Fatty acid biohydrogenation reactions

Garton etal, (10) demonstrated that rumen microbial suspensions could hydrolyze triglycerides. It was later established that virtually any ester link between fatty acid and glycerol was subject to hydrolytic cleavage by rumen organisms (7 7). As aconsequence of the activity of the lipolytic enzymes, high levels of free fatty acids are produced in the rumen. The unsaturated fatty acids are substrates in biohydrogenation reactions. 

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). 

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... 

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). 

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