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Comparison of the Effectiveness of Glycolic Acid and Glycine as Substrates for Photorespiration

^a Department of Biochemistry, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06504

Considerable evidence exists that the carboxyl-carbon atom of glycolic acid is the primary source of the CO₂ produced during photorespiration by leaves of many species of plants, including tobacco. Experiments were conducted to determine whether glyoxylate or glycine, both products of glycolic acid metabolism, is the more immediate precursor of photorespiratory CO₂.

Illuminated tobacco leaf disks were floated on 18 mM solutions of glycolate-1-¹⁴C or glycine-1-¹⁴C in CO₂-free air. The ¹⁴CO₂ released and the ¹⁴C content of several postulated intermediates were determined when the substrate solutions were provided alone or with one of the following: 9 mM -hydroxy-2-pyridine-methanesulfonic acid, an inhibitor of the oxidation of glycolate to glyoxylate; 9 mM isonicotinyl hydrazide, an inhibitor of the conversion of glycine to serine; or 18 mM nonradioactive glycine or glycolate with the other radioactive substrate.

Both inhibitors decreased the rate of photorespiration in tobacco leaf disks by the ¹⁴C-assay. The -hydroxy-2-pyridine-methanesulfonic acid severely blocked ¹⁴CO₂ production and labeling of the glycolate pathway from glycolate-1-¹⁴C. Isonicotinyl hydrazide had little effect on the ¹⁴CO₂ released from glycine-1-¹⁴C although the glycine to serine conversion was severely inhibited.

These results and other data in the literature indicate that the glycolate pathway of carbohydrate metabolism does not supply sufficient CO₂ during the synthesis of serine from glycine to account for the rates of photorespiration observed in many species. A direct decarboxylation of glyoxylate is more likely the main source of photorespiratory CO₂.