Carbonic anhydrase activity in leaves as measured in vivo by18O exchange between carbon dioxide and water

Carbonic anhydrase activity of intactCommelina communis L. leaves was measured using mass spectrometry, by following the¹⁸O-exchange kinetics between¹⁸O-enriched carbon dioxide and water. A gas-diffusion model (Gerster, 1971, Planta97, 155–172) was used to interpret the¹⁸O-exchange kinetics and to determine two constants, one (k) related to the hydration of CO₂ and the other (k_e), related to the diffusion of CO₂. Both constants were determined inCommelina communis L. leaves after stripping the lower epidermis to remove any stomatal influence. The hydration constant (k) was 17200 +2200 ·min^–1 (mean±SD, 12 experiments), i.e., about 8 600 times the uncatalyzed hydration of CO₂ in pure water, and was specifically inhibited by ethoxyzolamide, a powerful inhibitor of carbonic anhydrases, half-inhibition occurring around 10^–5 Methoxyzolamide. The diffusion constant (k_e) was 1.18±0.28·min^–1 (mean±SD, 12 experiments) and was only slightly inhibited (about 20%) by ethoxyzolamide. Carbonic anhydrase activity of stripped leaves was not affected by the leaf water status (up to 50% relative water deficits), was strongly inhibited by monovalent anions such as Cl^– or NO₃^–, and decreased by about 50% when the photon flux density during growth was increased from 100 to 500 mol photons·m^–2·s^–1. By studying the effect of ethoxyzolamide (10^–4 M) on photosynthetic O₂ exchange, measured using¹⁸O₂ and mass spectrometry, we found that inhibition of carbonic anhydrase activity by 92–95% had little effect on the response curves of net O₂ evolution to increased CO₂ concentrations. Ethoxyzolamide had no effect on the photosynthetic electron-transport rate, measured as gross O₂ photosynthesis at high CO₂ concentration (>350 l·^–1), but was found to increase both gross O₂ photosynthesis and O₂ uptake at lower CO₂ levels. The chloroplastic CO₂ concentration calculated from O₂-exchange data was not significantly modified by ethoxyzolamide. We conclude from these results that, under normal conditions of photosynthesis, most of the carbonic anhydrase activity is not involved in CO₂ assimilation. Measurement of carbonic anhydrase activity using¹⁸O-isotope exchange therefore provides a suitable model to study the in-vivo regulation of this chloroplastic enzyme in plants submitted to various environmental conditions.Abbreviations CA carbonic anhydrase - C_cc chloroplastic CO₂ concentration - C_e external CO₂ concentration - EZA ethoxyzolamide - k CO₂ hydration rate constant - k_e CO₂ diffusion rate constan - PPFD photosynthetic photon flux density - Rubisco ribulose-1,5 bisphosphate carboxylase oxygenase - RWD relative water deficitThe authors wish to thank P. Carrier for technical assistance with mass-spectrometric experiments and Dr. P. Thibault for helpful suggestions and comments. Dr. A. Vavasseur is gratefully acknowledged for supplyingCommelima communis. cultures. P.C., P.T. and A.V. are all from the CEA, Département de Physiologie Végétale et Ecosystèmes, Cadarache, France.