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 of Synechococcus 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 CO₂ transport 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.