Fig. 4. (Top ) The effect of substrate concentration on propofol hydroxylase activity in microsomes from β-lymphoblastoid cells expressing either CYP2B6 or CYP2C9. Activities are expressed as the rate of 4-hydroxypropofol formation normalized to incubation time and microsomal protein content. The solid line connects best-fit estimates of these data determined by nonlinear least-squares curve fit to either equation 1(CYP2B6) or equation 3(CYP2C9). Estimates for K  ^mand V^maxwere 10 ± 2 μm and 21 ± 1 nmoles · min⁻¹· nmole⁻¹CYP for CYP2B6, and were 41 ± 8 μm and 34 ± 4 nmoles · min⁻¹· nmole⁻¹CYP with a Hill exponent (n) of 1.5 for CYP2C9. (Bottom ) Eadie- Hofstee transformation of these data. V = reaction velocity, nmoles · mg⁻¹· min⁻¹; V/[S]= reaction velocity divided by propofol concentration at that velocity, nmoles · mg⁻¹· min⁻¹·μm⁻¹.