Effects of carbonic anhydrase inhibitors on proton exchange and photosynthesis in pea protoplasts

At concentrations of 100-200 muM, ethoxyzolamide, a lipophilic inhibitor of carbonic anhydrase, considerably (by 60%) inhibited light-induced CO2-depe ndent oxygen evolution in pea protoplasts at the optimum concentration of i norganic carbon (100 muM CO2) in the medium. At the same concentrations of the inhibitor, electron transport in isolated pea thylakoids was inhibited only by 6-9%. Acetazolamide, a water-soluble inhibitor of carbonic anhydras e, affected neither the rate Of CO2-dependent O-2 evolution in protoplasts nor electron transport in thylakoid membranes. A light-dependent proton upt ake by protoplasts was demonstrated. At pH 7.2, the induction kinetics and the rate of proton uptake were similar to those for CO2-dependent O-2 evolu tion. The rate of proton uptake was decreased twofold by I mM acetazolamide . This fact agrees with the notion that a membrane-bound carbonic anhydrase is operative in the plasma membrane of higher plant cells. A mechanism of its functioning is suggested. Possible functions of carbonic anhydrases in the cells Of C-3-plants are discussed.