Unsaturated fatty acids rumen biohydrogenation

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. 

Several other procedures have been developed to protect unsaturated fatty acids from ruminal biohydrogenation. Of these, only the amide derivative has extensive research documentation (Jenkins, 1998, 1999), but has not been applied commercially. Often, calcium soaps of palm oil or canola fatty acids are referred to as protected. These are not protected from ruminal biohydrogenation (Table 2.2), but rather are ruminally inert with regard to their effects on the rumen microbial population. 

The term conjugated linoleic add (CLA) refers to a mixture of positional and geometric isomers of linoleic add with a conjugated double bond system milk fat can contain over 20 different isomers of CLA. CLA isomers are produced as transient intermediates in the rumen biohydrogenation of unsaturated fatty acids consumed in the diet. However, cis-9, trans-11 CLA, known as rumenic acid (RA), is the predominant isomer (up to 90% of total) because it is produced mainly by endogenous synthesis from vaccenic acid (VA). VA is typically the major biohydrogenation intermediate produced in the rumen and it is converted to RA by A9-desaturase in the mammary gland and other tissues. 

Ruminants are equal to or superior to nonruminants in their ability to digest saturated fatty acids, but unsaturated fatty acids fed conventionally have lower digestibility due to rumen biohydrogenation. Unsaturated fatty acids, fed in protected form, are digested equally in ruminants or nonruminants. Moderate levels of added fat (up to 3% of feed dry matter) are 85% truly digestible (52). 

Because the presence of CLA in the human diet is reliant on ruminant products, this chapter first addresses the synthesis of CLA in ruminants. The presence of CLA in ruminant milk and meat is related to rumen fermentation and its synthesis by microorganisms through the process of biohydrogenation (BH) of dietary unsaturated fatty acids. Thus, the effect of diet and processes within the rumen is reviewed. The role of endogenous synthesis of CLA in mammalian tissues has been discovered, and this will be discussed also first as it contributes to the occurrence of CLA in ruminant products and second the significance of endogenous synthesis as a source of CLA in humans and other species. 

Harfoot, C.G., Noble, R.C., and Moore, J.H. 1973. Food particles as a site for biohydrogenation of unsaturated fatty acids in the rumen. Biochem. J. 132, 829-832. 

Produced During Microbial Biohydrogenation of Unsaturated Fatty Acids from Canola and Soya Bean Oil in the Rumen of Lactating Cows,/. Anim. Physiol A. Anim. Nutr. 86 422-432 (2002). 

In contrast to cis-9, trans-11 and trans-1, cis-9, the other isomers of CLA found in milk and body fat of ruminants appear to originate exclusively from rumen output. These are detected in rumen fluid (61) and duodenal fluid (39), and estimates of duodenal flow indicate that rumen output of these minor cis/trans, cis-cis, and trans-trans CLA isomers is greater than the trace amounts secreted in milk fat (39). The common theme to endogenously synthesized CLA isomers is A -desaturase and the cis-9 double bond that is added to trans-1 and trans- 1 monoenes. In contrast, there has been no demonstration that other mammalian desaturases act in a manner analogous to A -desaturase to synthesize CLA endogenously from mono-unsaturated fatty acids. Thus, these other CLA isomers found in trace levels in ruminant fat are of rumen origin and must represent intermediates in the ruminal biohydrogenation of linoleic and linolenic acids. 

Keeney M (1970) Lipid metabohsm in the rumen. In PhUlipson AT (ed) Physiology of digestion and metabolism in the ruminant. Oriel Press, Newcastle, pp 489-503 Kemp P, Lander DJ (1984) Hydrogenation in vitro of a-hnolenic add to stearic acid by mixed cultures of pure strains of rumen bacteria. J Gen Microbiol 130 527-533 Kemp P, White RW, Lander DJ (1975) The hydrogenation of unsaturated fatty adds by five bacterial isolates from the sheep rumen, including a new spedes. J Gen Microbiol 90 100-114 Kepler CR, Tove SB (1967) Biohydrogenation of unsaturated fatty acids III. Purification trans-isomerase from Butyrivibrio fibrisolvens. J Biol Chem 242 5686-5692... 

Maia MRG, Chaudary LC, Bestwick CS, Richardson AJ, McKain N, Larson TR, Graham lA, Wallace RJ (2010) Toxicity of unsaturated fatty acids to the biohydrogenating ruminal bacterium, Butyrivibrio flbrisolvens. BMC Microbiol 10 52. doi 10.1186/1471-2180-10-52 Makkar HPS, Becker K (1997) Degradation of quUlaja saponins by mixed culture of rumen microbes. Lett Appl Microbiol 25 243-245... 

Polan CE, McNeill JJ, Tove SB (1964) Biohydrogenation of unsaturated fatty acids by rumen bacteria. J Bacteriol 88 1056-1064... 

Figure 2. Biohydrogenation pathways of unsaturated fatty acids in the rumen and mammary gland.

Biohydrogenation of unsaturated fatty acids takes place in rumen microorganisms... 

The presence of Qg tram-fatty acids in milk fat is the result of incomplete biohydrogenation of the unsaturated dietary lipids in the rumen. These fatty acids have attracted attention because of their adverse nutritional affects. Clinical trials have shown that traus-octadecenoic acids, relative to the cis isomer, can increase the LDL-cholesterol and decrease the HDL-cholesterol, thus, producing an unfavourable affect on the LDL HDL ratio (Mensink and Katan, 1993). 

The degree of rumen-mediated fatty acid modification varies from species to species. For example, the biohydrogenation of dietary unsaturates is greater in sheep than in cattle, and thus mutton tallow contains 5% to 10% more stearic acid, and a correspondingly lower amount of oleic acid, than beef tallow. Table 1 illustrates this trend, although it is somewhat obscured by the necessarily wide ranges of values reported. 

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