chain FAs (SCFAs) or MCFAs at the sn-1 and sn-3 positions and PEFAs at the sn-2 position are rapidly hydrolyzed with pancreatic lipase (sn-1,3-specific lipase) and absorbed efficiently into mucosal cells. SCFAs or MCFAs are used as a source of rapid energy for infants and patients with fat malabsorption-related diseases. LCFAs, especially DHA and arachidonic acid, are important in both the growth and development of an infant, while n-3 PEFAs have been associated with reduced risk of cardiovascular disease in adults (Christensen et al., 1995 Jensen et al., 1995). 

Unfortunately, sueh formulas have resulted in poor absorption of fats and minerals, particularly when studied in infants dining the first few weeks of life [62, 91, 92]. This is because PA is present in the sn-1 position (human breast milk, lard native, enzyme-direeted and randomly ehemieally interesterified fats plant) eompared with the sn- and sn-3 positions (bovine milk, randomly ehemieally interesterified lard or crude palm oil). It was found the relative absorption of palmitic acid and full fat was linearly related to the proportion of palmitic acid in the sn-1 position of the TAG in human infants. Panereatie lipase selectively hydrolyses the fatty aeids at the sn- and sn-3 positions, yielding free fatty acids and monoacylglycerols (MGs). The TAG sn-1 position is absorbed more effieiently than free palmitic acid and it is conserved through the digestion, absorption and ehylomicron TAG synthesis [63, 91]. 

During the process of absorption, cholesterol dissolved in the lipid core of micelles is transported from the lumen of the small intestine across the intestinal wall and into the lymph. Because the solubility of cholesterol in aqueous systems is low, its absorption depends on the formation of detergent structures (mixed micelles) in the small intestine. These are composed mainly of bile salts, phospholipids, digestion products of fats such as fatty acids and monoacylglycerols, cholesterol (of which 90% is in free form), and fat-soluble micronutrients (Figure 6). 

The first description of the use of plant stanols to lower plasma cholesterol levels in humans was by Heinemann etal (1986) in a small uncontrolled study. They showed that the administration of capsules of sitostanol dispersed in monoacylglycerol and sunflower oil at a dose of 1.5g/day lowered LDL cholesterol levels by 15%. Similar concentrations of sitosterol and sitostanol infused directly into the small intestine decreased cholesterol absorption by 50% and 85%, respectively (Heinemaim et al, 1991). Becker et al (1993) obtained impressive results with low-dose sitostanol in an uncontrolled study with 9 children suffering from familial hypercholesterolemia. LDL cholesterol levels decreased by 33% when the children consumed 1.5 g sitostanol daily, and by 20% when they consumed 6 g sitosterol daily, suggesting that sitostanol is more effective than sitosterol at lowering LDL cholesterol levels. 

Lipid absorption in man occurs largely in the jejunum. The principal molecular species passing across the brush border membrane of the enterocyte are the monoacylglycerols and non-esterified long chain fatty acids. The bile salts themselves are not absorbed in the proximal small intestine but pass on to the ileum where they are absorbed and recirculated in the portal blood to the liver and then to the bile for re-entry at the duodenum. 

---