Inhibition of Photosynthesis by Azide and Cyanide and the Role of Oxygen in Photosynthesis

Abstract

Cyanide and azide inhibit photosynthesis and catalase activity of isolated, intact spinach (Spinacia okracea) chloroplasts. When chloroplasts are iUuminated in the presence of CN-or N3-, accumulation of H202 is observed, parallel to inhibition of photosynthesis. Photosynthetic 02 evolution is inhibited to the same extent, under saturating light, whether C02 or phosphoglycerate is present as electron acceptor.The illumination of chloroplasts with CN-or N3-inactivates the NADPH-and ATP-dependent phosphoglycerate reduction. This enzyme system can be reactivated by dithiothreitol. In reconstituted, envelope-less chloroplasts, the phosphoglycerate-dependent and the ribose 5-phosphate-dependent 02 evolution are inhibited to the same extent, while electron transport to NADP is unaffected.It is conduded that the inhibition of photosynthesis by CN-and N3-is due to H202 accumulation, which is a consequence of catalase inhibition. Azide and cyanide inhibit both respiration and photosynthesis, in vivo as well as in vitro. While the inhibition of respiration is accounted for by the inhibition of metal-containing terminal oxidases, primarily cyt oxidase, the mechanism of inhibition of photosynthesis is controversial. It is well known that cyanide and azide do not inhibit photosynthetic electron transport from H20 to NADP or to artificial electron acceptors, nor photosynthetic phosphorylation (1, 2). Therefore, the inhibition of photosynthesis by these poisons must be explained by an indirect mechanism of invoking the inhibition of one or more steps of the photosynthetic carbon cycle. Bamberger and Avron (3) have demonstrated that the steps between Rib5P2 and 3PGA are not affected by azide, at concentrations that completely inhibit photosynthetic CO2 fixation in isolated chloroplasts. These authors attribute the inhibition of photosynthesis by N3-to the inhibition of carbonic anhydrase, an enzyme abundant in chloroplast stroma (13), which has been proposed to have a function in the uptake of CO2 into the chloroplast (5, 7). Although this function ' This work was supported by the Consiglio Nazionale delle Richerche of Italy.2 Abbreviations: PEP: phosphoenolpyruvate; 3PGA: 3-phosphoglycerate: Rib-5-P: ribose 5-phosphate; DTT; dithiothreitol.is under dispute (13), the importance of carbonic anhydrase in photosynthesis is beyond doubt. The studies reported here on the mechanism of inhibition of photosynthesis by CN-and N3-lead us to conclude that the inhibition of photosynthesis in isolated chloroplasts by these reagents can be accounted for by their inhibition of catalase and the consequent accumulation of H202, in addition to the inhibition of carbonic anhydrase. It is known that H202 inhibits enzymes dependent on -SH groups for activity.The production of H202 by photosynthetic electron transport has been established by Mehler (1 1), and is the result of reoxidation by 02 of the primary reductant of PSI. On the other hand, no H202 is formed when NADP is available as the electron acceptor. The Mehler reaction has been sh...

Effects of Inhibitors of Catalase on Photosynthesis and on Catalase Activity in Unwashed Preparations of Intact Chloroplasts

“…In fact Table I ,UM would inhibit photosynthesis by slightly more than 50% according to the results of Forti and Gerola (11), while in the experiment presented here in Figure 1 no C02-dependent 02 evolution could be detected at cyanide concentrations greater than 30,1lM. It is difficult to reconcile this disparity.…”

“…DISCUSSION The similarity of the concentration ranges of inhibition by cyanide of photosynthesis and of catalase (Fig. 1) does not necessarily reflect the simple cause-and-effect relationship suggested by Forti and Gerola (11), since the shapes of the inhibition curves differ, as do the cyanide concentrations required for 50%o inhibition (17 tLM and 3.5 AM for photosynthesis and catalase, respectively).…”

“…Several investigators have recently described evidence that under certain circumstances 02 may compete with NADP as an electron acceptor in intact chloroplasts (4,10,11,15). Although three distinct mechanisms of photosynthetic 02 reduction have been proposed (reviewed in ref.…”

Long-Term Preservation of Microalgal Cells and their Optical Properties

“…This is particularly relevant during the freeze-thawing that constitutes a double stress for the cells [18][19][20]. Azide inhibits cell respiration and photosynthesis; it is also a strong antioxidant [21][22][23][24]. Furthermore, it's a selective and instantaneous inhibitor of mitochondrial metabolic activity [25].…”

Effects of washing and osmotic shock on catalase activity of intact chloroplast preparations

“…This effect has been demonstrated both in broken, washed chloroplasts [2] and in intact chloroplasts capable of CO2 fixation [3][4][5]. In the latter case endogenous catalase activity presents a difficulty [4] which cannot satisfactorily be overcome by addition of the catalase inhibitors, cyanide and azide, since these compounds also inhibit photosynthetic CO2 fixation [6].…”