Linoleic acid biohydrogenation

The process of linoleic acid biohydrogenation is depicted in Fig. 1.1. Linoleic acid from plant lipids is first acted upon by an isomerase of bacterial origin that creates new geometric and positional isomers, including conjugated dienes. However, no hydrogen addition occurs at this step and the same number of double bonds is maintained within the acyl chain. Additional transformations may follow where double bonds are eliminated from the fatty acyl chain via the action of mio-obial reductases. Reductase action may proceed until the fatty acyl chain is fully saturated. 

Most depictions of the pathways of biohydrogenation account for just a few of the trans monoenes and CLA isomers known to exist in the intestinal contents of ruminants (Fig. 1.5). In most cases, intermediates in the biohydrogenation of linoleic acid to stearic acid usually include only the cis-9, trans-11 and trans-11 isomers. A few other isomers for linoleic acid biohydrogenation have been proposed, but only in isolated citations. For instance, Griinari and Bauman (25) proposed that linoleic acid also could be converted to a trans- 0, cis- 2 diene, which in turn could be converted to a trans- 0 monoene followed by its conversion to stearic acid. Another study proposed the conversion of linoleic acid to trans-9, trans-, then to trans-9 monoene, and finally to stearic acid (26). 

The first step in the biohydrogenation of linoleic acid to stearic acid by the mmen microorganism, Butyrivibrio fibrisolvens, is the formation of the cis-9, trans-11 CLA isomer (Figure 1). This reaction is catalyzed by a membrane-bound enzyme, linoleate isomerase, which acts only on fatty acids possessing cis double bonds in positions and and a free carboxyl group (72). 

It is not certain that the presence of CLA in tissue lipids is due entirely to the production of cis-9, trans-11 as an intermediate during the biohydrogenation of linoleic acid in the rumen. However, the amount of CLA in milk (7 J) and butter (14) is positively correlated to the level of dietary linoleic acid. Some long chain fatty acid intermediates reach the small intestine and are normally absorbed and deposited into adipose tissue (75). There is seasonal variation in CLA content of milk, with the highest values occurring usually in summer (76). 

Cw-polyenoic acids are present at low concentrations in milk fat, because of the biohydrogenation reactions that take place in the rumen. These acids are comprised almost exclusively of linoleic acid (9c, 12c-18 2), about 1.2 to 1.7% and a-linolenic acid (9c, 12c, 15c-18 3), about 0.9 to 1.2% (Table 1.2). These two fatty acids are essential fatty acids they cannot be synthesised within the body and must be supplied by the diet. In recent times, the usage of the term essential has been extended to include derivatives of these fatty acids, which are not synthesised in significant quantities (e.g., eicosapentaenoic acid, 20 5 and docosahexaenoic acid, 22 6). The proportion of a-linolenic acid appears to be affected by the cow s diet the concentration is higher in milk from pasture-fed cows than in milk from barn-fed cows (Hebeisen et al., 1993 Wolff et al., 1995). In the case of linoleic... 

CLA refers to a mixture of positional and geometric isomers of linoleic acid (cis-9, cis-12 octadecadienoic acid) with a conjugated double bond system. The structure of two CLA isomers is contrasted with linoleic and vaccenic acids in Figure 3.1. The presence of CLA isomers in ruminant fat is related to the biohydrogenation of polyunsaturated fatty acids (PUFAs) in the rumen. Ruminant fats are relatively more saturated than most plant oils and this is also a consequence of biohydrogenation of dietary PUFAs by rumen bacteria. Increases in saturated fatty acids are considered undesirable, but consumption of CLA has been shown to be associated with many health benefits, and food products derived from ruminants are the major dietary source of CLA for humans. The interest in health benefits of CLA has its genesis in the research by Pariza and associates who first demonstrated that... 

