Medium triacylglycerol

Mortensen PB. C6-C10-dicarboxyUc aciduria in starved, fat-fed and diabetic rats receiving decanoic acid or medium-cbain triacylglycerol. An in vivo measure of the rate of beta-oxidation of fatty acids. Biochim BiophysActa, 1981, 664(2), 349-355. 

The nonpolar lipid core consists of mainly triacylglycerol and cholesteryl ester and is surrounded by a single surface layer of amphipathic phospholipid and cholesterol molecules (Figure 25-1). These are oriented so that their polar groups face outward to the aqueous medium, as in the cell membrane (Chapter 14). The protein moiety of a lipoprotein is known as an apo-lipoprotein or apoprotein, constituting nearly 70% of some HDL and as litde as 1% of chylomicrons. Some apolipoproteins are integral and cannot be removed, whereas others are free to transfer to other hpoproteins. 

Pasquier B, Armand M, Guillon F, Castelain C, Borel P, Barry JL, Pieroni G and Lairon D. 1996. Viscous soluble dietary fibers alter emulsification and lipolysis of triacylglycerols in duodenal medium in vitro. J Nutr Biochem 7 293—302. 

A summary of the sources and fates of fatty acids and ketone bodies is presented in Figure 7.1 and Table 7.1. A major problem with long-chain fatty acids and TAGs is their lack of solubility in the aqueous medium of the blood and interstitial fluid. How this is overcome for fatty acids is discussed in this chapter, and for triacylglycerol in Chapter 11. Unfortunately, the need to transport relatively large quantities of triacylglycerol in the blood can lead to pathological problems (Chapter 11). 

Medium-chain fatty acids are also present in bovine milk and some plant oils (e.g. coconut). After digestion of the triacylglycerol, they are taken up by the enterocytes in the small intestine but are not esterified. Instead they pass directly into the hepatic portal blood, from where they are taken up by the liver for complete oxidation or conversion to ketone bodies. 

Medium-chain acyl-CoA synthetase, which is present within the mitochondrial matrix of the liver, activates fatty acids containing from four to ten carbon atoms. Medium-chain length fatty acids are obtained mainly from triacylglycerols in dairy products. However, unlike long-chain fatty acids, they are not esterified in the epithelial cells of the intestine but enter the hepatic portal vein as fatty acids to be transported to the liver. Within the liver, they enter the mitochondria directly, where they are converted to acyl-CoA, which can be fully oxidised and/or converted into ketone bodies. The latter are released and can be taken up and oxidised by tissues. 

This latter situation is particularly beneficial for patients who are being fed intravenously because, if triacylglycerols containing medium-chain fatty acids are included in parenteral feeds, they are readily converted into ketone bodies so that a soluble fat fuel is rapidly made available in the blood that can be oxidised by most tissues. 

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. 

Unesterified fatty acids are carried in plasma by albumin (chapter 18). The plasma also transports more complex lipids (cholesterol, triacylglycerols) among the various tissues as components of lipoproteins (spherical particles composed of lipids and proteins). Because cholesterol and triacylglyc-erol are insoluble in an aqueous medium such as the plasma, these lipoproteins (which are soluble in plasma) have evolved for the purpose of transporting complex lipids among tissues. In this section we are concerned with the structure and metabolism of these lipoproteins. 

Christensen, J.O. et al. (2004) Solubilisation of poorly water-soluble drugs dnmitgo lipolysis of medium-and long-chain triacylglycerol ur. J. Pharm. Sci., 23 287-296. 

Full fat milk contains about 3% fat and triacylglycerols account for about 95% of the lipid fraction. Other components of the lipid fraction are diacylglycerols, cholesterol, phospholipids, and free fatty acids. The lipid structures contain many fatty acids from all major classes, that is, saturated, monounsaturated, and polyunsaturated fatty acids (Haug et al., 2007). More than 60% of the fatty acids in cow s milk and consequently in dairy products are saturated, including shorter and medium... 

Data on the proportions of different fatty acids in plasma lipid esters (cholesteryl esters, phospholipids, free fatty acids, or triacylglycerol), erythrocyte membranes, or adipose tissue may provide a more objective and accurate path to evaluating dietary fatty acid composition (Arab, 2003 Baylin and Campos, 2006). The fatty acid composition in blood and body tissues reflects the fatty acid composition of the diet at different time points after ingestion. Short and medium-term changes in the composition of dietary fatty acid intake are reflected in plasma lipids and erythrocyte membranes, weeks and months after intake, respectively. The incorporation of fatty acids in adipose tissue reflects long-term changes in the diet (years) (Baylin and Campos, 2006 Katan et al., 1997 Ma et al., 1995 Zock et al, 1997). 

With low polarity columns, separation is based on the number of carbon atoms, with unsaturation of the acyl substituent not detected. The degree of separation can be improved by the use of medium-polarity columns when the number of double bonds in the acyl molecule does have an effect on the separation. More recently there has been a move towards the use of longer capillary columns but even this does not give a very substantial improvement in separation of triacylglycerols than can be achieved by HPFC. 

The saturated and monoene triacylglycerol classes were dominant and each comprised about 35 to 40% of the total milk fat, while the approximate proportions of the high-, medium- and low-molecular weight fractions were 40, 20 and 40%, respectively. 

Studies in vitro and in rats showed that short and medium chain fatty acids and monoacylglycerols hydrolyzed from milk triacylglycerols and digestion products of sphingolipids possess strong anti-bacterial and anti-viral properties. 

Deckelbaum, R. J., Hamilton, J. A., Moser, A., Bengtsson-Olivecrona, G., Butbul, E., Carpentier, Y. A., Gutman, A., and Olivecrona, T. (1990), Medium-chain versus long-chain triacylglycerol emulsion hydrolysis by lipoprotein hpase and hepatic lipase Implications for the mechanisms of lipase action, Biochemistry, 29,1136-1142. 

Acidolysis. This reaction can be acid or enzyme catalyzed and may be used to modify triacylglycerol composition. Acidolysis of an oil containing only Cis and Cjg fatty acids with fatty acids rich in lauric acid (e.g., from palm-kemel oil) results in a triacylglycerol enriched in medium-chain fatty acids. 

Medium-chain fatty acids are saturated fatty acids because of the relatively shorter hydrocarbon chain, which does not facilitate unsaturation. The safety of medium-chain triacylglycerol (MCTs) in dietary oil has been debated, and associated effects on cholesterol metabolism remain unclear. Although some studies have shown that... 

---