| Abstract: | Reduced rabbit liver microsomal cytochrome P-450 binds nitrite to yield an optical absorbance spectrum characterized by a decrease 1n absorbance around 415 nm and an increase in absorbance at 446 nm. A spectral dissociation constant of 36[subscript mu]M was calculated for the reduced cytochrome P-450-nitrite complex. There is mutual inhibition of binding of carbon monoxide, metyrapone, nitromethane, oxygen, and nitrite to reduced cytochrome P-450. The extent of nitrite inhibition of ligand binding is dependent on the nitrite concentration and the preincubation time of microsomes in nitrite . This inhibition is reversed by ferricyanide which seems to dissociate the cytochrome P-450-nitrite complex. Nitrite does not inhibit aminopyrine, p-nitroanisole, or zoxazolamine binding; it does inhibit aniline binding to reduced cytochrome P-450. N itrite binding leads to significant inhibition of mixed function oxidase activity only when microsomes are incubated with nitrite anaerobically for 1-2 h. The inhibition is irreversible due to the formation of cytochrome P-420. I t is proposed that nitrite is reduced by cytochrome P-450 to form nitric oxide which binds to cytochrome P-450 to convert it to cytochrome P-420. Irreversible heme oxidation, nitrosothiol formation, and lip id peroxidation do not play a role in the inactivation of cytochrome P-450. Nitrobenzene reductase activity is also inhibited by nitrite but the inhibition is reversible, not involving the formation of cytochrome P-420. The inhibition constant is 39[subscript mu]M nitrite which indicates that nitrite binds to cytochrome P-450 inhibiting subsequent nitrobenzene binding. |
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