Hippuric Acid

Hippuric acid, benzoylglycine, C₆H₅.CO.NH.CH₂.COOH. It is contained in large quantities in the urine of herbivores. A person normally excretes an average of 0.7 g of hippuric acid per day in the urine; after the introduction of benzoic acid, toluene, bitter almond oil and other aromatic compounds, hippuric acid is excreted in significant quantities. Hippuric acid consists of prismatic crystals of the rhombic system, dissolves at 0° in 600 parts of water, more easily in hot water and ether, easily in alcohol; melts at 187° with decomposition. With silver, copper and lead it gives sparingly soluble salts, with iron-insoluble salt. The formation of hippuric acid after the introduction of benzoic acid, discovered in 1828 by Keller and Ure, was the first synthetically established process in the animal organism. The synthesis of hippuric acid occurs in the kidneys; in rabbits-also in other organs (liver); in these organs the enzyme histozyme (see) has been found, which splits hippuric acid into benzoic acid and glycine and probably participates in the synthetic formation of hippuric acid in the body. Along with hippuric acid, phenaceturic acid is found in the urine of herbivores-a conjugate of phenylacetic acid and glycine; in humans, phenylacetic acid is excreted as a conjugate with glutamine. Benzoic and phenylacetic acids are formed during the putrefaction of protein in the intestines, and the physiological significance of the formation of hippuric acid and conjugates of phenylacetic acid lies in the conversion of these poisonous aromatic acids into less harmful compounds.