In vitro activation of 7-benzyloxyresorufin O-debenzylation and nifedipine oxidation in human liver microsomes

Author： Niwa T. Shiraga T. Yamasaki S. Ishibashi K. Ohno Y. Kagayama A.

ISSN： 1366-5928

Source： Xenobiotica, Vol.33, Iss.7, 2003-07, pp. : 717-729

Disclaimer: Any content in publications that violate the sovereignty, the constitution or regulations of the PRC is not accepted or approved by CNPIEC.

Previous Menu Next

Abstract

1. The effects of substrate concentration and enzyme source (human liver microsomes and recombinant cytochrome P450s, CYP) on the activation of 7-benzyloxyresorufin O-debenzylation and nifedipine oxidation were investigated.2. 7-Benzyloxyresorufin O-debenzylase activity in human liver microsomes was inhibited by a monoclonal antibody against CYP2B6 and a polyclonal antibody against CYP3A2 by 53-69 and 19-44%, respectively, suggesting that CYP2B6 and CYP3A4 mainly catalyse 7-benzyloxyresorufin O-debenzylation in human liver microsomes.3. 7-Benzyloxyresorufin O-debenzylase activity at 0.2-5 μM substrate concentrations in human liver microsomes was increased by the addition of -naphthoflavone, quinidine, testosterone and progesterone, and the V_max of 7-benzyloxyresorufin O-debenzylation increased with increasing -naphthoflavone concentrations, whereas the K_m remained constant. Additionally, 7-benzyloxyresorufin O-debenzylation by recombinant CYP3A4 was increased by the addition of -naphthoflavone, testosterone and progesterone but not by quinidine, whereas no chemicals tested could activate the O-debenzylation of 7-benzyloxyresorufin by CYP2B6.4. The K_m for nifedipine oxidation activity by CYP3A4 decreased by the addition of progesterone, whereas the V_max remained constant. Quinidine and testosterone increased 7-benzyloxyresorufin O-debenzylase and nifedipine oxidase activities, respectively, in human liver microsomes, whereas activation was not observed in CYP3A4.5. The results suggest that in vitro activation patterns are substrate dependent and that selection of the enzyme source can influence the activation phenomenon.