Linoleic desaturation

Fig. 1. Effect of different concentrations of protein factor (Sp) and bovine serum albumin on the restoration of linoleate desaturation activity.

Linoleate desaturation. This desaturation could be observed in vivo with developing pea or spinach leaves. At the end of a 30 min pulse with oleate, clear precursor-product relationships could be observed during the following 48 hr chase period, between C-18 l PC and C-18 2 PC both in... 

The biosynthesis of linolenic acid, the dominant fatty acid in chloroplasts, has remained an unsolved problem despite continuous researches on this topic for more than forty years. Very little success has been obtained with attempts of in vitro linoleate desaturation [20, 2l]. 

To overcome progressively the numerous difficulties linked to linoleate desaturation studies, we began with in vivo studies [22j. As a labelled precursor of linolenic acid, we have used C-oleate, given externally to entire leaves, to follow the labelling of the metabolic intermediates of the sole eukaryotic pathway. To perform the labelling experiments, we have used... 

Browse, J.A. and Slack, C.R. 0 981). Catalase stimulates linoleate desaturation in microsome preparations from developing linseed cotyledons. FEBS Lett. 131,111-114. 

According to the current concept, plant and animal desa-turase systems include flavoprotein,Fe-containing protein and desaturase. Thus, oleate desaturase of potato microsomes is assumed to consist sequentially of NADH-ferricyanide reductase, cytochrome and desaturase 1. At present the involvement of photosynthetic electron transport components in the linoleate desaturation remains unknown. To study this problem the effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), the classical inhibitor of photosynthetic electron transport, on oleate and linoleate desaturation was investigated. 

DCMU inhibitory effect on linoleate desaturation appeared in 24 hours after the beginning of pea shoot incubation with l- C oleate the percentage of C-linolenate decreased twice in the presence of DCMU (Tabl. 1). On the one hand, the li-nolenate percentage decrease may be caused by the decrease of the reduced pyridine nucleotide level in the cells in the presence of DCMU. Besides, the possibility of electron transfer to linoleate desaturase both from NADPH and from H2O may be... 

ARE DIACYLGLYCEROLS POSSIBLE SUBSTRATES FOR LINOLEATE DESATURATION IN SAFFLOWER COTYLEDONS (Carthamus tinctorius L.) ... 

There results suggest two possibilities for linoleate desaturation ... 

A15-desaturation. Experiments were carried out with [l- C]linoleate and [l- " C]linolenate in order to eliminate the possibility that linoleate was desaturated as part of a different substrate and that the labeled linoleate and a-linolenate were merely transferred preferentially to MGDG. The results showed clearly that radiolabeled oleate, linoleate, and linolenate were incorporated in a similar manner, with PC being the most labeled lipid class there was no evidence for a preferential transfer of a-linolenate to MGDG. All the evidence pointed to MGDG being involved in linoleate desaturation and to this process being inhibited by Sandoz 9785. 

Organisms differ with respect to formation, processing, and utilization of polyunsaturated fatty acids. E. coli, for example, does not have any polyunsaturated fatty acids. Eukaryotes do synthesize a variety of polyunsaturated fatty acids, certain organisms more than others. For example, plants manufacture double bonds between the A and the methyl end of the chain, but mammals cannot. Plants readily desaturate oleic acid at the 12-position (to give linoleic acid) or at both the 12- and 15-positions (producing linolenic acid). Mammals require polyunsaturated fatty acids, but must acquire them in their diet. As such, they are referred to as essential fatty acids. On the other hand, mammals can introduce double bonds between the double bond at the 8- or 9-posi-tion and the carboxyl group. Enzyme complexes in the endoplasmic reticulum desaturate the 5-position, provided a double bond exists at the 8-position, and form a double bond at the 6-position if one already exists at the 9-position. Thus, oleate can be unsaturated at the 6,7-position to give an 18 2 d5-A ,A fatty acid. 

