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 1-hydroxylase act ivity have been studied. 2. The IC50 of 28 compounds for the pyrazole-treated male DBA/2 mouse and h uman liver microsomal coumarin 7-hydroxylation were determined at 10 mu M c oumarin concentration 15 rimes over K-m of coumarin. 3. The three most pote nt inhibitors for CYP2A5 were gamma-nonanoic lactone, gamma-decanolactone a nd gamma-phenyl-gamma-butyrolactone with an IC50 approximate to 1.9 +/- 0.4 , 2.1 +/- 0.2 and 2.4 +/- 0.3 mu M and for CYP2A6 7-methylcoumarin, butylcy clohexane and indan with an IC50 = 30 +/- 3.2, 43 +/- 9 and 50 +/- 11 mu M. 4. Among the 28 compounds studied, only 2-benzoxazolinone, 2-indanone and g amma-valerolactone showed similar inhibitory activity in both species. Inda n had a lower IC50 for human than for mouse coumarin 7-hydroxylation, where as the IC50 of 24 other compounds was higher for human than for mouse couma rin 7-hydroxylation. 5. The largest difference in IC50 between mouse and human activity was obse rved with 5-substituted phenyl pentyl, hexyl, heptyl or octyl gamma-lactone s or 6-substituted delta-lactones. IC50 of gamma-undecanolactone and gamma- decanolactone was 500 times lower for mouse than human coumarin 7-hydroxyla tion. 6. The difference in the IC50 between human and mouse coumarin 7-hydroxylat ion decreased substantially with the corresponding compounds without the la ctone ring. 7. It is concluded that certain 5- or 6-position substituted gamma- and del ta-lactones are potent inhibitors for mouse CYP2A5 but not for the ortholog ous human CYP2A6 and that the active site of CYP2A6 could be smaller than t he active site of CYP2A5.