One-electron photoreduction of bacterial cytochrome P-450 by ultraviolet light. I. Steady-state irradiations.

Bacterial ferric cytochrome P-450 can be photoreduced by ultraviolet light in presence of camphor as shown by the formation of the ferrous cytochrome P-450--CO complex absorbing at 446 nm in CO-saturated solutions. This photoreduction can only be performed with wavelengths (lambda less than 315 nm) corresponding to the absorption of light by aromatic residues and in particular, tryptophan. The primary process leading to the photoreduction is thus the photoionization of tryptophan residue(s) near the active site of the protein as deduced from competitive electron scavenging with N2O. In agreement with results obtained with chemical reduction, the quantum yield of photoreduction is proportional to the content of the high-spin state of the ferric cytochrome P-450 solution. It is also shown that the camphor concentration affects the CO binding.