PLANT PHYSIOLOGY , Vol 112, Issue 1 79-88, Copyright © 1996 by American Society of Plant Biologists
Ethoxyzolamide Differentially Inhibits CO2 Uptake and Na+-Independent and Na+-Dependent HCO3- Uptake in the Cyanobacterium Synechococcus sp. UTEX 625
P. N. Tyrrell, R. A. Kandasamy, C. M. Crotty and G. S. Espie
Department of Botany, Erindale College, University of Toronto, Mississauga, Ontario, Canada L5L 1C6
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 Synechococcus sp. UTEX 625 were examined.
Measurements of transport and accumulation using radiochemical,
fluorometric, and mass spectrometric 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 HCO3- 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.
