Effects of oxygen on the relative photodissociability of cytochrome P-450.CO complex in rat liver microsomes.

Flash photolysis of cytochrome P-450 in phenobarbital-induced rat liver microsomes was examined by observing the decay of absorbance change, A(t), at 450 nm after photolysis of the heme.CO complex by a depolarized laser flash. The relative photodissociability, A(0)/Ab, is significantly decreased upon an increase in oxygen concentration. A(0)/Ab is 96%, 86%, and 51% at oxygen concentrations of 90, 115, and 155 microM when compared with A(0)/Ab at 0 microM O2. In the presence of aminopyrine, the oxygen-dependent decrease in the relative photodissociability was enhanced about 2 times. The rate of CO recombination with cytochrome P-450 was decreased by increasing O2 concentration. By going from 0 to 155 microM O2, about 17% decrease and 58% decrease in the recombination rate were observed in the absence and presence of aminopyrine, respectively. In the absence of CO, no absorbance change A(t) at 450 nm was observed even at 155 microM O2, excluding a possible contribution of P-450.O2. The present observation leads to the speculation that oxygen molecules around the heme.CO complex affect the electronic state of the heme, resulting in an increase in Fe-CO bond strength.