TABLE 1

Apparent kinetic parameters for the N-demethylation of meperidine by human recombinant P450 enzymes

Formation rates were determined using HPLC as described under Materials and Methods. Values represent the mean ± S.E. of an experiment performed in duplicate. The measured intrinsic clearances were normalized using nominal amounts of P450 content in human liver microsomes (Mankowski, 1999; Rodrigues, 1999; Kassahun et al., 2001). All P450 preparations contained cDNA-expressed human P450 reductase. CYP2B6, CYP2C19, and CYP3A4 microsomes were also coexpressed with human cytochrome b5.

Microsomes
Km
Vmax
CLint
Nominal P450 Content in Human Liver Microsomes
Adjusted CLint
Percentage Adjusted CLint
μM pmol/min/pmolP450 μl/min/pmol P450 pmol/mg μl/min/mg
CYP2B6a 356 ± 34 138 ± 3 0.388 39 15.1 57
CYP3A4a 355 ± 81 25.0 ± 1.3 0.070 108 7.56 28
CYP2C19b 131 ± 12 26.9 ± 0.9 0.205 19 3.90 15
CYP2D6a 446 ± 71 4.62 ± 0.35 0.010 10 0.100 <1
CYP3A5c 977 ± 63 9.21 ± 0.24 0.009 1.0 0.009 <1
CYP2C18b
356 ± 47
26.4 ± 1.7
0.074
<2.5
N.D.
N.D.

  • N.D., not determined.

  • ↵ a Data fitted using a Michaelis-Menten model.

  • ↵ b Data fitted using a Michaelis-Menten model with uncompetitive inhibition (p < 0.0001, F test).

  • ↵ c Data fitted using the Hill allosteric model (p < 0.05, F test; Hill coefficient = 1.2 ± 0.1).