Hippuric acid

Hippuric acid (Gr. hippos, horse, ouron, urine) is an organic acid found in the urine of horses and other herbivores. High concentrations of hippuric acid can also indicate a toluene intoxication.

When many aromatic compounds such as benzoic acid and toluene are taken internally, they are converted to hippuric acid by reaction with the amino acid glycine. Justus von Liebig in 1829 showed that hippuric acid differed from benzoic acid, and in 1839 determined its constitution, while in 1873 Victor Dessaignes synthesized it by the action of benzoyl chloride on the zinc salt of glycine. It is also formed by heating benzoic anhydride with glycine, and by heating benzamide with monochloroacetic acid.

Hippuric acid crystallizes in rhombic prisms which are readily soluble in hot water, melt at 187 °C and decompose at about 240 °C. It is readily hydrolysed by hot caustic alkalis to benzoic acid and glycine. Nitrous acid converts it into benzoyl glycolic acid, C6H5C(=O)OCH2CO2H. Its ethyl ester reacts with hydrazine to form hippuryl hydrazine, C6H5CONHCH2CONHNH2, which was used by Theodor Curtius for the preparation of azoimide.

Uses in medicine
Para-amino-hippuric acid (PAH), a derivative of Hippuric Acid, is a compound useful in medical tests involving the kidney. PAH is completely filtered from plasma at the glomerulus and not reabsorbed by the tubules, in a manner identical to inulin. PAH is different from inulin in that the fraction of PAH that bypasses the glomerulus and enters the nephron's tubular cells (via the Peritubular capillaries) is completely secreted. Renal Clearance of PAH is thus useful in calculation of renal plasma flow (RPF), which empirically is (1-Hematocrit) times renal blood flow. Of note, the clearance of PAH is reflective only of RPF to portions of the kidney that deal with urine formation, and thus underestimates actually RPF by about 10%.