Mammals can add additional double bonds to unsaturated fatty acids in their diets. Their ability to make arachidonic acid from linoleic acid is one example (Figure 25.15). This fatty acid is the precursor for prostaglandins and other biologically active derivatives such as leukotrienes. Synthesis involves formation of a linoleoyl ester of CoA from dietary linoleic acid, followed by introduction of a double bond at the 6-position. The triply unsaturated product is then elongated (by malonyl-CoA with a decarboxylation step) to yield a 20-carbon fatty acid with double bonds at the 8-, 11-, and 14-positions. A second desaturation reaction at the 5-position followed by an acyl-CoA synthetase reaction (Chapter 24) liberates the product, a 20-carbon fatty acid with double bonds at the 5-, 8-, IT, and ITpositions. 

Macrolide aggregation pheromones produced by male cucujid beetles are derived from fatty acids. Feeding experiments with labeled oleic, linoleic, and palmitic acids indicate incorporation into the macrolide pheromone component [ 117 ]. The biosynthesis of another group of beetle pheromones, the lactones, involves fatty acid biosynthetic pathways. Japonilure and buibuilactone biosynthesized by the female scarab, Anomalajaponica, involves A9 desaturation of 16 and 18 carbon fatty acids to produce Z9-16 CoA and Z9-18 CoA,hydroxylation at carbon 8 followed by two rounds of limited chain shortening and cyclization to the lactone [118]. The hydroxylation step appears to be stereospecific [118]. 

FIGURE 3-7 Pathways for the interconversion of brain fatty acids. Palmitic acid (16 0) is the main end product of brain fatty acid synthesis. It may then be elongated, desaturated, and/or P-oxidized to form different long chain fatty acids. The monoenes (18 1 A7, 18 1 A9, 24 1 A15) are the main unsaturated fatty acids formed de novo by A9 desaturation and chain elongation. As shown, the very long chain fatty acids are a-oxidized to form a-hydroxy and odd numbered fatty acids. The polyunsaturated fatty acids are formed mainly from exogenous dietary fatty acids, such as linoleic (18 2, n-6) and a-linoleic (18 2, n-3) acids by chain elongation and desaturation at A5 and A6, as shown. A A4 desaturase has also been proposed, but its existence has been questioned. Instead, it has been shown that unsaturation at the A4 position is effected by retroconversion i.e. A6 unsaturation in the endoplasmic reticulum, followed by one cycle of P-oxidation (-C2) in peroxisomes [11], This is illustrated in the biosynthesis of DHA (22 6, n-3) above. In severe essential fatty acid deficiency, the abnormal polyenes, such as 20 3, n-9 are also synthesized de novo to substitute for the normal polyunsaturated acids. 

Although these are termed essential fatty acids, they are, in fact, precursors for the major polyunsaturated fatty acids that have essential roles in the body but are present only in small amounts in the diet. Linoleic acid is converted, via elongation and desaturation reactions, to dihomo-y-linolenic (20 3n-6) and then to arachidonic (20 4n-6) acid. a-Linolenic is converted to eicosapentaenoic (20 5n-3) and then docosahexae-noic (22 6n-3). The pathways for formation of these latter fatty acids, from their dietary precursors, are presented in Figures 11.11 and 11.12. Full details of one pathway are provided, as an example, in Appendix 11.4. For comparison of the two pathways, they are presented side by side in Figure 11.13. 

Figure 11.11 Outline of the pathway consisting of desaturation and elongation reactions that convert linoleic acid into arachidonic acid.

The question arises whether inhibition of the desaturase for this particular pathway can be overcome. The answer is yes. The product of the A desaturase when desaturating linoleic acid is y-linolenic acid. Supplementation of the diet with y-linolenic acid, which bypasses the A desaturase reaction, has been used to increase the formation of dihomo-y-linolenic acid and arachidonic acid (Figure... 

Figure 20.3 Essential fatty acids in the diet, production of physiological essential acids and their roles in the cell cycle. Essential fatty adds in the diet are mainly linoleic and a-linolenic but they are converted by desaturation and elongation reactions to the essential acids that are used in phospholipid formation and synthesis of eicosanoids. (For details of the elongation and desaturation reactions and eicosanoid formation, see Chapter 11.).

In human being, arachidonic acid is the most important precursor for the biosynthesis of eicosanoids. Arachidonic acid is formed from linoleic acid in most mammalians by desaturation and carbon elongation to dihomog-linolenic acid and subsequent desaturation. 

