Photoperoxidation in isolated chloroplasts: II. Role of electron transfer

Abstract

Isolated chloroplasts, upon illumination, undergo a cyclic peroxidation causing degradation of chlorophyll and tri-unsaturated fatty acids, and production of malondialdehyde. The role of electron transport in this process has now been investigated. Electron transport cofactors (potassium ferricyanide, manganous ion, phenazine methosulfate, and dichlorophenol indolphenol) inhibit malondialdehyde production. As pH increases, the linear rate of peroxidation increases concurrently with decreased activity of photosystem II reaching a maximum at pH 8.8. Only noncyclic cofactors manifest a marked pH dependence. Phenazine methosulfate retards loss of tri-unsaturated fatty acids of illuminated chloroplasts. Antioxidants (butylated hydroxytoluene, sodium ascorbate, purified β-carotene, α-tocopherol) inhibit, while bivalent ions (cupric, cobaltous) stimulate, consistent with the occurrence of a cyclic peroxidation. The results suggest that at high light intensities, overloading of the chlorophyll collection system results in energy being channeled into destructive cyclic peroxidation processes involving the membrane fatty acids. Acceleration of electron flow, e.g., by adding cofactors, simulates an “antioxidant” action perhaps by either removing the excess energy by electron transport or by direct inhibition of the peroxidation process.