Role of intracellular carbonic anhydrase in inorganic-carbon assimilation by Porphyridium purpureum.

Air-grown cells of Porphyridium purpurem contain appreciable carbonic-anhydrase activity, comparable to that in air-grown Chlamydomonas reinhardtii, but activity is repressed in CO2-grown cells. Assay of carbonic-anhydrase activity in intact cells and cell extracts shows all activity to be intracellular in Porphyridium. Measurement of inorganic-carbon-dependent photosynthetic O2 evolution shows that sodium ions increase the affinity of Porphyridium cells for HCO 3 (-) . Acetazolamide and ethoxyzolamide were potent inhibitors of carbonic anhydrase in cell extracts but at pH 5.0 both acetazolamide and ethoxyzolamide had little effect upon the concentration of inorganic carbon required for the half-maximal rate of photosynthetic O2 evolution (K0.5[CO2]). At pH 8.0, where HCO 3 (-) is the predominant species of inorganic carbon, the K0.5 (CO2) was increased from 50 μM to 950 μM in the presence of ethoxyzolamide. It is concluded that in air-grown cells of Porphyridium. HCO 3 (-) is transported across the plasmalemma and intracellular carbonic anhydrase increases the steady-state flux of CO2 from inside the plasmalemma to ribulose-1,5-bisphosphate carboxylase-oxygenase by catalysing the interconversion of HCO 3 (-) and CO2 within the cell.