Inhibition of cytochromes P450 by nitric oxide and a nitric oxide-releasing agent

Arch Biochem Biophys. 1993 Jan;300(1):115-23. doi: 10.1006/abbi.1993.1016.


The effect of nitric oxide (NO) on cytochrome P450-mediated benzyloxyresorufin and ethoxyresorufin O-dealkylase activity of rat hepatic postmitochondrial (S-9) or microsomal subfractions, or purified rat liver CYP2B1, was examined. Two distinct inhibitory phases were observed regardless of whether the NO was added prior to initiation of the reactions with NADPH or during the course of substrate turnover. The first was a reversible inhibition characterized by complete cessation of catalytic activity, the duration of which was NO concentration-dependent with an IC50 in the range of 8-60 microM. This phase was followed by a second, irreversible, inhibitory phase characterized by a varying extent of recovery of activity, with inhibition ranging from < 1 to approximately 100%. The extent of this diminution in substrate conversion rate was also NO concentration-dependent, with an IC50 in the range of 13-72 microM, and could be partially abrogated by the inclusion of bovine serum albumin in the reaction mixture. Lower IC50 values for both inhibitory phases were obtained in the case of benzyloxyresorufin O-dealkylase activity than in the case of ethoxyresorufin O-dealkylase activity, suggesting a differential susceptibility to inhibition by NO for the two O-dealkylase activities. A nitric oxide-releasing compound ([Et2NN(O)NO]Na,DEA/NO) caused qualitatively similar inhibitory effects on benzyloxyresorufin O-dealkylase activity when added prior to the initiation of the reactions with NADPH, or during the course of substrate turnover. Based upon these results, it is possible that NO may play a role in the regulation of P450 activity in vivo.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Cytochrome P-450 CYP1A1
  • Cytochrome P-450 CYP2B1
  • Cytochrome P-450 Enzyme System / metabolism*
  • Cytosol / metabolism
  • Kinetics
  • Liver / enzymology
  • Male
  • Mathematics
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology*
  • Mitochondria, Liver / drug effects
  • Mitochondria, Liver / enzymology*
  • Nitric Oxide / pharmacology*
  • Oxidoreductases / metabolism*
  • Phenobarbital / pharmacology
  • Rats
  • Rats, Inbred F344
  • Serum Albumin, Bovine / pharmacology
  • Spectrometry, Fluorescence


  • Serum Albumin, Bovine
  • Nitric Oxide
  • Cytochrome P-450 Enzyme System
  • Oxidoreductases
  • Cytochrome P-450 CYP1A1
  • Cytochrome P-450 CYP2B1
  • Phenobarbital