Figure 7.

Effect of cytochrome b₅on spectral interaction of cytochome P450s (P450s) with ketoconazole. P450 (2 μM, final) was mixed with ketoconazole in the absence or presence of 2 μM cytochrome b₅. A, P450 3A4 (±cytochrome b₅) mixed with 2 μM ketoconazole. B, second-order plot of binding data (from part A, the absence of cytochrome b₅). With equal amounts of P450 3A4 and ketoconazole, the reaction X + Y → Z (where X is P450 3A4, Y is ketoconazole, and Z is the complex) is mathematically equivalent to 2X → Z and a plot of 1/(unbound P450 [X]) versus time yields an apparent second-order rate constant as the slope (5 × 10⁵ M⁻¹ s⁻¹) (52).