The formation and metabolism of phenyl-substituted fatty acids in the ruminant

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Abstract
3-Phenylpropionic acid was isolated from sheep rumen liquor and characterized by gas-liquid chromatography, infrared spectroscopy, nuclear magnetic resonance and mass spectrography. Smaller amounts of phenylacetic acid and benzoic acid were identified in the same fluid by gas-liquid chromatography. The concentration of all three phenyl-substituted fatty acids was measured in the blood, rumen liquor, saliva and urine of the sheep. Uniformly Cl4-labelled L-phenylalanine is converted into radioactive phenylacetic acid by rumen micro-organisms. In the artificial rumen the yield is 30%. In the same system uniformly C14-labelled L-tyrosine is converted into radioactive fatty acid with a yield of 12-15%. 3-Phenylpropionic acid is the predominant product but p-hydroxyphenyl-acetic acid and 3-(p-hydroxyphenyl)propionic acid can also be detected. Uniformly C14-labelled 3-(p-hydroxyphenyl)-propionic acid is converted into C14-labelled 3-phenylpropionic acid by rumen micro-organisms, whereas there is much less conversion of uniformly C14-labelled p-hydroxyphenylacetic acid into C14-labelled phenylacetic acid. In the artificial rumen small yields of C14-labelled indol-3-ylacetic acid are obtained from tryptophan (indol-3[image]-yl[3-C14]alanine). Uniformly C14-labelled algal protein introduced into the intact rumen is converted into radioactive volatile fatty acids, including phenylacetic acid and a little 3-phenylpropionic acid. The rate of disappearance of C14-labelled 3-phenylpropionic acid and phenylacetic acid from the intact rumen was measured. On introducing uniformly C14-labelled L-phenvlalanine or uniformly C14-labelled phenylacetic acid into the rumen, radioactive phenylacetic acid is excreted in the urine. Very little is excreted on injecting uniformly c14-labelled L-phenylalanine into the circulation. When uniformly C14-labelled L-tyrosine or uniformly C14-labelled 3-phenylpropionic acid is added to the rumen, radioactive benzoic acid is excreted in the urine. None is found in the urine on injecting uniformly C14-labelled L-tyrosine into the circulation. Slices of liver and kidney from the sheep metabolize uniformly C14-labelled 3-phenylpho-pionic acid forming radioactive benzoic acid and CO2. No metabolism of uniformly C14-labelled phenylacetic acid could be detected. The significance of these results is discussed. It is concluded that dietary phenylalanine is the main precursor of rumen phenylacetic acid and one of the precursors of this acid in the urine. Dietary tyrosine is a minor precursor of rumen 3-phenylpropionic acid and one of the precursors of urinary benzoic acid.