Pn. Tyrrell et al., ETHOXYZOLAMIDE DIFFERENTIALLY INHIBITS CO2 UPTAKE AND NA-INDEPENDENT AND NA+-DEPENDENT HCO3- UPTAKE IN THE CYANOBACTERIUM SYNECHOCOCCUS SP UTEX-625(), Plant physiology, 112(1), 1996, pp. 79-88
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 radioc
hemical, fluorometric, and mass spedrometric assays indicated that act
ive CO2 transport and active Na+-independent HCO3- transport were inhi
bited by EZ. However, Na+-independent HCO3- transport was about 1 orde
r 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 a
nhydrase-like activity as part of their mechanism of action. In contra
st, Na+-dependent HCO3- transport was only partially (50% inhibition =
230 mu M) and noncompetitively inhibited by EZ. The collective eviden
ce suggested that EZ inhibition of Na+-dependent HCO,- transport was a
n 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 for
med by Na+-dependent HCO3- transport and the CO2 transport system. It
is argued that the Na+-independent HCO3- utilizing system was not dire
ctly involved in translocation, but converted HCO3- to CO2 for use in
CO2 transport.