Animal desaturases

After the first three molecular characterizations of animal desaturases from rat liver (Thiede et al., 1986), mouse adipose tissue (Ntambi et al., 1988) and carp (Tiku et al., 1996), a Drosophila desaturase was isolated in 1997 (Wicker-Thomas et al., 1997). Since then, numerous studies have been made on lepidopteran desaturases involved in nonhydrocarbon short-chain pheromones (Knipple et al., 1998 reviewed in Knipple and Roelofs, 2003). In Drosophila, there are seven fatty acyl-CoA desaturase genes, which are all located on chromosome III (Figure 4.1), but only three desaturases appear to be involved in hydrocarbon synthesis. In the cricket, a desaturase has been characterized but there is no evidence that this desaturase is involved in pheromone biosynthesis (Riddervold et al., 2002). On the other hand, the desaturase isolated from the housefly is probably involved in both lipid and pheromone biosynthesis (Eigenheer et al., 2002). 

Additional double bonds introduced into existing mo-nounsamrated fatty acids are always separated from each other by a methylene group (methylene interrupted) except in bacteria. Since animals have a desaturase, they... 

If the substrate is fully saturated, the first double bond is always inserted at position 9 by the A desaturase (so that, for example, stearic acid (18 0) is converted to oleic acid (18 co-9). Thus the A desaturase requires the presence of a cis double bond at position 9 before it can catalyse desaturation at position 6. Animals do not possess a desaturase that can insert a double bond at a position greater than nine. (Such desaturations are present in plants). It is this fact that determines that such unsaturated fatty acids must be provided in the diet, i.e. they are essential. 

In higher plants, animals, protozoa, and fungi, saturated fatty acids are acted upon by desaturases to introduce double bonds, usually of the cis (Z) configuration. The substrates may be fatty acyl-ACP, fatty acyl-CoA molecules, membrane phospholipids,97 or glycolipids.98 The A9 desaturase, isolated from liver or from yeast, converts stearoyl-CoA to oleoyl-CoA (Eq. 21-3).99-102 This membrane-associated enzyme system... 

Enzyme complexes occur in the endoplasmic reticulum of animal cells that desaturate at A5 if there is a double bond at the A8 position, or at A6 if there is a double bond at the A9 position. These enzymes are different from each other and from the A9-desaturase discussed in the previous section, but the A5 and A6 desaturases do appear to utilize the same cytochrome b5 reductase and cytochrome b5 mentioned previously. Also present in the endoplasmic reticulum are enzymes that elongate saturated and unsaturated fatty acids by two carbons. As in the biosynthesis of palmitic acid, the fatty acid elongation system uses malonyl-CoA as a donor of the two-carbon unit. A combination of the desaturation and elongation enzymes allows for the biosynthesis of arachidonic acid and docosahexaenoic acid in the mammalian liver. As an example, the pathway by which linoleic acid is converted to arachidonic acid is shown in figure 18.17. Interestingly, cats are unable to synthesize arachidonic acid from linoleic acid. This may be why cats are carnivores and depend on other animals to make arachidonic acid for them. Also note that the elongation system in the endoplasmic reticulum is important for the conversion of palmitoyl-CoA to stearoyl-CoA. 

Although E double bonds are uncommon in unsaturated fatty acids of animals, many have been found among the unsaturated components of lepidopteran pheromones. Early investigations of pheromone biosynthetic pathways showed that E isomers are not produced by isomerization of the Z double bonds, but rather are produced directly by the catalytic activity of desaturases possessing... 

Three major families of unsaturated fatty acids are seen in warm-blooded animals, that is, the n-9, monounsaturated fatty acids (e.g. oleic acid, OA), and the n-6 and n-3, both polyunsaturated fatty acids (PUFAs). However, only the n-6 and n-3 families, derived from LA and ALA, respectively, are EFA. These must be obtained from the diet since mammals lack the desaturase enzymes necessary for the insertion of a double bond in the n-6 and n-3 positions of the fatty acid carbon chain. Fatty acid nomenclature is as follows The first number denotes the number of carbon atoms in the acyl chain and the second refers to the number of unsaturated (double) bonds. This is followed by a symbol n or co and a number that denotes the number of carbon atoms from the methyl terminal of the molecule to the first double bond. Hence, LA is 18 2(n-6), while the more unsaturated ALA is denoted as 18 3(n-3) (Figure 26.1). These fatty acids must be metabolized to their longer chain derivatives before carrying out many of their activities. 

