Isolation and characterization of N-acetyl-S-[2-carboxy-1-(1H-imidazol-4-yl) ethyl]-L-cysteine, a new metabolite of histidine, from normal human urine and its formation from S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteine

N-Acetyl-S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteine (I), a new imidazole compound with a sulfur-containing side chain, was isolated from normal human urine by ion-exchange column chromatography, and characterized by physicochemical analyses involving ¹H-NMR spectrometry, mass spectrometry and high-voltage paper electrophoresis as well as chemical synthesis. Approximately five milligrams of crystals of the compound were obtained from 450 litres of the urine. Compound I was synthesized by the addition of N-acetyl-L-cysteine to urocanic acid. The compound was also formed by incubation of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteine (II) with acetyl-CoA in the use of rat kidney or liver homogenate as an enzyme source in a Tris buffer at pH 7.4. Rat brain and spleen homogenates were the less or no effective preparations as the enzyme source. On the other hand, little N-acetylation of a diastereomer of compound II occurred in enzymatic reactions with rat tissue homogenates. Compound I was degraded to compound II by rat kidney or liver homogenate. These results suggest that compound I is a new N-acetylated metabolite of compound II, a compound previously found in human urine, and that the acetylating enzyme recognizes stereoisomerism of asymmetric carbon atoms on the molecule of compound II. These findings support an alternative pathway of L-histidine catabolism initiated by the adduction of glutathione and/or cysteine to urocanic acid, the first catabolite of histidine.