Inhibition of nitrite assimilation by uncouplers of phosphorylation

Nitrite transport to the chloroplast in Chlamydomonas reinhardtii: molecular evidence for a regulated process

Nitrite transport to the chloroplast is not a well documented process in spite of being a central step in the nitrate assimilation pathway. The lack of molecular evidence, as well as the easy diffusion of nitrite through biological membranes, have made this physiological process difficult to understand in plant nutrition. The aim of this review is to illustrate that nitrite transport to the chloroplast is a regulated step, intimately related to the efficiency of nitrate utilization. In Chlamydomonas reinhardtii, the Nar1;1 gene has been shown to have this role in nitrate assimilation. NAR1;1 corresponds to a plastidic membrane transporter protein related to the bacterial formate/nitrite transporters. At least four Nar1 genes might exist in Chlamydomonas. The existence of orthologous Nar1 genes in plants is discussed.

The effect of the uncoupler carbonyl cyanide m-chlorophenylhydrazone on K+ transport, ATP level and intracellular pH of Chlorella fusca

1. Low concentrations of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) induced net K+ uptake by Chlorella fusca, optimal concentrations being 3 microM CCCP in the light and 1 microM CCCP in the dark. Higher concentrations increasingly stimulated K+ release. 2. Measurements of the unidirectional K+ fluxes showed that CCCP-induced net K+ uptake in the light was mainly a consequence of an inhibition of efflux. In the dark, influx was slightly stimulated in addition. 3. In conditions of CCCP-induced net K+ uptake, the ATP level was decreased by less than 10%. With higher CCCP concentrations it fell drastically. 4. By means of the 5,5-dimethyloxazolidine-2,4-dione distribution technique, an acidification of the cell interior on the addition of CCCP was found. 5. It is concluded that uncoupler-induced net K+ uptake is due to an enhanced proton leakage into the cell across the plasmalemma. Intracellular acidification by this process stimulates ATP-dependent K+/H+ exchange which, in itself, is not affected at low uncoupler concentrations.

The effect of nitrogen refeeding on starved cells of Platymonas striata Butcher

A rapid uptake of nitrogen was observed in nitrogen-starved cells of Platymonas striata after refeeding with ammonium or nitrate ions. This was followed by a net loss of nitrogen per cell. Cells initially grown in and then starved in a regime of continuous light showed greater increases in average cell nitrogen on refeeding with ammonium or nitrate ions than did cells initially grown in and then starved in a regime of alternating light and darkness. A particulate subcellular location was observed for nitrate reductase (EC 1.6.6.1) in broken cell suspensions prepared by sonication. Nitrite reductase (EC 1.6.6.4) was located in the soluble fraction of these cell suspensions. Broken cell preparations displayed a lowered nitrate reductase activity as compared with the particulate component of these preparations. This was shown not to be due to heat-stable inhibitors present in the soluble phase of the cell. It appeared to be an artefact produced by the high nitrite reductase activity of the broken cell preparations, which removed much of the nitrite as it was formed. Nitrogen starvation of nitrate-grown cultures produced cellular increases in nitrate reductase and nitrite reductase activities which were further increased after the addition of nitrate. The results are discussed.

[The nitrate- and nitrite-dependent O2-evolution in N 2 by Ankistrodesmus braunii]

O2-evolution in N2 by young cells of synchronous Ankistrodesmus braunii was measured manometrically in the presence and in the absence of nitrate or nitrite. Nitrate-starved cells produce O2 as a function of nitrate concentration with an optimum at 10 mM nitrate. The optimum rates are strongly dependent upon the pH of the medium culminating at pH 8.0.Nitrite excretion is initially slow and has its optimum at the same nitrate concentration as O2-evolution. It is slower under anaerobic conditions than in the presence of CO2 and slower at pH 5.6 than at pH 8.0. At pH 8.0 in N2 an accumulation of nitrite starts 40 to 100 min after the addition of nitrate to the algae.O2-evolution is faster with nitrite than with nitrate. No optimum curve is observed at pH 8.0 with various concentrations of nitrite in the medium; however, at pH 5.6 a distinct peak is found at 3 mM nitrite. This peak is found for a fast short-time reaction as well as for the following low rates of O2-evolution.The uncoupler carbonylcyanide-m-chlorophenylhydrazone (CCCP) equally inhibits nitrate- and nitrite-dependent O2-evolution and the incorporation of (32)P into cellular phosphate compounds. There is no indication of uncoupling in vivo of nitrite reduction which is completely independent of ATP in vitro. The inhibition of the nitrate-dependent O2-evolution by high concentrations of nitrate cannot be explained by an accumulation of products such as ammonia or nitrite.

[Dependence of nitrite assimilation in green algae on energy supplied by respiration and photosynthesis]

Inhibitors and uncouplers of phosphorylation, i.e., arsenate, 2.4-dinitrophenol (DNP), pentachlorophenol (PCP), and carbonyl cyanide m-chlorophenylhydrazone (CCCP), inhibit the assimilation of nitrite by the green alga Ankistrodesmus braunii in the dark and in the light. In a medium containing nitrate, these inhibitors interrupt nitrate reduction at the level of nitrite. In phosphatedeficient algae, the assimilation of nitrite can be decreased by a concomitant, energy-dependent uptake of chloride and phosphate ions. These results support the assumption that high-energy phosphate is required for the assimilation of nitrite.CO2 and glucose (after pre-illumination) increase nitrite assimilation in the light. Photosynthetic nitrite reduction is inhibited by 3-(3',4'-dichlorophenyl)-1,1-dimethyl urea (DCMU), an inhibitor of oxygen evolution, and by disalicylidene-propanediamine-(1,3) (DSPD), an inhibitor of the photosynthetic reduction of ferredoxin.