Observations on the Decomposition of Hemin by Fatty Acid Hydroperoxides. ∗

Discussion

The destruction of hemin by hydrogen peroxide has been demonstrated and some of its decomposition products have been identified (8). The decomposition of hemin and hemoglobin in the presence of unsaturated lipids usually has been investigated under conditions in which the porphyrin compound has served as a catalyst for the oxidation of polyunsaturated fatty acids in an aqueous emulsion (3,9,10). Studies also have been conducted using mixtures of crude peroxides in an aqueous medium. Tappel (10) found that in a colloidal system 300 moles of hydroperoxides were needed to destroy one moIe of hematin at 40°C. Dubouloz (11), using a different set of conditions, obtained a ratio of 50. In the homogenous system used in the present study, the destruction of 1 mole of hemin required only 4 moles of pure hydroperoxide.

The destruction of hematin catalysts which occurs during the oxidation of linoleate or during decomposition of linoleate peroxide has been attributed to random reactions between the hematin compounds and free radicals (10). However, these mechanisms do not satisfactorily account for the decomposition of hemin in acetic acid solution. As the peroxide preparations used in the present investigation were shown to be essentially free of linoleate, the destructive agents could not have been free radicals of this compound. Moreover, linoleate has been reported to be stable to hemincatalyzed oxidation in an acetic acid medium (3), a finding confirmed in this study (Table 11). It is unlikely that hydroperoxides would yield significant concentrations of free radicals in a strongly reducing solution of glacial acetic acid. MLHP was observed to be stable in this medium during the 4-hr incubation period employed in the present experiments, as indicated by the absence of any decrease in peroxide number or extinction at 235 mp and by a lack of absorption at 280 mp (10). The destruction of hemin under these conditions, therefore, is apparently due to the oxidative properties of hydroperoxides rather than to random encounter with free radicaIs formed in the process of their decomposition.