Hydroperoxides and superoxides in microsomal oxidations

Abstract

The following scheme summarizes our present knowledge on the mechanism of the reactions carried out by cytochrome P450:

X^δ+ can be a protein radical or a porphyrin radical cation.

Donor can be: (i) reduced cytochrome b₅ or NADPH:cytochrome c reductase; (ii) tetramethylphenylenediamine, dimethylphenylenediamine, diaminobenzidine; (iii) dimethylaniline or aminopyrine and resulting in N-dealkylation (Griffin, 1977); (iv) ROOH resulting in ROO^. production and thereby increasing cytochrome P450 destruction, lipid peroxidation, singlet oxygen formation and oxidation of peroxidase donors; (v) unsaturated lipids resulting in lipid peroxidation and singlet oxygen formation; and (vi) various antioxidants.

S substrate can be amines, drugs, steroids, carcinogens, antioxidants R·CH₂OH alcohols (from mechanism by Chance and Schonbaum, 1976). Hydroperoxides (e.g. ethyl hydroperoxide) that form alcohol substrates on reduction would be expected to be less effective in catalyzing hydroxylation reactions (Nordblom et al., 1976).

R·CHO Aldehyde product from alcohol oxidation ROOH Primary, secondary or tertiary hydroperoxides

The scheme explains the following three pathways involved in the formation of the oxenoid species:

• 1.

  (1) Organic hydroperoxide catalyzed. This involves first the formation of an enzyme-peroxide complex (Reaction 1) followed by a fast rearrangement by an outer sphere electron transfer mechanism (Chance and Schonbaum, 1976). Some of this complex may be dissociated to the corresponding aldehyde and the original enzyme. The complex may also be hydrated to compound I, the active hydroxylating species, and release the alcohol. The hydroperoxide catalyzed alcohol oxidation (Rahimtula and O'Brien, 1976) can be explained by the reversal of these changes from compound I to the complex and dissociation of this complex (Reaction 4). The hydroperoxide catalyzed substrate hydroxylation involves the transfer of activated oxygen from compound I to the substrate (Reaction 5). It is also possible that alcohol oxidation proceeds by a hydroxylation mechanism, followed by the rearrangement of the ‘hydroxylated’ intermediate to an aldehyde. The hydroperoxide catalyzed oxidation of hydrogen or electron donors, unsaturated lipids or antioxidants involves the protein or porphyrin free radical of compound I and the ferryl iron of compound II. In competition with these donors, ROOH can also convert compound I to compound II (Chance, 1952; Reaction 6) and the resulting peroxy radicals can also oxidize these donors. In the absence of these donors, cytochrome P450 destruction readily occurs as a result of the peroxy radicals or protein or porphyrin radicals.

• 2.

  (2) H₂O₂. The relatively high concentration of H₂O₂ required compared with that needed with the alkyl hydroperoxides, and the alkaline pH dependence, suggest that the hydroperoxide ion HO₂⁻ forms a reversible enzyme peroxide complex (Nordblom et al., 1976; Reaction 2). Further protonation and subsequent loss of a molecule of water forms compound I.

• 3.

  (3) NADPH-reductase catalyzed. Binding of a P450 drug substrate greatly enhances the reduction of P450 (Guengerich et al., 1975, 1976) with formation of the ferrous protein (Peterson et al., 1977). Phospholipid enhances this reduction (Guengerich and Coon, 1975). Binding of oxygen is very rapid (Guengerich et al., 1976) and protonation generates the same tertiary peroxide complex formed with H₂O₂.