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) H20 2 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 H20 2 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) H20 2 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 in­ dicating the involvement of photosystem I and of 0 2as an intermediate in the H20 2 producing reaction.