Dietary intake is of great importance. Linoleic acid (C18 2o)6) and a-linolenic acid (C18 3o)3) are the parent essential fatty acids for humans. Both fatty acids derive from vegetable oils. Higher fatty acids are then produced by chain elongation and desaturation. In addition, some of the prime essential fatty acids, AA (C20 4o)6), EPA (C20 5w3) and DHA (C22 6w3), can be obtained directly from the diet. Meat and eggs are rich in AA, whereas fish is a rich source of EPA and DHA [14]. 

Linoleic (C18 2) and linolenic (C18 3) acids cannot be synthesized by mammals and must be supplied in the diet, i.e. they are essential fatty acids (linoleic is the only true essential acid). These two polyenoic acids may then be elongated and/or further desaturated by mechanisms similar to stearic - oleic, to provide a full range of polyenoic acids. A summary of these reactions is given in Figure 3.12a, b. 

The dietary precursor of the prostaglandins is the essential fatty acid, linoleic acid. It is elongated and desaturated to arachidonic acid, the immediate precursor of the predominant class of prostaglandins (those with two double bonds) in humans (Figure 17.22). [Note Arachidonic acid is released from membrane-bound phospholipids by phospholipase Ap in response to a variety of signals (Figure 17.23).]... 

Figure 21-3 Major pathways of synthesis of fatty acids and glycerolipids in the green plant Arabidopsis. The major site of fatty acid synthesis is chloroplasts. Most is exported to the cytosol as oleic acid (18 1). After conversion to its coenzyme A derivative it is converted to phosphatidic acid (PA), diacylglycerol (DAG), and the phospholipids phosphatidylcholine (PC), phosphatidylinositol (PI), phosphatidylglycerol (PG), and phosphatidylethanolamine (PE). Desaturation also occurs, and some linoleic and linolenic acids are returned to the chloroplasts. See text also. From Sommerville and Browse.106 See also Figs. 21-4 and 21-5. Other abbreviations monogalactosyldiacylglycerol (MGD), digalactosyldiacylglycerol (DGD), sulfolipid (SL), glycerol 3-phosphate (G3P), lysophosphatidic acid (LPA), acyl carrier protein (ACP), cytidine diphosphate-DAG (CDP-DAG).

The conversion of oleoyl-CoA to linoleoyl-CoA is accomplished by some insects118 but does not take place in most animals. As a result of this biosynthetic deficiency, polyunsaturated fatty acids such as linoleic, linolenic, and the C20 arachidonic acid are necessary in the diet (Box 21-B). One essential function of linoleic acid is to serve as a precursor of prostaglandins and related prostanoids (Section D). Dietary linoleate is converted to its Co A derivative and then by sequential A6 desaturation,119 elongation, and then A5 desaturation, to the 20 4 (A5 8 11 14) arachidonoyl-CoA (Fig. 21-2, lower right). These acids are referred to as 0)6 because of the position of the last double bond. Linolenic acid can be converted in an analogous fashion to the CoA derivative of the 20 5 (A5 8 11 14 17 co6) eicosapentaenoic acid (EPA). The 22 6 docasahexaenoic acid (DHA Fig. 21-2) is apparently formed by elongation of the 22 5 acyl-CoA to 24 5, desaturation, transfer to a peroxisome or mitochondrion, and p oxidation to shorten the chain.953... 

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. 

Arachidonic acid (5,8,11,14-eicosatetraenoic acid), a polyunsaturated fatty acid derived from dietary sources or by desaturation and chain elongation of the essential fatty acid linoleic acid, is found widely in the body. It is transported in a protein-bound state and stored in the phospholipids of cell membranes in all tissues of the body [108] from where it can be changed into biologically... 

Emken, E.A., Adlof, R.O., and Gulley, R.M. 1994. Dietary linoleic acid influences desaturation and acylation of deuterium-labeled linoleic and linolenic acids in young adult males. Biochim. Bio-phys. Acta 1213, 277-288. 

Emken, E.A., Adlof, R.O., Duval, S.M., and Nelson, G.J. 1999. Effect of dietary docosahexaenoic acid on desaturation and uptake in vivo of isotope-labeled oleic, linoleic, and linolenic acids by male subjects. Lipids 34, 785-791. 

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