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A Study of Formate Production and Oxidation in Leaf Peroxisomes during Photorespiration

Botany School, Department of Botany, University of Cambridge, Cambridge, United Kingdom

When glycolate was metabolized in peroxisomes isolated from leaves of spinach beet (Beta vulgaris L., var. vulgaris) formate was produced. Although the reaction mixture contained glutamate to facilitate conversion of glycolate to glycine, the rate at which H₂O₂ became "available" during the oxidation of [1-¹⁴C]glycolate was sufficient to account for the breakdown of the intermediate [1-¹⁴C]glyoxylate to formate (C₁ unit) and ¹⁴CO₂. Under aerobic conditions formate production closely paralleled ¹⁴CO₂ release from [1-¹⁴C]glycolate which was optimal between pH 8.0 and pH 9.0 and was increased 3-fold when the temperature was raised from 25 to 35 C, or when the rate of H₂O₂ production was increased artificially by addition of an active preparation of fungal glucose oxidase.

When [¹⁴C]formate was added to these preparations it was oxidized directly to ¹⁴CO₂ by the peroxidatic action of peroxisomal catalase; however, the breakdown of formate was slow relative to the rate of formate production. For example, when [¹⁴C]formate was generated from [2-¹⁴C]glycolate it was not readily oxidized to ¹⁴CO₂ in these organelles. Because the activity of formate-NAD⁺ dehydrogenase in cell-free leaf extracts was low compared with that of formyl tetrahydrofolate synthetase it is suggested that most of the formate produced during glycolate oxidation could be metabolized via the one carbon pool and not oxidized directly to CO₂.

At 25 C the rate of release of ¹⁴CO₂ from [2-¹⁴C]glycolate in leaf discs was 40 to 50% of the rate from [1-¹⁴C]glycolate. Isonicotinyl hydrazide inhibited ¹⁴CO₂ release from both [1-¹⁴C]- and [2-¹⁴C]glycolate; but this inhibitor was more effective in blocking ¹⁴CO₂ release from [2-¹⁴C]glycolate. It is argued that the oxidation of the methylene carbon group of glycolate does not occur as a direct consequence of formate (C₁ unit) breakdown, but is a product of the further metabolism of formate and glycine, possibly, via serine.

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