Identification of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-3-mercaptopyruvic acid with a metabolic intermediate between S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteine and S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-3-mercaptolactic acid

S-[2-Carboxy-1-(1H-imidazol-4-yl)ethyl]-3-mercaptopyruvic acid (I) was chemically synthesized in 15% yield by incubating a reaction mixture of trans-urocanic acid and 3-fold excess of 3-mercaptopyruvic acid at 45°C or 6 days. The synthesized compound was characterized by fast-atom-bombardment mass spectrometry and high-voltage paper electrophoresis. Compound I was identified with a product of an enzymatic reaction of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-L-cysteine (II) with rat liver homogenate in a phosphate buffer, pH 7.4. Compound I was degraded to S-[2-carboxy-1(1H-imidazol-4-yl)ethyl]-3-mercaptolactic acid (III), a compound previously found in human urine, by incubation with rat liver homogenate. From these results, we suggest that compound I is a metabolic intermediate for the formation of compound III from compound II. The present pathway follows a formation of compound II from S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]gluthathione, a proposed metabolite of L-histidine.