GENERATION OF H2O2 IN BIOMEMBRANES T. Ramasarma Biochemica et Biophysica Acta, 694 (1982) 69-93 From pages 70-71 I. Introduction Knowledge of the generation of H2O2 in cellular oxidations has existed for many years. It has ben assumed that H2O2 is toxic to cells and the presence of catalase is indicative of a detoxication mechanism. Other radicals of oxygen were recently recognized to be more potent destructive agents of biological material than H2O2. Also catalase and other peroxidases utilize H2O2 in some cellular oxidation processes leading to several important metabolites. Thus, the generation of H2O2 in cellular processes seems to be purposeful and H2O2 can not be dismissed as a mere undesirable byproduct. Biological formation of H2O2 is not limited to the previously known flavoprotiens and some copper enzymes, but other redox systems, particulrly heme and non-heme iron proteins, are now found to undergo auto-oxidation yeilding H2O2. The capacity for generation of H2O2 is now found to be widespread in a variety of organisms and in the organelles of the cells. The reduction of oxygen to H2O by mitochondrial cytochrome oxidase beingthe predominant oxygen-utilizing reaction had overshadowed the importance of the quantitatively minor pathways. Under aerobic conditions generation of H2O2 by a variety of biomembranes has now been found to be a physiological event interlinked with phenomena such as phagocytosis, transport processes and thermogenesis in some as yet unidentified way. The underlying mechanisms of of these processes seem to involve generation and utilization of H2O2 in mitichondria, microsomes, peroxisomes or plasma membranes. This review gives an account of the potential of the biomembranes to generate H2O2 and its implication in the cellular processes.