Determination of chiral catabolites from S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]glutathione, a proposed metabolite of l-histidine, by capillary electrophoresis

Abstract A new method for simultaneous determination of two diastereomers in each of S -[2-carboxy-1-(1 H -imidazol-4-yl)ethyl]- l -cysteine ( I ) and N -{ S -[2-carboxy-1-(1 H -imidazol-4-yl)ethyl]- l -cysteinyl}glycine ( II ) was developed by electrophoresis using a neutral coated capillary with a separation buffer, pH 6.00, containing 80 m M hydroxypropyl-β-cyclodextrin at a field strength of 500 V cm −1 at 20°C. This method was applied to establishment of a catabolic pathway from S -[2-carboxy-1-(1 H -imidazol-4-yl)ethyl]glutathione ( III ) to compound I . Incubation of either of compound II diastereomers as an enzyme substrate with rat kidney homogenate in a phosphate buffer, pH 7.4, resulted in a formation of compound I only having correspondent configurations on asymmetric carbon atoms of its molecule with those of the substrate, i.e. no occurrence of isomerization in the catabolism. Additionally, little difference in action as the substrate between two diastereomers of compound II was found. When an equimolar mixture of two diastereomers of compound III was allowed to react with the homogenate in the presence of glycylglycine, two diastereomers of compound II were formed in the same yield with each other and then these were catabolized gradually to both isomers of compound I . These results suggest that compound II is a metabolic intermediate for the formation of compound I from compound III , and that little variation in reactivities of two diastereomers of compound III as well as compound II with enzymes is given by the difference in stereoisomerism of asymmetric carbon atoms on their molecules.