Hydrogen peroxide reacts with reduced transition metals to generate the highly reactive hydroxyl radical (·OH), most often proposed as the predominant species for initiating microsomal lipid peroxidation. To assess the potential involvement of ·OH, generated from hydrogen peroxide, in microsomal lipid peroxidation, we have altered the concentration of microsomal hydrogen peroxide and measured the resulting rates of malondialdehyde production. Hydrogen peroxide concentration in microsomes was changed by adding exogenous catalase, by washing to reduce both endogenous catalase activity and hydrogen peroxide-dependent glutathione oxidase activity, and by inhibiting endogenous catalase activity with azide in either the presence or absence of exogenous hydrogen peroxide. In only one instance was the rate of lipid peroxidation affected; exogenous hydrogen peroxide added to microsomes, previously incubated with azide, inhibited lipid peroxidation, the opposite effect from that predicted if ·OH, generated from hydrogen peroxide, is actually the major initiating species. Neither these results, nor the inability of known ·OH traps to inhibit microsomal lipid peroxidation, support the role of free hydrogen peroxide in the initiation of microsomal lipid peroxidation.
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