Comparative studies of N-hydroxylation and N-demethylation by microsomal cytochrome P-450.
The N-hydroxylation of representative aromatic amines by rabbit liver microsomes was mediated by cytochrome P-450 as demonstrated by the sensitivity to carbon monoxide and other cytochrome P-450 inhibitors. The rate of N-hydroxylation was increased by induction with phenobarbital. Involvement of isozyme LM2 (P-50IIB1) was demonstrated in reconstituted systems. Aromatic N-hydroxylation was substantially faster and more efficient than aliphatic N-hydroxylation, while N-demethylation of aromatic and aliphatic dimethylamines was comparable in rate and efficiency. Aliphatic N-hydroxylation showed no rate increase with increasing pH despite the predicted increase in the concentration of the neutral substrate. The relative rates of N-hydroxylation and N-demethylation were compared for a series of para-substituted aromatic amines. The rate of demethylation of para-substituted N,N-dimethylanilines, as measured both by product formation and by NADPH consumption, correlated with the electronic parameter sigma and with the Hansch lipophilicity parameter pi. N-Hydroxylation of a similar series of anilines did not show a dependence on the electronic parameter but was dependent on the lipophilicity parameter. The differing dependence on the electronic parameter suggests that there are different rate-determining processes of N-oxidation for these two reactions.
Burstyn
J N
JN
Department of Pharmacology, UCLA School of Medicine 90024-1735.
Iskandar
M
M
Brady
J F
JF
Fukuto
J M
JM
Cho
A K
AK
D000588:Amines / Q000737:chemistry / Q000378:metabolism; D000814:Aniline Compounds / Q000378:metabolism; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D006900:Hydroxylation; D007700:Kinetics; D008297:Male; D008745:Methylation; D008862:Microsomes, Liver / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D010084:Oxidation-Reduction; D011817:Rabbits
D000588:Amines; D000814:Aniline Compounds; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; C004658:4-chloroaniline