The reversal of the carbon monoxide inhibition by bands of monochromatic light was determined for the oxidative demethylation of codeine and monomethyl-4-aminopyrine and the hydroxylation of acetanilide by rat liver microsomes and for the hydroxylation of 17-hydroxyprogesterone at carbon-21 by bovine adrenocortical microsomes. Maximum reversal occurred at 450 millimicrons, the light absorption maximum of the CO compound of the CO-binding pigment of microsomes. The agreement between photochemical action spectrum and spectrophotometric difference spectrum supports the conclusion that the CO-binding pigment is the terminal oxidase of mixed function oxidase systems of mammals.

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Quenching of the tryptophanyl fluorescence of cytochrome P-450C-21 by acrylamide and its relationship to substrate binding are investigated by using steady-state and time-resolved data. The average collisional quenching constant was 0.4 M whereas the quenching constant for the total fluorescence was 10.8 +/- 0.9 M. This indicates that the quenching is essentially static. The quencher inhibited the binding of the substrate apparently competitively. The inhibition constant was 0.092 M, giving rise to an association constant of 10.9 M which is remarkably similar to the static quenching constant. It is suggested that tryptophan(s) may represent a key to the substrate-binding site in P-450C-21.

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