Relationship between Effects of Phenolic Compounds on the Generation of Free Radicals from Lactoperoxidase-Catalyzed Oxidation of NAD(P)H or GSH and their DPPH Scavenging Ability.

Abstract

Reactive oxygen species such as superoxide radicals (O 2 · Ϫ ), hydroxyl radicals, and singlet oxygen have been implicated both in the aging process and in degenerative disease. [1][2][3][4] Phenolic compounds are known to act as poisons under certain conditions. The mechanism of toxicity of phenolic compounds such as phenol 5) and 17b -estradiol 6) is suggested to be the generation of O 2 · Ϫ during the reaction of the compounds with oxidative enzymes (peroxidases, tyrosinase, prostaglandin synthase, etc.) in the presence of biological reductants such as reduced b -nicotinamide adenine dinucleotide (NADH), reduced b -nicotinamide adenine dinucleotide phosphate (NADPH) or glutathione (GSH). On the other hand, phenolic compounds that scavenge O 2 · Ϫ may be of potential therapeutic use as antioxidants. 7,8) In consideration of these points, we are interested in elucidating how phenolic compounds can enhance the generation of O 2 · Ϫ from the reaction of lactoperoxidase (LPO) with NAD(P)H and GSH, and whether there is a relationship between the enhancing activity of the phenolic compounds and their electrochemical properties. LPO is a mammalian hemoprotein found in saliva, tears, and milk, and forms part of the antimicrobial defense system. As the electrochemical data for phenolic compounds have been obtained under a wide range of experimental conditions, however, it is difficult to make direct comparisons. Thus, the ability of a phenolic compound to scavenge a stable radical, 1,1-diphenyl-2-picrylhydrazyl (DPPH), was used in this study to assess the electrochemical properties of the compounds. 1,1-Diphenyl-2-picrylhydrazyl is a radical widely used for the rough estimation of the antioxidant activity of phenolic compounds. The ability of a phenolic compound to scavenge DPPH is recently suggested to be related to the redox potential.9) The chemical structure of the nineteen phenolic compounds used in this study are depicted in Fig. 1. The compounds are 17b -estradiol, phenol, p-chlorophenol, p-eugenol, isoeugenol, 3,4-dimethylphenol, quercetin, Trolox C, diethylstilbestrol, bisphenol A, 4-hydroxyphenylacetic acid, 4-hydroxybenzoic acid, acetaminophen, guaiacol, p-methoxyphenol, curcumin, 2-hydroxyestradiol, gallic acid, and 3(2)-tert-butyl-4-hydroxyanisole. Phenolic dental medicaments including phenol, pchlorophenol, p-eugenol, isoeugenol, and guaiacol have been used for disinfection and sedative treatment for pulpitis in dental practice.10) We report here that the ability of phenolic compounds to enhance the LPO/H 2 O 2 -catalyzed oxidation of NAD(P)H or GSH is in inverse proportion to their quenching activity against the DPPH. Materials and MethodsMaterials 17b -Estradiol was purchased from Steraloids Inc. (U.S.A.). p-Chlorophenol, diethylstilbestrol (DES), reduced b -nicotinamide adenine dinucleotide (NADH), lactoperoxidase (LPO), catalase, superoxide dismutase (SOD), and reduced glutathione (GSH) were purchased from Sigma Chemical Co. (U.S.A.). Trolox ® (abbreviated as Trolox C hereinafter) was obtained fro...

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