Table 2

Kinetic analysis of the inhibition effects when diazepam and testosterone are incubated simultaneously with B-lymphoblastoid microsomes expressing CYP3A4 and CPR

Model/EquationVmaxKsKaKiαβ
pmol/min/pmol CYP μM
3HDZ formation and activation by TSModel 1C/eq.5 2-a 21  (1.2)638 (41)148 (40)0.1 (0.02)22-b
Model 1B/eq. 4 24  (2)694 (112)137 (55)0.1 (0.04)2.0 (0.2)
NDZ formation and activation by TSModel 1B/eq. 4 3.9  (0.1)407 (41)38 (3)0.2 (0.04)1.8 (0.1)
6β-HTS formation and inhibition by DZModel 1D/eq. 6 19  (0.2)137 (12)186 (6)0.1 (0.02)22-b

The kinetic parameters were determined using rate equation models as listed above using GRAFIT v3.0 (Erithacus Software Ltd). Values in parentheses represent standard error values.

  • 2-a  The data set at the highest testosterone concentration is excluded from the fit to model 1C because there is a combination of activation and inhibition occurring at the high concentrations suggesting a competitive process, as described by model 1B.

  • 2-b  Assuming that the catalytic sites are equivalent, β is equal to 2 in the equation and cancels out because Vmax is equal to 2Kp/[E]t, where [E]t is the total enzyme concentration.