Evidence for 18O labeling of photorespiratory CO2 in photoautotrophic cell cultures of higher plants illuminated in the presence of 18O2

Abstract

The 18O-enrichment of CO2 produced in the light or during the post-illumination burst was measured by mass spectrometry when a photoautotrophic cell suspension of Euphorbia characias L. was placed in photorespiratory conditions in the presence of molecular 18O2. The only 18O-labeled species produced was C18O16O; no C18O18O could be detected. Production of C18O16O ceased after addition of two inhibitors of the photosynthetic carbon-oxidation cycle, aminooxyacetate or aminoacetonitrile, and was inhibited by high levels of CO2. The average enrichment during the post-illumination burst was estimated to be 46 ± 15% of the enrichment of the O2 present during the preceding light period. Addition of exogenous carbonic anhydrase, by catalyzing the exchange between CO2 and H2O, drastically diminished the 18O-enrichment of the produced CO2. The very low carbonio-anhydrase level of the photoautotrophic cell suspension probably explains why the 18O labeling of photorespiratory CO2 could be observed for the first time. These data allow the establishment of a direct link between O2 consumption and CO2 production in the light, and the conclusion that CO2 produced in the light results, at least partially, from the mitochondrial decarboxylation of the glycine pool synthesized through the photosynthetic carbon-oxidation cycle. Analysis of the C18O16O and CO2 kinetics provides a direct and reliable way to assess in vivo the real contribution of photorespiratory metabolism to CO2 production in the light.