Rates and efficiencies of reactions of ruminal biohydrogenation of linoleic acid according to pH and polyunsaturated fatty acids concentrations.

Data from a previous study about the effects of pH and of linolenic acid (C18:3n-3) and linoleic acid (C18:2n-6) concentrations on C18:2n-6 biohydrogenation in ruminal cultures were used to calculate the rates and efficiencies of the three reactions of C18:2n-6 biohydrogenation (isomerisation of C18:2n-6 to CLA; reduction of CLA to trans-octadecenoic acids; reduction of trans-octadecenoic acids to stearic acid). First, low pH was confirmed to inhibit isomerisation and was shown to inhibit the second reduction, leading to an accumulation of vaccenic acid. This later effect had only been observed in some in vivo studies using high concentrate diets, because in in vitro experiments, the very low pH frequently used depresses isomerisation which consequently generates very low amount of substrates for reductions whose variations become difficult to ascertain. Second, C18:2n-6 at high concentration was confirmed to saturate its own isomerisation and the increase of CLA production due to high initial C18:2n-6 was shown to inhibit the two subsequent reductions. Third, C18:3n-3 at high concentrations was confirmed to inhibit C18:2n-6 isomerisation. Moreover, the second reduction was shown to be saturated, probably by all trans-octadecenoic acids intermediates of C18:2n-6 and C18:3n-3 biohydrogenation, leading to an accumulation of trans-octadecenoic acids, especially vaccenic acid. This fatty acid is partly desaturated into CLA in the mammary gland, which explains the synergy between C18:2n-6 and C18:3n-3 for milk CLA noticed by others in vivo. This approach helped explain the actions of pH and of C18:2n-6 and C18:3n-3 concentrations on C18:2n-6 biohydrogenation and allows some explanations about differences noticed between studies.