2-Monoacylglycerols formation

Figure 24-2. Biosynthesis of triaq/lglycerol and phospholipids. ( , Monoacylglycerol pathway (D, glycerol phosphate pathway.) Phosphatidylethanolamine may be formed from ethanolamine by a pathway similar to that shown for the formation of phosphatidylcholine from choline.

Figure 4.9 Formation of a micelle and its role in the uptake of fatty acids and monoacylglycerol into enterocytes. The micelle is stable in the aqueous environment of the intestinal lumen and is necessary for movement of the monoacylglycerol and fatty acids in the lumen.

Figure 4.10 Monoacylglycerol pathway for synthesis of triacylglycerol and formation of chylomicrons within the enterocyte.

Metabolism of the monoacylglycerol and fatty acids in the enterocyte formation 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 22.5. Chylomicron Formation. Free fatty acids and monoacylglycerols are absorbed by intestinal epithelial cells. Triacylglycerols are resynthesized and packaged with other lipids and apoprotein B-48 to form chylomicrons, which are then released into the lymph system.

Digestion of 1,3- and 1,2-DAG (70% 30%, respectively, as diolein) results in the preferential formation of 1(3)-MAG and FFA in rats (6). 1(3)-MAG is 65% of the monoacylglycerol after 60 minutes of interaction with the small intestine. This observation has been demonstrated also in mice using labeled fatty acids incorporated into 1,3-DAG (7). After exposure of labeled 1,3-DAG to the small intestine in mice, the percentage of 1(3)-MAG from total lipid content increased to 14.2% compared with 1.5% (p < 0.001), 2-MG decreased to 7.2% compared... 

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. 

The source of TG used for assembly with apo B has been proposed to originate primarily (-70%) from the cytosolic TG storage pool rather than from the pool of TG made by de novo synthesis in the ER [10]. One model for the assembly of TG with apo B is that cytosolic TG is hydrolyzed, perhaps by the microsomal TG hydrolase (R. Lehner, 1999), to diacylglycerol/monoacylglycerol, which are subsequently re-esterified in the ER lumen to TG which assembles with apo B. However, many questions remain regarding the topology of this proposed lipolysis/re-esterification cycle and the molecular identity of the players. For example, it is not known if the active site of the enzyme that makes TG for assembly with apo B resides on the lumenal or cytosolic side of the ER membrane. If the formation of TG by re-esterification occurred within the ER lumen, one would also need to explain how fatty acids entered the ER lumen. 

Fig. 5. Mechanism of remnant lipoprotein formation at the endothelial surface. Apo B is not illustrated. FFA, unesterified fatty acid MG, monoacylglycerol closed triangles, apo C2 closed circles, apo E. This model reflects the appearance of partially lipolyzed lipoprotein particles in the circulation during lipoprotein lipase-mediated lipolysis of triacylglycerol-rich lipoproteins.

Fig. 32.10. Formation and secretion of chylomicrons. The triacylglycerol is produced in the smooth endoplasmic reticulum (SER) of intestinal epithelial cells from the digestive products, fatty acids, and 2-monoacylglycerols. The protein is synthesized in the rough endoplasmic reticulum (RER). The major apoprotein in chylomicrons is B-48. Assembly of the lipoproteins occurs in both the ER and the Golgi complex.

The digested fat will be absorbed as FA and monoacylglycerols through the intestinal epithelium and esterihed back to TAG. The TAG produced in the hens bloodstreams are associated with lipoproteins. The lipoprotein and TAG synthesis occurs de novo in the liver and is, among other things, necessary for the formation of egg yoUc lipid in the ovaries. Dne to this synthesis, the FA distributions in the 1-, 2-, and 3-positions in the TAG are similar in the liver and egg yolk (Hirata et al.,... 

Unlike their short-chain counterparts, long-chain fatty acids are not absorbed directly from the rumen. When they reach the small intestine they are mainly saturated and unesterified, but some - in the bacterial lipids - are esterified. Monoacylglycerols, which play an important role in the formation of mixed micelles in non-ruminants, are replaced in ruminants by lysophosphatidyl choline. 

Figure 7. Formation of monoacylglycerol and acrolein by pyrolysis or hydrolysis (adapted from Pokomy, 1989).

Saponification of Hpid ester bonds is based on the reaction of aqueous solutions of alkali metal hydroxides and the formation of soaps (alkali metal salts of fatty acids) and glycerol as a byproduct. Intermediates of saponification of triacylglycerols are di- and monoacylglycerols. 

Lipids (fats and oils) and monoacylglycerols used as emulsifiers form inclusion compounds with amylose, slow down the swelling of starch granules and the extent of starch gelatinisation. For example, around 96% of starch is fully gelatinised in white bread, which is low in fat. Bakery products rich in fat, especially in the surface layers with lower water activity, contain a considerable proportion of ungelatinised starch. Small concentrations of sodium chloride have only limited impact on gel formation. 

Unsaturated lipid substrates are the prime compounds of deterioration in food and cosmetics. They are constituted of mixtures of tri-, di-, and monoacylglycerols, free fatty acids, glycolipids, phospholipids, sterols, and other substances (Chen et ah, 2011). Hydrolysis, polymerization, and isomerization reactions can cause the deterioration of food, and the resulting compounds are harmful to health (Chen et ah, 2011 Ng et ah, 2014). However, the most important reaction that occurs with lipids is oxidation, because lipids are easily oxidized, even by atmospheric oxygen. This process results in the destruction of essential triglycerides and fat-soluble vitamins (A, D, E, and K), which leads to the formation of aldehydes, ketones, and free fatty acids. This is undesirable because the palatability, odor, texture, consistency, appearance, and nutritional value of the food are altered (Takemoto et ah, 2009 Andre et al., 2010). 

Degradation of MGDG - Autoradiography and enzyme activities calculated from radioactivity countings (Fig.l) show that the degradation of MGDG leads to the formation of free fatty acids (FFA), monoacylglycerols (MG), and to a... 

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). 

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