EVIDENCE FOR O-18 LABELING OF PHOTORESPIRATORY CO2 IN PHOTOAUTOTROPHIC CELL-CULTURES OF HIGHER-PLANTS ILLUMINATED IN THE PRESENCE OF O-18(2)

The O-18-enrichment of CO2 produced in the light or during the post-il lumination burst was measured by mass spectrometry when a photoautotro phic cell suspension of Euphorbia characias L. was placed in photoresp iratory conditions in the presence of molecular O-18(2). The only O-18 -labeled species produced was (COO)-O-18-O-16; no (COO)-O-18-O-16 coul d be detected. Production of (COO)-O-18-O-16 ceased after addition of two inhibitors of the photosynthetic carbon-oxidation cycle, aminooxya cetate or aminoacetonitrile, and was inhibited by high levels of CO2. The average enrichment during the post-illumination burst was estimate d to be 46+/-15% of the enrichment of the O2 present during the preced ing light period. Addition of exogenous carbonic anhydrase, by catalyz ing the exchange between CO2 and H2O, drastically diminished the O-18- enrichment of the produced CO2. The very low carbonio-anhydrase level of the photoautotrophic cell suspension probably explains why the O-18 labeling of photorespiratory CO2 Could be observed for the first time . These data allow the establishment of a direct link between O2 consu mption and CO2 production in the light, and the conclusion that CO2 pr oduced in the light results, at least partially, from the mitochondria l decarboxylation of the glycine pool synthesized through the photosyn thetic carbon-oxidation cycle. Analysis of the (COO)-O-18-O-16 and CO2 kinetics provides a direct and reliable way to assess in vivo the rea l contribution of photorespiratory metabolism to CO2 production in the light.