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Alternate Pathways of Glycolate Synthesis in Tobacco and Maize Leaves in Relation to Rates of Photorespiration

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

After a preliminary period in light, leaf disks floated on 10 mM -hydroxy-2-pyridinemethanesulfonic acid to inhibit glycolate oxidase accumulate glycolate at average initial rates of 67 micromoles in tobacco and 8 micromoles per gram fresh weight per hour in maize under optimal conditions in air. In the presence of ¹⁴CO₂, the glycolate synthesized has a high specific radioactivity in illuminated tobacco and a low one in maize. Isonicotinic acid hydrazide also inhibits glycolate oxidation and causes a slow accumulation of glycolate in maize but not in tobacco, while it inhibits glycolate synthesis in tobacco but not in maize. Radioactive carbon in acetate-2-¹⁴C and especially pyruvate-3-¹⁴C is incorporated predominantly into the C-2 of glycolate in both species, but the specific radioactivity is much greater in maize. Glyoxylate-2-¹⁴C is readily converted to glycolate-2-¹⁴C in both species. The addition of phosphoenolpyruvate stimulated glycolate formation in maize and inhibited its synthesis in tobacco, and in the presence of ¹⁴CO₂ the specific radioactivity in glycolate-¹⁴C was decreased greatly by the added phosphoenolpyruvate only in maize.

Thus, unsymmetrically labeled glycolate is mainly synthesized from pyruvate-3-¹⁴C by a slow pathway in maize. Tobacco possesses an additional rapid pathway that produces equally labeled glycolate more directly from fixed CO₂ during photosynthesis. Glycolate is believed to be the primary substrate of photorespiration, and sufficiently rapid rates of glycolate synthesis have been observed in tobacco to account for this function. Hence the high rates of photorespiration observed in tobacco leaves compared with maize result partly from differences between these species in the pathway of glycolate synthesis.

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