Medium-chain fatty acids triacylglycerols

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. 

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. 

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. 

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... 

In neonate, suckling mammals, short- and medium-chain fatty acids are preferentially split at the sn-3 triacylglycerol position by oral and gastric lipases and are absorbed in the stomach, while the long-chain fatty acids are hydrolyzed at the sn- and sn-2 positions and by pancreatic lipases and are absorbed in the small intestine (50, 51). With growth, the neonate fat digestion system becomes less active, and is replaced by the small intestine-pancreatic lipase pathway. But residual oral and gastric lipase activities and direct absorption of short-chain fatty acids in the... 

Digestion and absorption of triacylglycerols with medium-chain fatty acids 12 carbons) proceed by a different pathway. Medium-chain triacylglycerols are partly water-soluble, are rapidly hydrolyzed by lingual and pancreatic lipases, and do not require the participation of bile acids. Some are absorbed intact and hydrolyzed inside the absorptive cell. Medium-chain fatty acids enter the portal... 

At least three acyl-CoA synthases, each specific for a particular size of fatty acid, exist acetyl-CoA synthase acts on acetate and other low-molecular-weight carboxylic acids, medium-chain acyl-CoA synthase on fatty acids with 4-11 carbon atoms, and acyl-CoA synthase on fatty acids with 6-20 carbon atoms. The activity of acetyl-CoA synthase in muscle is restricted to the mitochondrial matrix. Medium-chain acyl-CoA synthase occurs only in liver mitochondria, where medium-chain fatty acids obtained from digestion of dietary triacylglycerols and transported by the portal blood are metabolized. Acyl-CoA synthase, the major activating enzyme, occurs on the outer mitochondrial membrane surface and in endoplasmic reticulum. The overall reaction of activation is as follows ... 

Eccleston VS, Ohlrogge JB (1998) Expression of lauroyl-acyl crirrier protein thioesterase in Brassica napus seeds induces pathways for both fatty acid oxidation and biosynthesis and imphes a set point for triacylglycerol accumulation. Plant Cell 10 613-621 Eccleston VS, Cranmer AM, Voelker TA, OUrogge JB (1996) Medium-chain fatty acid biosynthesis and utilization in Brassica napus plants expressing lauroyl-acyl crurier protein thioesterase. Planta 198 46-53... 

The fatty acid composition of different milk fats is shown in Table 3.220. It will be seen clearly that palmitic and oleic acids are the main components. Other acids usually present as significant components are capric, lauric, myristic, palmitoleic and linoleic acids. Although medium-chain acids (Cg-Cia) usually represent 8-15% of the total acids of milk triacylglycerols, in rabbit they may comprise up to 65% and in elephant about 100% of the total. The chain-termination mechanism by which medium-chain fatty acids are released from fatty acid syn-... 

Figures 1 and 2 show the capacity of insect homogenates from various stages of development to incorporate medium chain fatty acids (10 0 and 12 0) (fig. lA) as well as unsaturated 18C (18 1 and 18 2) (fig. 2A). In all cases a crossing-zone between the radioactivity patterns of free fatty acids and triacylglycerols is defined in clear coincidence with the larval-pupal apolysis stage. This metabolic difference between larval and pharate adult homogenates could not be explained through differences in the acyl-CoA synthetase activity of the insect furthermore,the enzyme activity increases notably from the larval to the adult stage of development in a manner similar for each fatty acids (figs. IB and 2B).

Deerberg, S., von Twickel, J., Forster, H.-H., Cole, T. and Fuhrmann, J. (1990) Synthesis of medium-chain fatty acids and their incorporation into triacylglycerols by cell-free fractions from Cuphea embryos. Planta 180, 440-444. 

Using a modified system as described by Ergan et al. (1990), an 87% conversion of medium chain fatty acids into medium chain triacylglycerols (MCTs) has been observed. However, Kim and Rhee (1991) obtained marginal amounts of tricaprin in their experiments with lipozyme and various methods of removing water from the reaction. The current interest in the use of MCTs in various food applications as fatless fats could trigger more activity in their enzymatic synthesis. Attention might be focused on short-chain specific enzymes such as lipases from Rhizopus delemar (Tahoun and Ali, 1986). 

L = long chain fatty acids, M = medium chain fatty acids, S = short chain fatty acids, MAG = monoacylglycerols, DAG = diacylglycerols, TAG = triacylglycerols. Substrate specificity to acylglycerols. 

Carvajal, O., Sakono, M., Sonoki, H., Nakayams, M., Kishi, T., Sato, M., Ikeda, I., Sugano, M., and Imaizumi, K. Structured triacylglycerol containing medium-chain fatty acids in sn 1(3) facilitates the absorption of dietary long-chain fatty acids in rats. Biosci. Biotechnol. Biochem. 64, 793-798, 2000. 

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