Antagonistic action of imidazolineoxyl N-oxides against endothelium-derived relaxing factor/.NO through a radical reaction

Biochemistry. 1993 Jan 26;32(3):827-32. doi: 10.1021/bi00054a013.


A labile inorganic free radical, nitric oxide (.NO), is produced by nitric oxide synthase from the substrate L-arginine in various cells and tissues. It acts as an endothelium-derived relaxing factor (EDRF) or as a neurotransmitter in vivo. We investigated the reactivity of stable radical compounds, imidazolineoxyl N-oxides such as 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), carboxy-PTIO, and carboxymethoxy-PTIO against .NO/EDRF in both chemical and biological systems. By using electron spin resonance (ESR) spectroscopy, imidazolineoxyl N-oxides were found to react with .NO in a stoichiometric manner (PTIO/.NO = 1.0) in a neutral solution (sodium phosphate buffer, pH 7.4) with rate constants of approximately 10(4) M-1 s-1, resulting in the generation of NO2-/NO3- and imidazolineoxyls such as 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl (PTI), carboxy-PTI, or carboxymethoxy-PTI. Furthermore, the effects of imidazolineoxyl N-oxides on acetylcholine- or ATP-induced relaxation of the smooth muscle of rabbit aorta were tested. The vasorelaxations were inhibited by all three imidazolineoxyl N-oxides markedly. The inhibitory effects of carboxy-PTIO was almost 2-fold stronger than those of .NO synthesis inhibitors, N omega-nitro-L-arginine and N omega-monomethyl-L-arginine. Generation of EDRF/.NO was identified by reacting the PTIO in aortic strips and quantitating the reaction product with ESR spectroscopy. Thus, it was clarified that imidazolineoxyl N-oxide antagonize EDRF/.NO via a unique radical-radical reaction with .NO.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetylcholine / pharmacology
  • Animals
  • Aorta / drug effects*
  • Benzoates / chemical synthesis
  • Benzoates / pharmacology
  • Cyclic N-Oxides / chemical synthesis
  • Cyclic N-Oxides / metabolism
  • Cyclic N-Oxides / pharmacology
  • Dose-Response Relationship, Drug
  • Female
  • Free Radicals
  • Imidazoles / chemical synthesis
  • Imidazoles / chemistry
  • Imidazoles / metabolism
  • Imidazoles / pharmacology
  • Muscle Relaxation / drug effects*
  • Muscle, Smooth, Vascular / drug effects*
  • Nitric Oxide / antagonists & inhibitors*
  • Nitric Oxide / metabolism*
  • Rabbits


  • Benzoates
  • Cyclic N-Oxides
  • Free Radicals
  • Imidazoles
  • 1,3-dihydroxy-4,4,5,5-tetramethyl-2-(4-carboxyphenyl)tetrahydroimidazole
  • 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide
  • 2-phenyl 4,4,5,5-tetramethylimidazoline-1-oxyl
  • Nitric Oxide
  • imidazole
  • Acetylcholine