Superoxide- and Ethane-Formation in Subchloroplast Particles: Catalysis by Pyridinium Derivatives

Oxygen reduction by chloroplast lamellae is catalyzed by low potential redox dyes with E'0 values between -0 .38 V and -0 .6 V. Compounds of E'0 values o f -0 .67 V and lower are inactive. In subchloroplast particles with an active photosystem I but devoid of photosynthetic electron transport between the two photosystems, the active redox compounds enhance chlorophyll bleaching, superoxide formation and ethane production independent on exogenous substrates or electron donors. The activities of these compounds decrease with decreasing redox potential, with one exception: 1-methyl-4,4'-bipyridini urn bromide with an E'0 value of lower -1 V (and thus no electron acceptor of photosystem I in chloroplast lamellae with intact electron transport) stimulates light dependent superoxide formation and unsaturated fatty acid peroxidation in sub­ chloroplast particles, maximal rates appearing after almost complete chlorophyll bleaching. Since this activity is not visible with compounds with redox potentials below -0 .6 V lacking the nitrogen atom at the 1-position of the pyridinium substituent, we assume that 1 -methyl-4,4'-bipyridinium bromide is “activated” by a yet unknown light reaction.