Recently, we proposed that singlet oxygen (
1O
2) plays an essential role in microsomal cytochrome P450 (P450)-dependent
p-hydroxylation of aniline and
O-deethylation of 7-ethoxycoumarin. We then examined whether the role of
1O
2 is general in the P450-dependent substrate oxidations. In the present study, we examined ω- and (ω-1)-hydroxylations of lauric acid,
O-demethylation of
p-nitroanisole, and
N-demethylation of aminopyrine in rat liver microsomes. The addition of β-carotene and NaN
3 significantly suppressed these reactions in a concentration-dependent manner, and
1O
2 during the reactions was detected by ESR spin-trapping using 2,2,6,6-tetramethyl-4-piperidone (TMPD) as a
1O
2-spin trapping reagent, where the addition of
1O
2 quenchers, SKF-525A as a P450 inhibitor, or
p-nitroanisole decreased ESR signal intensities due to TMPD-
1O
2 adduct. Next, we examined the effect of
1O
2 quenchers on P450-dependent reactions in the human liver microsomes, and
1O
2 was also indicated to be an active species in substrate hydroxylations and dealkylations such as nifedipine oxidation by CYP3A4. On the basis of the results, we concluded that
1O
2 is an essentially important active oxygen species in both rat and human P450-dependent substrate oxidations.
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