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 oleracea) chloroplasts. When chloroplasts are illuminated in the presence of CN- or N3-, accumulation of H2O2 is observed, parallel to inhibition of photosynthesis. Photosynthetic O2 evolution is inhibited to the same extent, under saturating light, whether CO2 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 O2 evolution are inhibited to the same extent, while electron transport to NADP is unaffected. Apparently the inhibition of photosynthesis by CN- and N3- is due to H2O2 accumulation, which is a consequence of catalase inhibition. The inhibition of phosphoglycerate reduction, but not of CO2 reduction, is abolished under conditions where ATP is available in excss of NADPH (low light, supply of ATP). Possibly electron flow from photosystem I is diverted to O2 (Mehler reaction, which produces H2O2) when the unavailability of ATP is limiting the rate of reoxidation of NADPH. The Mehler reaction is considered a physiological process supplying ATP for photosynthesis.