====== CO inhibition and photochemical action spectrum ======

Binding of CO to reduced P450 results in a peak at 450 nm in the [[p450_spectrum|difference spectrum]].

Inhibited, CO-bound P450 can be reactivated by light irradiation at 450 nm. Historically, reversal of CO inhibition was taken as definitive evidence of P450 involvement in a reaction (Estabrook et al., 1963, for the steroid C21 hydroxylation, Cooper et al., 1965, for the p-hydroxylation of acetanilide and the demethylations of codeine and methylaminopyrine). See the reviews by Omura et al., 1965 and Cooper et al., 1979. Similarly, in insects, the //Manduca sexta// ecdysone 20-monooxygenase shows a photochemical action spectrum with a maximum reversal of inhibition at 450 nm (Smith et al. 1979).

Reversal of CO inhibition by white light is experimentally easier, and was shown for instance for the //Locusta migratoria// ecdysone 20-monooxygenase ([[https://doi.org/10.1111/j.1432-1033.1978.tb12420.x|Feyereisen and Durst, 1978]]) and methyl farnesoate epoxidase (see below, from [[https://doi.org/10.1111/j.1432-1033.1981.tb06391.x|Feyereisen et al., 1981]]).{{ ::co-mfelocusta.jpg?600 |}}


Light (450nm) reversal of CO inhibition remains a defining property of P450-catalyzed reactions, and can therefore be used to distinguish P450 enzymes from other enzymes. 

Note that experiments to show CO inhibition should be carried out in CO/O<sub>2</sub> atmosphere, not pure CO, because P450 as monooxygenase needs O<sub>2</sub>. Pure CO would be equivalent to anaerobiosis. Note also that CO is a highly poisonous, odorless, colorless gas and should only be handled in a fume hood.