Mixed triacylglycerol

FIGURE 24.4 In the small intestine, fatty acids combine with bile salts in mixed micelles, which deliver fatty acids to epithelial cells that cover the intestinal villi. Triacylglycerols are formed within the epithelial cells. 

Two possible pathways for the biosynthesis of 2-AG have been proposed (1) a phospholipase C (PLC) hydrolysis of membrane phospholipids followed by a second hydrolysis of the resulting 1,2-diacylglycerol by diacylglycerol lipase or (2) a phospholipase Ai (PLA,) activity that generates a lysophospholipid, which in turn is hydrolyzed to 2-AG by lysophospholipase C (Fig. 5) (Piomelli, 1998). Alternative pathways may also exist from either triacylglycerols by a neutral lipase activity or lysophosphatidic acid by a dephosphorylase. The fact that PLC and diacylglycerol lipase inhibitors inhibit 2-AG formation in cortical neurons supports the contention that 2-AG is, at least predominantly, biosynthesized by the PLC pathway (Stella, 1997). However, a mixed pathway may also be plausible. 

Saponification (see Section 7.4) is carried out to extract more recalcitrant lipids, and the yields are higher than for conventional solvent extraction (Stern et al. 2000). 3 ml of 0.5 M methanolic NaOH is added to 0.1 g of the shard powder and heated at 70°C for 3 hours in a sealed glass vial. After cooling, the supernatant is acidified with HC1 and extracted with three aliquots of 3 ml //-hexane. The hexane will not mix with the methanolic solution (unlike the DCM MeOH used above), but will absorb the lipids and can be transferred into a new clean vial. The removal of excess hexane is carried out as above. Saponification will hydrolyze and methylate any ester functionalities, which removes the requirement to derivatize the samples (Section 7.4) unless other molecules are suspected of being present. However, any wax esters or triacylglycerols will also be hydrolyzed to their fatty acid methyl esters and alcohols therefore, if information on their composition is required, then conventional solvent extraction is recommended as a first step. For subsequent characterization of the lipid extract, see Chapter 7. 

Phospholipids are digested and absorbed in a similar manner to that of triacylglycerol. Pancreatic lipase has some hydrolytic activity towards phospholipids and removes the fatty acid from the 1-position. The product is a lysophospholipid such as lysolecithin (Figure 4.12). It also acts as a detergent and contributes to the stability of the mixed micelles. 

Stearoyl, 2-linoleoyl, 3-palmitoyl glycerol, a mixed triacylglycerol... 

FIGURE 10-2 Glycerol and a triacylglycerol. The mixed triacylglycerol shown here has three different fatty acids attached to the glycerol backbone. When glycerol has two different fatty acids at C-1 and C-3, the C-2 is a chiral center (p. 76). 

Correct answer = E. Chylomicrons contain a lipid core that is composed of dietary lipid and lipid synthesized in the intestinal mucosal cells. Free fatty acids are esterified primarily to 2-monoacylglycerol. forming triacylglycerol, prior to export from the intestinal mucosal cells in chylomicrons. Mixed micelles are found only in the lumen of the small intestine. 

As an aid in the identification of such triacylglycerols in butterfat, Marai et al. (128) have investigated the reversed-phase LC-MS behavior of randomized butterfat, which contains the various isomeric triacylglycerols in known and sufficient amounts for analysis. The results show that conventional C, 8 reversed-phase columns would not resol ve the positional and reverse isomers of mixed acid triacylglycerols, but that the resolution of isologous triacylglycerols is retained also when two or three short-chain fatty acids occur per molecule. 

Fig. 1. Structure of (a) a simple triacylglycerol (1,2,3-tripalmitoyl-glycerol) and (b) a mixed triacylglycerol (1 -palmitoyl-2,3-dioleoyl-glycerol).

In summary, triacylglycerols from the diet are digested by lipase and associate with bile salts into mixed micelles. The free fatty acids are absorbed by the cells of the small intestine, from which they are transported via the lymph system to the liver. From the liver, they are released as apolipoproteins in the circulation, carrying fatty acids and cholesterol to the cells throughout the body. 

Several excellent reviews have summarized the well-established ability of phytosterols to lower serum cholesterol concentrations in humans.The consumption of food products supplemented with 1.6 to 3.5 g/d of phytosterols has been shown to consistently decrease serum total cholesterol and LDL-C concentrations by up to 15%, without affecting HDL-C and triacylglycerol concentrations (Table 8.6). About 1 g of mixed phytosterols per day is needed to decrease cholesterol by at least 5%, but phytosterol intakes exceeding 2.5 g/d decrease plasma cholesterol and LDL-C by up to 15%. 

Simple triacylglycerols are composed of three identical fatty acid side chains, whereas mixed triacylglycerols have two or three different fatty acids. Table 29.2 lists the most common fatty acids used to form triacylglycerols. 

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