Reductive titration of cytochrome P-450 in rat liver microsomes.

Microsomal cytochromes b5 and P-450 of rat liver have been titrated with standardized sodium dithionite solution with a newly developed titrating apparatus that allows spectrophotometric monitoring of the reduction process while strictly O2-free conditions are maintained throughout the procedure. Cytochrome b5 and other electron acceptors in the microsomal preparation were saturated with reducing equivalents prior to addition of carbon monoxide to the system. Continued titration in the presence of CO revealed that 1 electron equivalent was required for the formation of P-450(Fe2+)-CO. These results are in agreement with previous findings of 1 electron equivalence for cytochrome P-450 of adrenocortical mitochondria and P-450CAM.