H2O2 destruction by ascorbate-dependent systems from chloroplasts.

Washed lamellae from isolated spinach chloroplasts exhibited peroxidative activity with 3,3'-diaminobenzidine or ascorbate as electron donors. By heat treatment or by incubation of the chloroplasts with pronase a heat-labile enzymic activity (system A) and a heat-stable non-enzymic peroxidative activity (system B) could be differentiated. System A is membrane-bound, reacts with 3,3'-diaminobenzidine and with ascorbate as electron donors, shows a sharp pH optimum between 7.5 and 8.0 with both substrates and is inhibited competitively by cyanide. The heat-stable factor can be extracted from the chloroplast lamellae by heat treatment, reacts only with ascorbate as electron donor, shows increasing activity with higher pH values but no optimum and is not inhibited by cyanide. Both peroxidative systems in connection with a relatively high concentration of ascorbate in chloroplasts should represent an important tool for the detoxification of H2O2 which is produced in these organelles by photosynthetic O2 reduction.