Ruminal Biohydrogenation
Ruminal biohydrogenation was first described in 1951 (Reiser, 1951); the well-known mechanism for biohydrogenation of dietary linoleic acid was worked out by Tove and colleagues at North Carolina State (Kepler et al., 1966). Biohydrogenation of 18:3 n-3 fatty acids, naturally abundant in fresh forages, is more complex (Harfoot and Hazlewood, 1997), and leads to formation of numerous isomers, many poorly characterized.
The first step in biohydrogenation of linoleic acid is mediated by delta cis-12, delta trans-11 isomerase, with a CLA isomer as the product. This isomer, cis-9, trans-11 18:2, has been given the trivial name rumenic acid, and is common in milk fat; it was found by Pariza and associates to be strongly anticarcinogenic (Pariza, 1999). The cis-9 bond of this intermediate is quickly hydrogenated in the rumen to form trans-11 18:1 (vaccenic acid) and most is further hydrogenated to form stearic acid. Initially it was believed that all CLA in milk was derived directly from ruminal synthesis; more recent thinking is that as much as 3/4 of milk CLA may be synthesized endogenously by mammary )-9 desaturation of vaccenic acid (Corl et al., 2000).
As noted above, most of the 400 or so fatty acids that have been identified in milk fat are products of ruminal microbial metabolism. Though identified, most have not been characterized as to pathways of metabolic origin, dietary and ruminal conditions that may influence amounts synthesized, or possible effects on milk fat synthesis and function, or other biological effects. Our experience with the tremendous amount of research that has emanated from the discovery of biological effects of CLA suggests that there may be other isomers with equally important biological effects and implications for human health. All that is needed are scientists with the chemical knowledge and creative curiosity of Stuart Patton that we are celebrating today.