Opklaring van 'n kataboliese weg vir metielsuksiensuur in die mens

Abstract

ELUCIDATION OF A CATABOLIC PA111WAY FOR METHYLSUCCINIC ACID IN MAN. (Original title: Opklaring van 'n kataboliese weg vir metielsuksiensuur in die mens). Methylsuccinic acid (3-carboxybutyric acid) is an induced metabolite excreted in high concentrations in the urine of patients with isovaleric acidemia, propionic acidemia, glutaric aciduria type II and ethylmalonic adipic aciduria. This substance is produced by w-oxidation and carboxylation respectively. It is not known whether methylsuccinic acid can be catabolized although the presence of methylfumaric acid (mesaconic acid) in isovaleric acidemia has been described which indicates the possible degradation of methylsuccinic acid. The presence of mesaconic acid in glutaric aciduria type II and ethylmalonic adipic aciduria has however not yet been documented. By identifying the isomers of methylsuccinic acid produced by the different anabolic pathways an indication could be obtained whether methylsuccinic acid will be catabolized, since o-methylsuccinic acid inhibits succinate dehydrogenase while L-methylsuccinic acid acts as a substrate for this enzyme. Gaschromatographic-mass spectrometric (GC-MS) analyses were conducted on the urine of patients with propionic acidemia, isovaleric acidemia, glutaric aciduria type II and ethylmalonic adipic aciduria. Firstly the presence of the isomers of methylsuccinic acid could by determined by these analyses. Secondly and more importantly a catabolic pathway for methylsuccinic acid could be ascertained by employing a biochemical analytical approach. In this way the induced metabolites were predicted on the basis of the knowledge of the mechanisms of existing metabolic pathways. The suggested pathway was investigated by further analyses of the involved substances in the urine of patients with the inborn metabolic disorders mentioned, as well as by studying the products of in vitro enzyme reactions.

Both isomers of methylsuccinic acid were present in isovaleric acidemia as well as propionic acidemia. This indicates that the formation of methylsuccinic acid by w-oxidation does not occur stereospecifically. Although the isomers of methylsuccinic acid in the urine of patients with glutaric aciduria type II and ethylmalonic adipic aciduria could not be investigated completely it appears that at least the L-isomer is present in ethylmalonic adipic aciduria. This indicates that carboxylation of butyryl-CoA produces at least L-methylsuccinic acid. According to this study the L-methylsuccinic acid is converted to mesaconic acid by succinate dehydrogenase. Mesaconic acid is converted to 3-methylmalic acid by fumarase. This substance is dehydrogenated by malate dehydrogenase to 3-methyloxaloacetic acid. The latter could be catabolized by two possible pathways, namely (i) by pyruvate carboxylase to 2- ketobutyric acid and (ii) by condensation with propionyl-CoA of acetyl-CoA to give rise to 1,3-dimethylcitric acid and methylcitric acid respectively. Citramalic acid (2-methylmalic acid) is sometimes excreted in very high concentrations by propionic acidemia patients. It appears from this study that citramalic acid is not produced in the catabolism of methylsuccinic acid. It was therefore proposed that it originates through the condensation of lactyl-CoA with acetyl-CoA by 3-methylglutaryl-CoA synthase as the responsible enzyme for the reaction. The presence of 2,3-dimethylmalic acid in the urine of propionic acidemia patients supports this suggestion. The latter substance possibly arises through a condensation similar to the above mentioned with propionyl-CoA and pyruvyl-CoA or lactyl-CoA as the substrates. Methylsuccinic acid and its catabolic products may through inhibition of the Krebs cycle be associated with clinical-biochemical phenomena like hyperammonemia, hypoglycemia and ketoacidosis. Further studies could be conducted to fully elucidate and confirm the catabolism of methylsuccinic acid in man by stable isotope labeling analyses.