Biohydrogenation of 22:6n-3 by Butyrivibrio proteoclasticus P18

Background: Rumen microbes metabolize 22:6n-3. However, pathways of 22:6n-3 biohydrogenation and ruminal microbes involved in this process are not known. In this study, we examine the ability of the well-known rumen biohydrogenating bacteria, Butyrivibrio fibrisolvens D1 and Butyrivibrio proteoclasticus P18, to hydrogenate 22:6n-3. Results: Butyrivibrio fibrisolvens D1 failed to hydrogenate 22:6n-3 (0.5 to 32 mu g/mL) in growth medium containing autoclaved ruminal fluid that either had or had not been centrifuged. Growth of B. fibrisolvens was delayed at the higher 22:6n-3 concentrations; however, total volatile fatty acid production was not affected. Butyrivibrio proteoclasticus P18 hydrogenated 22:6n-3 in growth medium containing autoclaved ruminal fluid that either had or had not been centrifuged. Biohydrogenation only started when volatile fatty acid production or growth of B. proteoclasticus P18 had been initiated, which might suggest that growth or metabolic activity is a prerequisite for the metabolism of 22:6n-3. The amount of 22:6n-3 hydrogenated was quantitatively recovered in several intermediate products eluting on the gas chromatogram between 22:6n-3 and 22:0. Formation of neither 22:0 nor 22:6 conjugated fatty acids was observed during 22:6n-3 metabolism. Extensive metabolism was observed at lower initial concentrations of 22:6n-3 (5, 10 and 20 mu g/mL) whereas increasing concentrations of 22:6n-3 (40 and 80 mu g/mL) inhibited its metabolism. Stearic acid formation (18:0) from 18:2n-6 by B. proteoclasticus P18 was retarded, but not completely inhibited, in the presence of 22:6n-3 and this effect was dependent on 22:6n-3 concentration. Conclusions: For the first time, our study identified ruminal bacteria with the ability to hydrogenate 22:6n-3. The gradual appearance of intermediates indicates that biohydrogenation of 22:6n-3 by B. proteoclasticus P18 occurs by pathways of isomerization and hydrogenation resulting in a variety of unsaturated 22 carbon fatty acids. During the simultaneous presence of 18:2n-6 and 22:6n-3, B. proteoclasticus P18 initiated 22:6n-3 metabolism before converting 18:1 isomers into 18:0

Isomerization of Vaccenic Acid to cis and trans C18:1 Isomers During Biohydrogenation by Rumen Microbes

International audienceIn ruminants, cis and trans C18:1 isomers are intermediates of fatty acid transformations in the rumen and their relative amounts shape the nutritional quality of ruminant products. However, their exact synthetic pathways are unclear and their proportions change with the forage:concentrate ratio in ruminant diets. This study traced the metabolism of vaccenic acid, the main trans C18:1 isomer found in the rumen, through the incubation of labeled vaccenic acid with mixed ruminal microbes adapted to different diets. [1-13C]trans-11 C18:1 was added to in vitro cultures with ruminal fluids of sheep fed either a forage or a concentrate diet. 13C enrichment in fatty acids was analyzed by gas-chromatography-mass spectrometry after 0, 5 and 24 h of incubation. 13C enrichment was found in stearic acid and in all cis and trans C18:1 isomers. Amounts of 13C found in fatty acids showed that 95% of vaccenic acid was saturated to stearic acid after 5 h of incubation with the concentrate diet, against 78% with the forage diet. We conclude that most vaccenic acid is saturated to stearic acid, but some is isomerized to all cis and trans C18:1 isomers, with probably more isomerization in sheep fed a forage diet