Fatty acyltransferase activities

The esterification of fatty acids in the mammary cell has been reported as a function of the microsomes and mitochondria (Bauman and Davis 1974 Moore and Christie 1978). While both microsomes and mitochondria may have acyltransferase activity, it has been observed to be 10 times greater in the microsomal fraction of the rat mammary cell (Tanioka et al. 1974). Based on autoradiographic studies, it appears that most synthesis of milk TG occurs in the rough endoplasmic reticulum of mouse mammary tissue (Stein and Stein 1971). 

Fig. 7 Biosynthesis of NATs and TRP channel activation by NATs. (a) Evidence for a fatty acyl CoA taurine A-acyltransferase activity was detected in mouse tissue by incubating taurine and arachidonoyl-CoA with various tissue lysates, (b) arachidonyl NAT was tested as an activator of the TRPV1 (black line), TRPV4 (gray line), and TRPM8 (dashed line) ion channels. Channel activation was measured using a Fura-2-based calcium-imaging assay, where the ratio between the fluorescence at 340 and 380 nm is reflective of cellular calcium concentrations...

Mathur, S.N., Simon, L, Lokesh, B.R., Specter, AA. Phospholipid fatty acid modification of rat liver microsomes affects acylcoenzyme A cholesterol acyltransferase activity. Biochem. Biophys. Acta 1983 751 401-411... 

Rumsey, S. C., Galeano, N., Lipschitz, B., and Deckelbaum, R. J. (1995). Oleate and other long-chain fatty acids stimulate low-density-lipoprotein receptor activity by enhancing acyl coenzyme A cholesterol acyltransferase activity and altering intracellular regulatory cholesterol pools in cultured cells. /. Biol. Chem. 270,10008-100016. 

Despite the presence of acetyl-CoA ACP acyltransferase activity in plant fatty acid synthase preparations, acetyl-ACP does not appear to play a major role in plant fatty acid synthesis (J. Jaworski, 1993). Instead, the first condensation takes place between acetyl-CoA and malonyl-ACP. This reaction is catalyzed by P-ketoacyl-ACP synthase III, one of three ketoacyl synthases in plant systems (Fig. 2). The acetoacetyl-ACP product then undergoes the standard reduction-dehydration-reduction sequence to produce 4 0-ACP, the initial substrate of ketoacyl-ACP synthase I. KAS I is responsible for the condensations in each elongation cycle up through that producing 16 0-ACP. The third ketoacyl synthase, KAS II, is dedicated to the final plastidial elongation, that of 16 0-ACP to 18 0-ACP. 

The enzymes in the pathways of fatty acid activation and p-oxidation (the synthetases, the carnitine acyltransferases, and the dehydrogenases of p-oxidation) are somewhat specific for the length of the fatty acid carbon chain. The chain length specificity is divided into enzymes for long-chain fatty acids (C20 to approximately C12), medium-chain (approximately C12 to C4), and short-chain (C4-C2). The major lipids oxidized in the liver as fuels are the long-chain fatty acids (palmitic, stearic, and oleic acids), because these are the lipids that are synthesized in the liver, are the major lipids ingested from meat or dairy sources, and are the major form of fatty acids present in adipose tissue triacylglycerols. The liver, as well as many other tissues, uses fatty acids as fuels when the concentration of the fatty acid-albumin complex is increased in the blood. 

Katavic, V., D.W. Reed, D.C. Taylor, et al. 1995. Alteration of seed fatty acid composition by an ethyl methanesulfonate-induced mutation in Arabidopsis thaliana affecting diacylglycerol acyltransferase activity. Plant Physiol. 108 399M 09. 

The lipid acyl hydrolase from potato tubers (described above) will transfer fatty acids from a donor lipid to a suitable alcohol acceptor. This effect is illustrated in Fig. 3 which shows the results obtained on adding increasing amounts of methanol to a system containing a lipid substrate and the enzyme at 30% methanol, the major reaction product is fatty acid methyl ester, and a relatively small amount of free fatty acid is produced. The affinity of the enzyme for methanol is approximately 10 times that for water, hence the acyltransferase activity is greater than the acyl hydrolase activity. 

Hg.3. Effect of methanol concentration on acyltransferase activity of lipolytic acyl hydrolase. Amounts of free fatty acids (circles) and fatty acid methyl esters formed (triangles) or lyso-phosphatidylcholine deacylated (squares) in 10-min incubations at 2S°C are given as percentage of substrate (lysophosphatidylcholine) added. (Reproduced from Galliard and Dennis, 1974, by permission.)... 

Bafor, M., Wiberg, E. and Stymne, S. (1990c) Palm kernel (Elaeis guineensis), lipid accumulation, fatty acid changes and acyltransferase activities, in Plant Lipid Biochemistry, Structure and Utilization, eds. P.J. Quinn and J.L. Harwood, Portland, London, pp. 198-200. 

