Cyanate inhibition of nitrate reductase

Abstract

In the nonvacuolated acidophilic thermophilic red alga, Cyanidium caldarium, nitrate uptake and reduction can be separated by measuring disappearance of nitrate from the suspension medium using, in in vivo experiments, cyanate, a competitive inhibitor of algal nitrate reductase. Cyanate selectively inhibited nitrate reduction at concentrations that did not significantly affect nitrate uptake, photosynthesis or respiration. Its use proved that short-term control of intracellular nitrate through the increase of a carrier-mediated nitrate efflux took place when nitrate reduction was inhibited. The occurrence of the high- and low-affinity nitrate uptake systems in cells grown in nitrogen-limited conditions, as previously reported, suggests a ‘pump and leak’ mechanism operating at the plasmalemma level to regulate nitrate uptake and intracellular nitrate: the high-affinity nitrate transport system mediated by proton cotransport (irreversible) operates the influx, while the low-affinity transport (reversible) operates influx or efflux according to cell requirements. Kinetic analysis of cyanate inhibition in cells taken from low-nitrate medium supports this hypothesis and reveals that, in Cyanidium, intracellular nitrate is probably compartmented in the cytosol.