Chirality of urinary metabolites in inherited metabolic disorders.

An important group of inherited metabolic disorders in man produce abnormal excretion of organic acids in the urine. Diagnosis is usually based on identification of the abnormal metabolites, working back from there to characterise the defect at enzymatic level. The chirality of these metabolites may be significant in that the different enantiomers of a substance usually have different metabolic origins. Thus, knowledge of the chirality of a metabolite aids in the understanding of the mechanism of the disorder. The chirality of a number of urinary metabolites in inherited metabolic disease was examined using gas chromatography-mass spectrometry, most of the analyses being performed using single ion monitoring. This analytical method requires the use of chiral reference compounds of known configuration. Chiral lactic and glyceric acids are available commercially, but a range of chiral beta-hydroxy acids were prepared by the reduction of the corresponding beta-ketocarboxylic acid esters with fermenting baker's yeast. After hydrolysis of the esters, separation of the enantiomers is based on their reaction with a suitable chiral reagent to form a volatile mixture of the diastereoisomers which are resolved by gas-liquid chromatography using a capillary column with a non-chiral stationary phase. Information from the analysis of the chiral standards was used to assign the absolute configuration of the following urinary metabolites; lactic acid in an unusual case of lactic aciduria, glyceric acid in the glyceric acidurias, beta-hydroxybutyrate in ketonuria, beta-hydroxyvalerate in propionic acidaemia, 2-methyl beta-hydroxybutyrate in beta-ketothiolase deficiency and beta-hydroxyadipic acid in hydroxydicarboxylic aciduria. The biochemical significance of the chirality of each of these metabolites is discussed. The assigning of configuration to urinary beta-hydroxyvalerate in propionic acidaemia, 2-methyl beta-hydroxybutyrate in beta-ketothiolase deficiency and beta-hydroxyadipic acid in hydroxydicarboxylic aciduria represents original work and has helped to elucidate the metabolic origins of these compounds.