Studies on the mechanism of the NADPH-catalyzed peroxidation of endogenous microsomal lipid

The importance of metal chelation in the mechanism of microsomal lipid peroxidation has been studied using both phosphate- and sulfhydryl-containing compounds. The optimal concentration for maximum stimulation by each of these compounds has been determined, and the decrease in stimulation observe at concentrations above the maxima has been related to the ability of these compounds to form stable chelation complexes with non-heme iron. Of the compounds tested, only ADP and ATP facilitated the cooperative binding of NADPH to the membrane and thus suggested the possibility of three binding sites for NADPH. Neither of the other two phosphate-chelating agents (Pi or PPi) and neither of the two thiols (cysteine or dithiothreitol)facilitated cooperative binding of NADPH. These data suggested that the adenine ring of ADP or ATP is directly involved in the cooperativity of NADPH binding. They also emphasized that the binding of the chelation complex to the protein is an important parameter in the mechanism of the NADPH-catalyzed peroxidation of endogenous microsomal lipids. Furthermore, stimulation of the rat of lipid peroxidation by sulhydryl-containing compounds, by freezing thawing the microsomal protein, and by treatment of the protein with detergent may be due to a decrease in this cooperative binding effect. Since cysteine and deoxycholate as well as freezing and thawing alter membrane structure, the stimulation of lipid peroxidation seems to involve some alteration to the structure of the microsomal membrane prior to the onset of enzymatic lipid peroxidation.