Interindividual variability in catalytic activity and immunoreactivity of three major human liver cytochrome P450 isozymes.
OBJECTIVE
Interindividual variations in immunoreactivity and function of three major human drug metabolising P450 monooxygenases has been investigated in liver microsomes from 42 Caucasians (kidney donors or liver biopsies).


METHODS
Diclofenac 4'-hydroxylation, dextromethorphan O-demethylation and midazolam 1'-hydroxylation, measured by HPLC in incubates, were used as probes to determine CYP2C9, CYP2D6 and CYP3A4 function kinetics, respectively. Immunoquantification of the three isoforms was achieved by Western blotting, using rabbit polyclonal antibodies raised against human CYP2C9 and human CYP3A4, and mouse monoclonal antibody raised against human CYP2D6.


RESULTS
Diclofenac 4'-hydroxylation exhibited Michaelis-Menten kinetics with kM = 3.4 mumol.l-1 and Vmax = 45 nmole.mg-1 P.h-1. Relative immunoreactivity of CYP2C9 was correlated with Vmax and CL(int). Dextromethorphan O-demethylation in EM (extensive metabolisers) liver microsomes also showed Michaelis-Menten kinetics, with kM = 4.4 mumol.l-1 and Vmax = 5.0 nmol.mg-1 P.h-1. Relative immunoreactivity of CYP2D6 was correlated with Vmax and CL(int). Midazolam 1'-hydroxylation also exhibited Michaelis-Menten kinetics with kM = 3.3 mumol.l-1 and Vmax = 35 nmol.mg-1 P.h-1. Relative immunoreactivity of CYP3A4 was correlated with Vmax and CL(int). Immunoreactivity and function were correlated for each isozyme, but there was no cross correlation between isozymes.


CONCLUSIONS
The velocity of metabolite formation (Vmax) by the three major human drug metabolising P450 monoxygenases is correlated with their immunoreactivity in liver microsomes. Interindividual variation was much larger for Vmax than kM. Interindividual variability was more pronounced for CYP2D6, probably due to the presence of several different functional alleles in the population of extensive metabolisers.
Transon
C
C
Division of Clinical Pharmacology, University Hospitals, Geneva, Switzerland.
Lecoeur
S
S
Leemann
T
T
Beaune
P
P
Dayer
P
P
D000328:Adult; D000367:Age Factors; D018686:Anesthetics, Intravenous / Q000378:metabolism; D000818:Animals; D000996:Antitussive Agents / Q000378:metabolism; D001189:Aryl Hydrocarbon Hydroxylases; D016861:Cyclooxygenase Inhibitors / Q000378:metabolism; D065729:Cytochrome P-450 CYP2C9; D019389:Cytochrome P-450 CYP2D6 / Q000276:immunology / Q000378:metabolism*; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000276:immunology / Q000378:metabolism*; D003915:Dextromethorphan / Q000378:metabolism; D004008:Diclofenac / Q000378:metabolism; D005260:Female; D006801:Humans; D006900:Hydroxylation; D007527:Isoenzymes / Q000276:immunology / Q000378:metabolism*; D008297:Male; D008745:Methylation; D051379:Mice; D008862:Microsomes, Liver / Q000201:enzymology*; D008874:Midazolam / Q000378:metabolism; D006899:Mixed Function Oxygenases / Q000276:immunology / Q000378:metabolism*; D011817:Rabbits; D012044:Regression Analysis; D012737:Sex Factors; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000276:immunology / Q000378:metabolism*; D044465:White People
D018686:Anesthetics, Intravenous; D000996:Antitussive Agents; D016861:Cyclooxygenase Inhibitors; D007527:Isoenzymes; D004008:Diclofenac; D003915:Dextromethorphan; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; C104464:CYP3A protein, human; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; D039181:Steroid 16-alpha-Hydroxylase; C510163:CYP3A4 protein, human; D008874:Midazolam