Use of carbon oxysulfide, a structural analog of CO2, to study active CO2 transport in the cyanobacterium synechococcus UTEX 625

Abstract

Carbon oxysulfide (carbonyl sulfide, COS) is a close structural analog of CO2. Although hydrolysis of COS (to CO2 and H2S) does occur at alkaline pH (>9), at pH 8.0 the rate of hydrolysis is slow enough to allow investigation of COS as a possible substrate and inhibitor of the active CO2 transport system ofSynechococcus UTEX 625. A light-dependent uptake of COS was observed that was inhibited by CO2 and the ATPase inhibitor diethylstilbestrol. The COS taken up by the cells could not be recovered when the lights were turned off or when acid was added. It was concluded that most of the COS taken up was hydrolyzed by intracellular carbonic anhydrase. The production of H2S was observed and COS removal from the medium was inhibited by ethoxyzolamide. Bovine erythrocyte carbonic anhydrase catalysed the stoichiometric hydrolysis of COS to H2S. The active transport of CO2 was inhibited by COS in an apparently competitive manner. When Na+-dependent HCO3− transport was allowed in the presence of COS, the extracellular [CO2] rose considerably above the equilibrium level. This CO2 appearing in the medium was derived from the dehydration of transported HCO3− and was leaked from the cells. In the presence of COS the return to the cells of this leaked CO2 was inhibited. These results showed that the Na+-dependent HCO3− transport was not inhibited by COS, whereas active CO2transport was inhibited. When COS was removed by gassing with N2, a normal pattern of CO2 uptake was observed. The silicone fluid centrifugation method showed that COS (100 micromolar) had little effect upon the initial rate of HCO3−transport or CO2 fixation. The steady state rate of CO2 fixation was, however, inhibited about 50% in the presence of COS. This inhibition can be at least partially explained by the significant leakage of CO2 from the cells that occurred when CO2 uptake was inhibited by COS. Neither CS2 nor N2O acted like COS. It is concluded that COS is an effective and selective inhibitor of active CO2 transport.