Hippuric acid is an acyl glycine formed from the conjugation of benzoic acid with glycine. Acyl glycines are produced through the action of glycine N-acyltransferase (EC 2.3.1.13) which is an enzyme that catalyzes the chemical reaction: acyl-CoA + glycine < -- > CoA + N-acylglycine. Hippuric acid is a normal component of urine and is typically increased with increased consumption of phenolic compounds (tea, wine, fruit juices). These phenols are converted into benzoic acid which is then converted into hippuric acid and excreted in the urine. Hippuric acid is the most frequently used biomarker in the biological monitoring of occupational exposure to toluene. This product of solvent biotransformation may be also found in the urine of individuals who have not been exposed to the solvent. A smaller fraction of the absorbed toluene is oxidized into aromatic compounds including ortho-cresol, which is not found in the urine of nonexposed individuals in a significant amount. The concentration of hippuric acid in the urine of individuals exposed to a low toluene concentration does not differ from that of individuals not exposed to the solvent. This has led to the conclusion that hippuric acid should not be utilized in the biological monitoring of occupational exposure to low levels of toluene in the air. Protein-bound organic acids such as hippuric acid are markedly accumulated in uremic plasma and produce defective protein binding of drugs. Hippuric acid has been identified as a uremic toxin according to the European Uremic Toxin Working Group. Hippuric acid is also found to be associated with phenylketonuria, propionic acidemia, and tyrosinemia I, which are inborn errors of metabolism. Hippuric acid is an endogenous phenolic acid metabolite detected after the consumption of whole grain.