Several systems for formation and hydrolysis of cholesteryl esters in rat liver are known. Microsomes contain an acyl-CoA cholesterol acyltransferase, which requires coenzyme A and ATP for fatty add activation, and operates at neutral pH. Enzymic transfer of fatty adds from lecithin to cholesterol occurs in the soluble fraction of rat liver. A third enzyme, cholesterol esterase, occurs in rat liver and its main function is probably hydrolytic. Although human liver apparently does not have acyl-CoA cholesterol acyltransferase activity, it does have a reversible cholesterol esterase (E.C.3.1.1.13) with optimal... 

To promote uses of rapeseed ( Brassica. nopus) oil, some new oil fatty acid profiles are needed. In this paper, we report the development of low linolenic acid (Cl8 3) iines. The triacylglycerol (TAG) lipid structure of high and low Cl8 3 rapeseed oils has been determined to test Sn-2 specific acyltransferase activities. No differences were detected among the lines for the TAG lipid structure even with a 0.9% Cl8 3 content. 

The fatty acid profile of seed oil,( e.g. triacylglycerols or TAGs), is very different from membrane lipids. This difference can be assigned to a small number of enzymes (2) some of which being acyltransferases that are specific to the Sn-2 position of glycerol backbone(3). One objective of this study was fo verify if deficiency of Sn-2 specific acyltransferase activities correlates with low linolenic acid content. The TAG lipid structure of 3 low linolenic acid lines, variety Stellar (SL, St T and St PI) and 2 high linolenic acid lines, variety Drakkar, (Drk C and Drk P2) are reported. 

The oil fatty acid composition of low Hnolenic acid lines were widely modified. This transformation affected only the Cl8 unsaturated fatty acids ( Cl8 1, Cl8 2 and Cl8 3). But no major consequence was observed on TAG lipid structure, with this change in the ratios among these 3 fatty acids, We did not find a decrease acyltransferase activities specific to Sn-2, in this experiment. It doesn t mean that there is no one, but even there is one, it does not clearly modify the TAG lipid structure at the end of the storage lipid biosynthesis. It will be interesting to determine the lipid structure of some key components like Phosphatidylcholine (PC) or LysoPC, involved in previous steps of lipid biosynthesis. 

While storage in membrane phospholipids is, from the standpoint of signal transduction, a predominant fate of arachidonoyl-CoA, additional pathways do exist. For instance, as shown in Figure 2.3, arachidonoyl-CoA may be converted back to arachidonate by a fatty acyl-CoA hydrolase activity, whose biological roles, if any, remain unknown. Moreover, arachidonoyl-CoA is an acceptable substrate for other acyltransferase activities, and it may therefore be used for the synthesis of cholesterol esters (a reserve supply of cholesterol) and triacylglycerols. Arachidonic acid in... 

ICAT (or PCAT, phosphatidylcholine-cholesterol acyltransferase) is an enzyme in the blood that is activated by apoA-1 on HDL. LCAT adds a fatty add to cholesterol, producing cholesterol esters, which dissolve in the core of the HDL, allowing HDL to transport cholesterol from the periphery to the liver. This process of reverse cholesterol transport is shown in Figure 1-15-7. 

The inner mitochondrial membrane has a group-specific transport system for fatty acids. In the cytoplasm, the acyl groups of activated fatty acids are transferred to carnitine by carnitine acyltransferase [1 ]. They are then channeled into the matrix by an acylcar-nitine/carnitine antiport as acyl carnitine, in exchange for free carnitine. In the matrix, the mitochondrial enzyme carnitine acyltransferase catalyzes the return transfer of the acyl residue to CoA. 

Esterification of glycerol 3-phosphate with a long-chain fatty acid produces a strongly amphipathic lysophosphatidate (enzyme glycerol-3-phosphate acyltransferase 2.3.1.15). In this reaction, an acyl residue is transferred from the activated precursor acyl-CoA to the hydroxy group at C-1. 

Regulation of the LDL receptor gene involves a hormone-response element (HRE, see p. 238).] Third, if the cholesterol is not required immediately for some structural or synthetic purpose, it is esterified by acyl CoA cholesterol acyltransferase (ACAT, AC AT transfers a fatty acid from a fatty acyl CoA derivative to cholesterol, producing a cholesteryl ester that can be stored in the cell (Figure 18.21). The activity of ACAT is enhanced in the presence of increased intracellular cholesterol. 

Esterification of cholesterol When cholesterol is taken up by HDL, it is immediately esterified by the plasma enzyme phos-phatidylcholine cholesterol acyltransferase (PCAT, also known as LCAT, in which "L" stands for lecithin). This enzyme is synthesized by the liver. PCAT binds to nascent HDLs, and is activated by apo A-l. PCAT transfers the fatty acid from carbon 2 of phosphatidyl-... 

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