Information on the effect of diet on the production of minor isomers of CLA in the rumen and alterations in their content in milk fat is limited. Diet-induced changes in trans-10, cis-12 CLA have been best described, and its biological effects in the dairy cow will be discussed in Section 3.6.1. Griinari and Bauman (1999) presented a putative pathway for the biohydrogenation of linoleic acid where the initial isomerization involved the cis-9 double bond, thereby resulting in the production of trans-10, cis-12 CLA and trans-10 18 1 as intermediates. As discussed earlier, rumen bacteria have been identified that produce trans-10, cis-12 CLA when incubated with linoleic acid (Verhulst et al., 1987 Kim et al., 2002), and the addition of trans-10, civ-12 CLA to the rumen results in the increased formation of trans-10 18 1 (Loor and Herbein, 2001). 

Figure 3.5. Generalized scheme of ruminal biohydrogenation of linoleic acid under normal conditions (solid line) and during diet-induced milk fat depression (dotted line). Adapted from Griinari and Bauman (1999).

Duckett, S. K., Andrae, J.G., Owens, F.N. 2002. Effect of high-oil corn or added corn oil on ruminal biohydrogenation of fatty acids and conjugated linoleic acid formation in beef steers fed finishing diets. J. Anim. Sci. 80, 3353-3360. 

Fatty acids with two or more conjugated double bonds are found in some plants and animals. In tissues of ruminant animals (and, hence, in meat and dairy products), fatty acids with conjugated diene system were detected as intermediates or by-products in the biohydrogenation of linoleic acid by microorganisms in the rumen. The main isomer, 9-cis, ll-fran -octadecadienoic acid, may account for up to 1% of the total fatty acids of milk fat. 9-cis, ll-fran5-15-cw-octadecatrienoic acid, derived from a-linolenic acid, is present in ruminant tissues only in trace levels. This fatty acid has been shown to have several medical properties, especially anti-cancer and anti-atherosclerosis effects. 

FIG. 3 Scheme for the biohydrogenation of linoleic acid group A and group B refer to the two classes of biohydrogenating bacteria. (Used, with permission, from Harfoot and Hazlewood, 1997.)... 

Boufaied, H., Chouinard, P.Y., Tremblay, G.F., Petit, H.V., Michaud, R., and Belanger, G. 2003. Fatty acids in forages. II. In vitro ruminal biohydrogenation of linolenic and linoleic acids from... 

Kepler CR, Hirons KP, McNeill JJ, Tove SB. (1966) Intermediates and products of the biohydrogenation of linoleic acid by Butyrinvibriofibrisolvens. J Biol Chem 241 1350-1354. 

Conjugated linoleic acid (CLA) is naturally present in milk, dairy products, and the meat of ruminants (1). Ruminants are the major dietary source of this fatty acid because of the unique abihty of rumen bacteria to convert linoleic acid into cis-9,trans- CLA (c9,tll-CLA) (1). This reaction is part of a process that takes place in the rumen it is called biohydrogenation and it converts linoleic acid [or, less efficiently, other 18-carbon polyunsaturated fatty acids (PUFA) with double bonds located at 9 and 12 positions] to stearic acid (1). During this process, vaccenic acid (tl 1-18 1) is formed. This acid can be converted to CLA in all organisms that possess A9-desaturase (2). 

Elaidic acid (18 1 9t) is easily made from oleic acid (Section 10.9) but is very rare among natural acids. Vaccenic acid (18 1 Ilf), present in small quantities in depot and milk fats of ruminant animals is probably produced from dietary linoleic acid by biohydrogenation occurring in the rumen. The cis isomer is a major bacterial fatty acid (Section 3.8). 

It has been estimated that the daily intake of trans acids (in West Germany) is 4.5 6.4 g per day. About 35-45% of this comes from ruminant fats (biohydrogenation) and the balance from partially hydrogenated fat. Some of the trans acids are deposited in the tissues which may contain about 2% (1.0-4.3%) of these acids. Another study concludes that such acids are harmless if accompanied by an adequate dietary intake of linoleic acid. 

Bessa, R. J. B., Santos-Silva, J., Ribeiro, J. M. R., Portugal, A. V. (2000). Reticulo-rumen biohydrogenation and the enrichment of ruminant edible products with linoleic acid conjugated isomers. Livestock Production Science, 63, 201-211. 

In the mid-1900s there was increased interest in the process of biohydrogenation, which occurs in the rumen, and is responsible for the formation of both the trans monoenoic acids and the conjugated linoleic acids found in the rumen and milk. 

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