Biphasic O-deethylation of phenacetin and 7-ethoxycoumarin by human and rat liver microsomal fractions.
Human and rat liver microsomal fractions exhibit non-linear Michaelis-Menten kinetics in the O-deethylation of both phenacetin and 7-ethoxycoumarin. Comparison of various models indicated that the data were best described by a biphasic plot, which could be interpreted in terms of two populations of cytochrome P-450. The K(m)'s of the high affinity phase of 7-ethoxycoumarin O-deethylase activity were 1.8 +/- 0.4 microM and 2.3 +/- 0.4 microM for human and rat respectively while the K(m)'s of the low affinity phase were 205 +/- 20 microM and 237 +/- 59 microM in the two species respectively. V(max) of the high affinity phase of human 7-ethoxycoumarin O-deethylase activity was 96.9 +/- 19.0 pmol mg(-1) min(-1) and the activity of the corresponding phase in the rat was 2.7 times greater. The activities of the low affinity phase were 10-15 times greater than the respective activity of the high affinity phase. Rat and human also had similar values for the K(m)'s of the two phases of phenacetin O-deethylase activity, around 5 microM for the high affinity phase and 300 microM for the low affinity phase. Total activity was very similar in the two species, 1500-1750 pmol mg(-1) min(-1) and the difference between the two phases of activity was only 2.5-fold in man and 10-fold in rat. Studies on the effects of the in vitro modifiers of monooxygenase activity alpha-naphthoflavone and metyrapone further supported the hypothesis that the two phases of O-deethylase activity represent two different forms or populations of cytochrome P-450.
Department of Clinical Pharmacology, Royal Postgraduate Medical School, Ducane Road, London W12 0HS, UK.
A R
AR
Boobis
G C
GC
Kahn
C
C
Whyte
M J
MJ
Brodie
D S
DS
Davies
D015660:7-Alkoxycoumarin O-Dealkylase / Q000378:metabolism; D000818:Animals; D001571:Benzoflavones / Q000378:metabolism; D003374:Coumarins / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008297:Male; D008797:Metyrapone / Q000378:metabolism; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D010615:Phenacetin / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D013347:Subcellular Fractions; D013379:Substrate Specificity
D001571:Benzoflavones; D003374:Coumarins; C017299:7-ethoxycoumarin; C011512:alpha-naphthoflavone; D003577:Cytochrome P-450 Enzyme System; D015660:7-Alkoxycoumarin O-Dealkylase; D010615:Phenacetin; D008797:Metyrapone