Inhibition of hepatic microsomal monooxygenase activity by cinchocaine: mechanistic studies and effects of ionization.
The quinoline-based local anaesthetic cinchocaine (dibucaine) was found to be a mixed-type inhibitor of microsomal aminopyrine N-demethylase and 7-ethoxycoumarin O-deethylase activities from control and phenobarbitone-induced rat liver in-vitro. Cinchocaine also elicited a characteristic type I optical difference spectrum in oxidized liver microsomes (Ks = 24 microM; delta Amax = 3.4 X 10(-3) absorbance units (nmol cytochrome P450)-1) but did not appear to bind to the reduced form of the cytochrome. Additional studies indicated that cinchocaine competitively inhibited the type I spectral binding of substrate (aminopyrine) to ferric cytochrome P450. Studies of monooxygenase inhibition by cinchocaine over a relatively narrow pH range (6.5-8.5) indicated that, as might be expected, the un-ionized form of the drug is associated with inhibitory potency superior to that of the ionized form. Thus 40% inhibition of aminopyrine N-demethylase activity was observed with 100 microM cinchocaine at pH 8.0 and 8.5 (24% and 50% un-ionized drug, respectively), whereas only 16% inhibition was observed at pH 6.5 (1% un-ionized drug). These findings suggest that the inhibitory action of cinchocaine is mediated exclusively via an interaction with ferric cytochrome P450 and that the extent of ionization is a determinant of mixed function oxidase inhibition.
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D015660:7-Alkoxycoumarin O-Dealkylase; D000633:Aminopyrine N-Demethylase / Q000037:antagonists & inhibitors; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003992:Dibucaine / Q000494:pharmacology*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors*; D010105:Oxygenases / Q000037:antagonists & inhibitors; D051381:Rats; D011919:Rats, Inbred Strains
D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D010105:Oxygenases; D015660:7-Alkoxycoumarin O-Dealkylase; D000633:Aminopyrine N-Demethylase; D003992:Dibucaine