Photorespiration: Ribulose Diphosphate Oxygenase or Hydrogen Peroxide?

Abstract

Oxygen uptake and evolution in illuminated cells of Hydrodictyon qfricanum have been measured using ¹⁸O₂ mass spectrometry. Earlier work showed that the light-stimulated O₂ uptake under conditions of light and CO₂ saturation was insensitive to cyanide and to CCCP. This was interpreted as due to a pseudocyclic electron flow, possibly associated with glycolate synthesis by the pathway proposed by Coombs and Whittingham. The work reported here was carried out at the CO₂ compensation point, where light-dependent O₂ uptake is higher than at CO₂ saturation. The component of O₂ uptake which is stimulated by lowering the CO₂ concentration is inhibited by either cyanide or CCCP. This component is attributed to the activity of ribulose diphosphate (RuDP) oxygenase, which is completely inhibited by CO₂ at high CO₂ levels. The rate of pseudocyclic electron flow seems to be insensitive to changes in CO₂ concentration between saturation and the compensation point