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Membranes and Bioenergetics

Action of Heavy Metals on Hill Activity and O₂ Evolution in Anacystis nidulans¹

Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi-221 005, India, Department of Botany, Banaras Hindu University, Varanasi-221 005, India

Addition of 5 micromolar Cu²⁺, Cd²⁺, and Zn²⁺ was inhibitory to 10 micromolar H₂O₂-supported Hill activity (dichlorophenolindophenol reduction) and O₂ evolution in membrane preparation from Anacystis nidulans. The reversal of Cd²⁺ and Zn²⁺ inhibition, in contrast to Cu²⁺, by exogenously added catalase (EC 1.11.1.6) suggested that the former cations were inhibitory to H₂O₂ degradation. Ascorbic acid (20 micromolar) supported 27% of the Hill activity which was insensitive to DCMU (10 micromolar) and the remaining activity, attributable to the DCMU sensitive process, was sensitive to inhibition by Cu²⁺ only. It is suggestive that the action site of Cd²⁺ and Zn²⁺ is located between the electron donation sites of H₂O₂ and ascorbic acid, while that of Cu²⁺ is located beyond it. Electron donation by reduced glutathione was insensitive to DCMU and Cu²⁺, indicating that the action site of Cu²⁺ is prior to its electron donation site. Further, the phenanthroline (10 micromolar) reversal of Cu²⁺ inhibition of Hill activity suggested a tentative action site of Cu²⁺ at the level of cytochrome.