Enzymic synthesis of oxalic acid in Oxalis pes-caprae

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Abstract
The enzymic reactions involved in the synthesis of oxalic acid and the metabolism of related compounds have been studied with cell-free extracts of O. pes-caprae. The extracts contain isocitrate-lyase activity, which accounts for the biosynthesis of glyoxylic acid, the immediate precursor of oxalic acid in Oxalis. In Oxalis, the combined actions of isocitrate dehydrogenase and glyoxylate reductase result in the synthesis of glycollic acid. The former provides reduced nicotinamide-adenine dinucleotide phosphate necessary for the reduction of glyoxylic acid. Both enzymes have been demonstrated in Oxalis extracts. Glycollic acid and glyoxylic acid were oxidized by a flavin-linked enzyme. This enzyme differed from glycollate oxidase in the relative rates of oxidation of the 2 substrates and the lack of inhibition of oxalic acid on the oxidation of glyoxylic acid. The lack of inhibition observed may account for the preferential oxidation of glyoxylic acid to oxalic acid rather than a diversion to glycine synthesis in Oxalis. Extracts catalyse a reduced nicotinamide-adenine dinucleotide phosphate- and coenzyme A-dependent reduction of oxalic acid. It is probable that oxalyl-coenzyme A is formed as an intermediate in this reduction.