HYDROGEN PEROXIDE AND ORGANIC HYDROPEROXIDES

The ratios of two typical A and G products are compiled in Table 12.13 for y-radiolysis in N20/02-saturated solutions which produces 90% OH and 10% 0 and for H202. If in the reaction of DNA-bound transition metal ions with H202 OH would be the only agent that is responsible for the observed damage one would expect that these product ratios should be identical, but they are markedly different (in order to avoid potential uncertainties, data from the same group where chosen). 

Since in both cases the primary damage is set by OH, these differences can be explained if the transition metal ion which must be still near the damaged site modifies the damage prior to a reaction of oxygen. Alternatively, H202 which was present at rather high concentrations in these experiments (0.1 to 0.4 mol 

For the Fenton reaction to proceed, the transition metal ion must be in its low oxidation state. For example, iron must be present as Fe2+ [reaction (48)]. 

When all the Fe2+ is consumed (the intracellular labile iron pool is around micromolar Epsztejn et al. 1997), the reaction should come to a standstill. In the cellular systems that have been investigated, this seems not to be the case, and it has been postulated that Fe3+ (possibly complexed to DNA) is reduced by H202 in a slow reaction [reaction (49) Ward et al. 1985], 

With Cu2+, H2O2 and ascorbate, a Fenton-type reaction is initiated. In the presence of DNA, the copper is bound to DNA and an addition of OH scavengers such as DMSO suppresses this reaction only marginally as shown with a polyly-sine-bound fluorescent OH probe (Makrigiorgos 1999). 

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Hatakeyama, S H. Lai, S. Gao, and K. Murano, Production of Hydrogen Peroxide and Organic Hydroperoxides in the Reactions of Ozone with Natural Hydrocarbons in Air, Chem Lett, 1287-1290 (1993). 

Hydrogen peroxide and organic hydroperoxides can be reduced by molybdate-activated iodide [Allen et al. 1952 reactions (13) and (14)]. The iodine atoms formed in these reactions combine to iodine which complexes with iodide [reactions (15) and (16)], and it is this I3- complex which is measured (s(3540 nm) = 25,000 dm3 mol-1 cm-1)... 

Hydrogen peroxide and organic hydroperoxides are relatively poor oxidants in the absence of radical initiators or other specific reagents. No reaction occurs with alkenes unless a reagent capable of producing electrophilic intermediates, such as a peracid or metal peroxo complex, is used. 

Glutathione (GSH) peroxidase (EC 1.11.1.9) is one of several selenoproteins that contain a unique selenocysteine residue at the active site (1). This enzyme catalyzes the GSH-dependent reduction of both hydrogen peroxide and organic hydroperoxides, including fatty acid hydroperoxides formed during lipid peroxidation. GSH peroxidase activity has been measured in both protozoan and helminth parasites (11,12). The enzyme from Schistosoma mansoni has been cloned (13). The presence of the unique selenocysteine codon (UGA) confirms that it is a selenoprotein. Although GSH peroxidase activity appears to be absent from Hymenolepsis dimimta and M. expansa... 

Frew JE, Jones J, Scholes G. Spectrophotometric determination of hydrogen peroxide and organic hydroperoxides at low concentrations in aqueous solution. Analytica Chimica Acta 1983 155 139-150. 

Hua W, Chen ZM, et al. Atmospheric hydrogen peroxide and organic hydroperoxides during PRIDE-PRD 06, China their concentration, formation mechanism and contribution to secondary aerosols. Atmos. Chem Phys. 2008 8 6755-73. 

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