Pronounced differences in inhibition potency of lactone and non-lactone compounds for mouse and human coumarin 7-hydroxylases (CYP2A5 and CYP2A6)

1. The structural requirements for a compound to be a potent inhibitor for mouse CYP2A5 and human CYP2A6 enzymes catalysing coumarin 7-hydroxylase activity have been studied. 2. The IC₅₀ of 28 compounds for the pyrazole-treated male DBA/2 mouse and human liver microsomal coumarin 7-hydroxylation were determined at 10 muM coumarin concentration 15 times over K_m of coumarin. 3. The three most potent inhibitors for CYP2A5 were gamma-nonanoic lactone, gamma-decanolactone and gamma-phenyl-gamma-butyrolactone with an IC₅₀=1.9 +/- 0.4, 2.1 +/- 0.2 and 2.4 +/- 0.3 muM and for CYP2A6 7-methylcoumarin, butylcyclohexane and indan with an IC₅₀=30 +/- 3.2, 43 +/- 9 and 50 +/- 11 muM. 4. Among the 28 compounds studied, only 2-benzoxazolinone, 2-indanone and gamma- valerolactone showed similar inhibitory activity in both species. Indan had a lower IC₅₀ for human than for mouse coumarin 7-hydroxylation, whereas the IC₅₀ of 24 other compounds was higher for human than for mouse coumarin 7-hydroxylation. 5. The largest difference in IC₅₀ between mouse and human activity was observed with 5-substituted phenyl, pentyl, hexyl, heptyl or octyl gamma-lactones or 6-substituted delta-lactones. IC₅₀ of gamma-undecanolactone and gamma-decanolactone was 500 times lower for mouse than human coumarin 7-hydroxylation. 6. The difference in the IC₅₀ between human and mouse coumarin 7-hydroxylation decreased substantially with the corresponding compounds without the lactone ring. 7. It is concluded that certain 5- or 6-position substituted gamma- and delta -lactones are potent inhibitors for mouse CYP2A5 but not for the orthologous human CYP2A6 and that the active site of CYP2A6 could be smaller than the active site of CYP2A5.