Ethoxyzolamide differentially inhibits CO2 uptake and Na+-independent and Na+-dependent HCO3- uptake in the cyanobacterium Synechococcus sp UTEX 625

The effects of ethoxyzolamide (EZ), a carbonic anhydrase inhibitor, on the active CO2 and Na+-independent and Na+-dependent HCO3- transport systems of the unicellular cyanobacterium Syn-echococcus sp. UTEX 625 were examined. Measurements of transport and accumulation using radiochemical, fluorometric, and mass spedrometric assays indicated that active CO2 transport and active Na+-independent HCO3- transport were inhibited by EZ. However, Na+-independent HCO3- transport was about 1 order of magnitude more sensitive to EZ inhibition than was CO2 transport (50% inhibition = 12 mu M versus 80 mu M). The data suggest that both the active CO2 (G.D. Price, M.R. Badger [1989] Plant Physiol 89: 37-43) and the Na+-independent HCO3- transport systems possessed carbonic anhydrase-like activity as part of their mechanism of action. In contrast, Na+-dependent HCO3- transport was only partially (50% inhibition = 230 mu M) and noncompetitively inhibited by EZ. The collective evidence suggested that EZ inhibition of Na+-dependent HCO,- transport was an indirect consequence of the action of EZ on the CO2 transport system, rather than a direct effect on HCO3- transport. A model is presented in which the core of the inorganic carbon translocating system is formed by Na+-dependent HCO3- transport and the CO2 transport system. It is argued that the Na+-independent HCO3- utilizing system was not directly involved in translocation, but converted HCO3- to CO2 for use in CO2 transport.