Incorporation of Oxygen into Glycolate, Glycine, and Serine during Photorespiration in Maize Leaves

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Abstract
Glycolate, glycine, and serine extracted from excised Zea mays L. leaves which had been allowed to photosynthesize in the presence of 18O2 were analyzed by gas chromatography-mass spectrometry. In each case, only one of the oxygen atoms of the carboxyl group had become labeled. The maximum enrichment observed in glycine and serine was attained after 5 minutes and 15 minutes of exposure to 18O2 at the CO2 compensation point; the labeling was very high, reaching 70 to 73% of that in the applied O2. Thus, it appears that all or nearly all of the glycine and serine are synthesized in maize leaves via fixation of O2. In the presence of CO2 (380 or 800 microliters per liter), 18O-labeling was markedly slower. Glycolate enrichment was variable and much lower than that in glycine and serine. It is possible that there are additional pathways of glycolate synthesis which do not result in the incorporation of 18O from molecular oxygen. An estimation of the metabolic flow of O2 through the photorespiratory cycle was made. It appeared that less than 75% of the O2 taken up by maize leaves is involved in this pathway. Therefore, other processes of O2 metabolism must occur in the light.