Analysis of Different Mechanisms of Photosynthetic Oxygen Reduction

Three mechanisms of oxygen reduction by chloroplast lamellae in the presence of autoxidizable electron acceptors can be differentiated by product analyis in the presence or absence of either dibromothymoquinone (DBMIB) or superoxide dismutase (SOD) : 1) H₂O₂ is the product of two-electron oxygen reduction by 2, 3-dimethyl-5, 6-methylenedioxy-pbenzoquinone, involving only photosystem II. This reaction is not inhibited by either DBMIB or SOD. 2) Superoxide anion, and H₂O₂ as the product of its dismutation, are products of monovalent oxygen reduction following autoxidation of certain low potential electron acceptors (herbicides) of photosystem I. These reactions are not inhibited by SOD but are blocked by DBMIB. 3) H₂O₂ is the product of and “apparent” two-electron photoreduction of oxygen, mediated by certain o-diphenols (caffeic acid). These reactions are inhibited by both DBMIB and SOD indicating the involvement of photosystem I and of [formula omitted] as an intermediate in the H₂O₂ producing reaction.