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.

I A. Steps in the reduction of oxygen

Molecular oxygen has two unpaired electrons each of which goes into 
separate antibonding pi-orbitals with parallel spins giving the 
molecule the stability and paramagnetic property in the ground state.  
The reductions of O2 by addition of one ,two and four electrons lead 
to formation of superoxide anion(O2-), H2O2 and H2O, respectively.

(a)     O2  >> &1e->>  O2- >> &1e->>  O2--

(b)   O2 >> H+ &1e->> HO2- >> H+&1e->> H2O2 >> H+&1e->> H2O & HO

        H2O & HO >> H+&1e->> 2H2O

Only two electrons can be accomodated by each oxygen atom.  The 
antibonding orbital of molecular oxygen recieves the added electrons 
and each addition weakens and increases the length of the O to O bond, 
from 1.274 angstrom in O2 to 1.480 angstrom in H2O2, leading to 
rupture [1].

The two electron reduction of oxygen dierectly to H2O2 is restricted 
by symmetry considerations [2] that can be overcome by binding of O2 
to the electron donor and consequent pertubution of the molecular 
orbitals.

(c)     O2 & 2H+ & 2e-  >>  H2O2

(d)     O2 & 1e-  >>  O2-

(e)     O2-  & O2-  & 2H+  >>  H2O2  & O2

The flavoprotein oxidases appear to follow this type of direct two 
electron reduction process (reaction c) with no intermediate step [3].  
Other H2O2 generating systems seem to use one electron reductions 
forming superoxide anions (O2-)(reaction d)[4] two of which then 
dismutate yielding a molecule each of H2O2 and O2 either spontaneously 
or catalyzed by the enzyme superoxide dismutase (reaction e)[5].  The 
flavoprotein dehydrogenases and possibly the theiron protein 
generating H2O2 seem to adopt this mechanism ans are mostly membrane-
localized.  It is now found that superoxide formation is a property 
shared by  large number of redox components.  In view of the 
ubiquitous nature of superoxide dismutase and easy nonenzymatic 
dismutation of superoxide, generation of H2O2 accompanying oxidation 
of these redox components with molecular oxygen becomes equally 
widespread.


References

1  Samuel, D. and Steckel, F. (1974) in Molecular Oxygen in Biology - 
   Topics in Molecular Oxygen Research (Hayashi,O., ed.), pp. 1-27, 
   North-Holland Pbl. Co, Amsterdam

2  Taube, H. (1965) J. Gen. Physiol. 49, 29-35

3  Massey, V., Strickland, S., Mayhew, S.G., Howell,L.G., Engle, P.C., 
   Mathews, R.g., Schuman, M. and sullivan, P.A. (1969) Biochem. 
   Biophys. Res. Commun. 36,891-897

4  Fridovich, I. and Handler, P. (1961) J. Biol. Chem. 236,1836-1840

5  McCord, J.M. and Fridovich, I. (1969) J. Biol. Chem. 244,6049-6055


