The metabolism of glyoxylate by human- and rat-liver mitochondria

Abstract

The metabolism of [1-C14] glyoxylate by rat and human-liver mitochondria suspended in 0-25 M-sucrose (pH 7.4) has been studied in the presence of an equimolar amount of L-glutamate. Decarboxylation, oxidation to oxalate and animation to glycine account for virtually all of the glyoxylate that is metabolized. Glyoxylate-decarboxylase activity and cytochrome-oxidase activity were also studied. Human-liver mitochondria had a lower cytochrome-oxidase activity than rat-liver mitochondria, but no other appreciable species differences were detected. Non-enzymic reactions did not contribute significantly to glyoxylate degradation under our experimental conditions. L-Glutamate was needed for decarboxylation and formation of glycine but not for production of oxalate. In the presence of equimolar amounts of glyoxylate and L-glutamate, about 40% of the evolved CO2 was derived from the C-1 atom of glyoxylate, and the remainder was formed from the C-l atom of L-ghltamate. De-carboxylation occurred if L-ghitamate was replaced by 2-oxo-glutarate (but not by other tricarboxylic acid-cycle intermediated or related compounds), and the glyoxylate-decarboxylase activity of the mitochondria was enhanced by this substitution. 2-[5-C14 ] Oxoglutarate did not yield [C14] CO2- Phosphate ions inhibited, but Mg2+ ions, adenosine diphosphate, adenosine triphosphate and Ca2+ ions did not influence the glyoxylate-decarboxylase activity of the mitochondria. Malonate inhibited the decarboxylation of [1-C14]-glyoxylate competitively but did not influence oxalate or glycine formation. The metabolism of [1-C14]glyoxylate was unaffected by fluoroacetate or by 3 cycles of freezing and thawing of the mitochondria. Anaerobiosis depressed decarboxylation, glycine formation and oxalate formation from [1-C14]glyoxylate by approximately 20, 10 and 40% respectively. Some glyoxylate-decarboxylase activity was demonstrated when glycollate was substituted for, glyoxylate. We have been unable to demonstrate that N-formyl-L-glutamate is an intermediate in glyoxvlate metabolism under our experimental conditions. When [2-C14]glyoxylate was used, the proportion of the C-2 atom of glyoxylate that was recovered as formate or carbon dioxide was considerably less than the proportion of the C-l atom of glyoxylate incorporated into CO2 in the experiments with [1-C14]giyoxylate. It appears that glyoxylate and L-glutamate (or a derivative thereof, which we suggest is 2-oxo-glutarate) are de-carboxylated synergistically in this system. The details of the mechanism by which this is effected have not, however, been elucidated.