Barber, M.C., Ward, R.J., Richards, S.E., Salter, A.M., Buttery, P.J., Vernon, R.G., Travers, M.T. 2000. Ovine adipose tissue monounsaturated fat content is correlated to depot-specific expression of the stearoyl-CoA desaturase gene. J. Anim. Sci. 78, 62-68. 

Cameron, P.J., Rogers, M., Oman, J., May, S.G., Lunt, D.K., Smith, S.B. 1994. Stearoyl coenzyme A desaturase enzyme activity and mRNA levels are not different in subcutaneous adipose tissue from Angus and American Wagyu steers. J. Anim. Sci. 72, 2624-2628. 

Animal and human tissues contain the enzyme A9-desaturase that can introduce a cA-double bond at carbon 9 in VA to produce RA. Santora et al. (2000) fed VA to mice and found that 12% of this VA or 50% of the VA stored in carcase adipose tissue was converted to RA. Humans fed increasing... 

Pyridoxal phosphate has a clear role in lipid metabolism as the coenzyme for the decarboxylation of phosphatidylserine, leading to the formation of phosphatidylethanolamine, and then phosphatidylcholine (Section 14.2.1), and membrane lipids from vitamin Bg-deficient animals are low in phosphatidylcholine (She et al., 1995). It also has a role, less well defined, in the metabolism of polyunsaturated fatty acids vitamin Bg deficiency results in reduced activity of A desaturase and impairs the synthesis of eicosapentanoic and docosahexanoic acids (Tsuge et al., 2000). 

Research demonstrated that CLA isomers reduce body fat in growing animals (29-31) and its actions on fat and energy metabolism may, in part, be directed through changes in both PPARa and PPARy (32, 33). In addition, specific effects of CLA isomers on activity and expression of enzymes associated with anabohc pathways of lipid metabolism are reported (34). Eor example, CLA was observed to decrease the mRNA level of the A9-desaturase enzyme in both liver tissue and hepatocyte cultures (35). 

Alpha-linolenic acid (18 3n-3) is an 18-carbon fatty acid with three double bonds at carbons 9, 12, and 15. It is an essential n-3 fatty acid that is a required nutrient for human beings and can be obtained through diets including both plant and animal sources. Alpha-linolenic acid can be converted by elongases and desaturases to other beneficial n-3 fatty acids, such as eicosapentaenoic acid (EPA) and docosa-hexaenoic acid (DHA), which are implicated in normal brain development, normal vision, and a decreased risk of heart disease. Novel dietary sources of n-3 fatty acids are desired for those who do not consume adequate amounts of fish or fish-based food products rich in long-chain n-3 fatty acids. This section summarized fruit, spice, and herb seed oils rich in a-linolenic acid (18 3n-3). These include black raspberry, red raspberry, boysenberry, marionberry, blueberry, cranberry, sea buckthorn, basil, and hemp seed oils. 

Both plants and animals use mixed function oxidases (simultaneously oxidize two substrates) Acyl-CoA desaturases localized on the ER. Similar mixed function oxidases are also used to modify structural components of cells, hormones etc. so we will use the acyl-CoA desaturase as an example for this group of enzymes. In the acyl-CoA desaturase reaction molecular oxygen is used to oxidize both a fatty acid and NADH, each providing two of the the four electrons needed by the oxygen ... 

As is outlined in the following section, it is now apparent that endogenous synthesis of RA occurs in most, if not all, animal species. The classic studies of Mahfouz et al. (1980) and Pollard et al. (1980) showed independently that positional isomers of trans-octadecenoic acids are desaturated by the enzyme A-9 desaturase in rat liver microsomal systems. All tram monoenes, A-4 to A-13, except A-8, 9, and 10, were substrates, with products being trans-A-x, cis-9 dienes. The rate of A-9 desaturation increased as the trans bond was removed further from the A-9 position, so that trans-A-4 and A-13 monoenes were most rapidly desaturated (Mahfouz et al., 1980). The trans-5, cis-9 18 2 was isomerized rapidly to the cis, cis diene without changing bond positions tram-4, cis-9 18 2 was isomerized similarly, but at a slower rate (Mahfouz et al., 1980). Significant amounts of some of the cis/trans dienes were desaturated further at A-6, to yield cis, tram, cis trienes (Pollard et al., 1980). 

Beswick, N.S. and Kennelly, J.J. 2000. Influence of bovine growth hormone and growth hormonereleasing factor on messenger RNA abundance of lipoprotein lipase and stearoyl-CoA desaturase in the bovine mammary gland and adipose tissue. J. Anim. Sci. 78, 412-419. 

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