TABLE 2

Estimated contribution of P450 isozymes in human female liver microsomes toward the 2-hydroxylation of ethinylestradiol (EE)

P450 Form
Rate of 2-Hydroxylation of EE
pmol P450/mg proteina
 pmol/min/pmol rP450
pmol/min/mgb
 %TNRc
 %1d
CYP1A1 NAe 0.246 NDf NDf NDf
CYP1A2 8.9 0.013 0.1 2.4 6.0
CYP2C8 NDf 0.008 NDf NDf 26
CYP2C9 44 0.025 1.1 23 24
CYP2C19 19 0.021 0.4 8.0 NDf
CYP3A4 22 0.134 2.9 61 52
CYP3A5 4.1 0.068 0.3 5.8 NDf
TNR
4.8

  • ↵ a The abundance of cytochrome P450 protein in native human liver microsomes was quantified by Western blot with P450 isozyme-specific polyclonal antibody

  • ↵ b Rate was normalized using the specific content of each P450 isozyme in native human liver microsomes (Table 1). For example, the normalized rate for CYP3A4 = rate produced by rCYP3A4 (0.134 pmol/min/pmol rP450) × CYP3A4 content in liver microsomes (22 pmol/mg protein) = 2.9 pmol/min/mg

  • ↵ c Data expressed as percentage of TNR, where TNR = 4.8 pmol/min/mg

  • ↵ d Inhibition of 2-hydroxylation of [3H]EE (5 μM) by isozyme-specific monoclonal antibodies (5 μl) in a pool (n = 7) of female human liver microsomes

  • ↵ e NA, not applicable

  • ↵ f ND, not determined since no isozyme-specific antibodies were available for the inhibition study or the immunoblotting assays