Polyunsaturated fatty acid biohydrogenation

Hay and Morrison (1970) identified the monoenoic positional and geometric isomers in milk fat and determined the amounts of each total acid class and percentage of trans isomers. The geometric and positional isomers of the monoenes are primarily the result of biohydrogenation of polyunsaturated fatty acids in the rumen. Stearate is also produced, and cis-9-18 l accounts for most of the monoenes. The several positional isomers in trans 16 1 and 18 1 are due to the positional isomerization of double bonds which accompanies elaidinization. 

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... 

Although polyunsaturated fatty acids are biohydrogenated to stearic acid (Cl8 0) in the rumen, they are desaturated to oleic acid in the small intestine, mucosa adipose tissue, and mammary gland. Thus the ratio of C18 l to C18 0 is greater in the fatty tissue and the milk lipids than in plasma triacylglycerols. Ruminant milk also contains 40-50% C4 0-C14 0 fatty acids by weight synthesized in the mammary glands from acetate (52). 

Fats not protected for escape or bypass will be altered if fed to ruminants. Within limits, triacylglycerols are hydrolyzed, and the polyunsaturated fatty acids are biohydrogenated by rumen bacteria enzymes. 

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). 

Rumen Biohydrogenation of n-3 Polyunsaturated Fatty Acids and their Effects on Microbial Efficiency and Nutrient DigestibUity in Sheep, J. Agric. Sci., Camh. 135 419— 428 (2000). 

In food, CLA is found in milk fat, the tissue fat of ruminant animals, and products derived from them, although there are exceptions. About 90% of the CLA is represented by the cis-9, trani-11-isomer or rumenic acid (RA) (4). This isomer is produced in the rumen by the action of a linoleic acid isomerase on dietary linoleic acid as a first step in the biohydrogenation process. Further hydrogenation produces trans- - %. or vaccenic acid (VA), the predominant trans monounsaturat-ed fatty acid of milk and animal tissue fat. The major polyunsaturated fatty acids of pasture, a-and y-linolenic acid, cannot be converted to RA, but they do produce VA. A proportion of RA and VA escape further hydrogenation in the rumen and after absorption pass via the circulatory system to adipose tissue and the mammary gland where A -desaturases convert VA to RA. About 70% of RA in milk fat is produced by this pathway (5). 

The production of low-fat milk has traditionally been explained by a lack of coarse roughage in the diet resulting in lower acetate and higher propionate production in the rumen. This ruminal volatile fatty acid profile reduces mammary supply of acetate for milk fat synthesis and/or increases body fat deposition. More recently, intermediaries in the biohydrogenation of polyunsaturated fatty acids in the rumen have been proposed as the main mechanism controlling milk fat reduction. 

Experiments with specifically labeled synthetic fatty acids have given conclusive evidence that no biohydrogenation of mono- and polyunsaturated fatty acids occurs in the mammalian liver (Stoffel et al. 1964). Feeding experiments with doubly-labeled linoleic, arachidonic and stearic acid disprove the often suggested hypothesis of a partial degradation and elongation of the carboxyl end of fatty acids. From these experiments, the conclusion has been drawn that a fatty acid molecule is completely degraded on contact with the 8-oxidation multienzyme of the mitochondrion. 

Fatty acid saturation metabolism, so called biohydrogenation, is considered to be a detoxif ting metabolism of gut bacteria to transform toxic-free polyunsaturated fatty acids to less toxic-free saturated fatty acids. We have revealed the complex metabolic... 

Because of animal biohydrogenation, the content of polyunsaturated acids in milk is low, currently reported at about 5% (Smith et al 1978), and is associated mostly with the phospholipids. While quantitatively unimportant, these acids are the most susceptible targets of oxidation and provide the essential fatty acids (EFA), mostly cis, cis-9, 12-18 2. 

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