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

Abstract N- Acetyl -S-[2- carboxy -1-(1 H- 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 1 H-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-(1 H- 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.