Cytochrome b5 as electron donor for oxy-cytochrome P-450.

The steady-state concentration of the oxycomplex of microsomal cytochrome P-450 in the presence of NADPH could be elevated by increasing pH and ionic strength or by adding anti-(cytochrome b5) immunoglobulin. Low pH and low ionic strength as well as incorporation of cytochrome b5 into microsomal membranes decreased the steady-state level. Parallel to these effects, the b5 antibody monooxygenase activity at low pH and low ionic strength, but was not effective at high pH and high ionic strength. This establishes a role for cytochrome b5 in donating electrons for the reduction of oxy-cytochrome P-450 to the active oxygen complex of cytochrome P-450 but also points to large variations in the importance of this role depending on the experimental conditions, the species of P-450 involved and the substrates employed.