Inhibition of photosynthesis by azide and cyanide and the role of oxygen in photosynthesis.

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 N(3) (-), accumulation of H(2)O(2) is observed, parallel to inhibition of photosynthesis. Photosynthetic O(2) evolution is inhibited to the same extent, under saturating light, whether CO(2) or phosphoglycerate is present as electron acceptor.The illumination of chloroplasts with CN(-) or N(3) (-) 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 O(2) evolution are inhibited to the same extent, while electron transport to NADP is unaffected.It is concluded that the inhibition of photosynthesis by CN(-) and N(3) (-) is due to H(2)O(2) accumulation, which is a consequence of catalase inhibition.The inhibition of phosphoglycerate reduction, but not of CO(2) reduction, is abolished under conditions where ATP is available in excess of NADPH (low light, supply of ATP). This is taken as an indication that electron flow from photosystem I is diverted to O(2) (Mehler reaction, which produces H(2)O(2)) 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.