Other CLA Isomers

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). 

---

A broad overview of the biological effects of CLA is presented elsewhere in this volume (Chapter 17), so the emphasis in the following section will be two-fold. Firstly, the biology of trans-10, cis-12 CLA in the dairy cow will be summarized because under certain dietary conditions, production of this isomer in the rumen can profoundly affect milk fat synthesis. Secondly, the biological effects of RA when supplied as a natural component of the diet will be reviewed because this CLA isomer represents a functional component of milk fat that has potential health benefits. Although other CLA isomers are present in milk fat, they are present at concentrations much too low to have a significant effect. 

Yurawecz. Synthesis and Isolation of trans-7,cis-9 Octadecadienoic Acid and Other CLA Isomers by Base Conjugation of Partially Hydrogenated y-Linolenic Acid, 38 ... 

Different groups (8,11,34) have smdied the CLA composition of cheese by GC-MS, often in combination with several other complementary analytical techniques, hi one study (34), CLA fractions, isolated by RP-HPLC, were examined as the methyl esters by GC-MS on a column of medium polarity (Supelcowax 10) with the mass spectrometer in Cl mode using isobutane as reactant gas. Seven peaks comprising nine components (all possible geometrical isomers of 9,11- and 10,12-18 2 and llc,13c-18 2) were identified. The % tl 0t, 2t peak was the most abundant, but this was a result of using harsh acid conditions for methylation, which increased the levels of trans,trans isomers at the expense of the other CLA isomers. Incidentally, there have been many subsequent studies on developing the most suitable conditions for methylating CLA these have been reviewed in some detail (24,25,76,77). 

Other rumen isomerases produce small amounts of other CLA isomers. The amount and type depend on factors such as diet and rumen conditions. For example, increased intake of linoleic acid increased the proportion of RA (6), whereas low-fiber diets increased the proportion of trans- 0, cm-12-CLA (5). 

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. 

Tonalin 22.6% trans- 0,cis- 2, 23,6% cis-, trans- 3, 17.6% cis-9,trans-, 16.6% trans-S,cis- 0, 7,7% trans-9,trans- and tram- 0jrans- 2, and 11,9% other CLA isomers. FFA free fatty acid HDL high density lipoprotein LDL low density lipoprotein NEFA nonesterified fatty acid TAG triacylglycerol VLDL very low density lipoprotein. 

Delmonte, P., Roach, J.A.G., Mossoba, M.M., Morehouse, K.M., Lehmann, L., and Yurawecz, M.P., Synthesis and isolation of franj-7,cw-9-octadecadienoic acid and other CLA isomers by base conjugation of partially hydrogenated gamma-linolenic acid. Lipids, 38, 579-583, 2003. 

---