Toxicology 177 (2002) 67 – 80
Plant phenolic antioxidant and prooxidant activities:
phenolics-induced oxidative damage mediated by metals in
plants
Yasuko Sakihama
a
, Michael F. Cohen
a
, Stephen C. Grace
b
,
Hideo Yamasaki
a,c,
*
a
Laboratory of Cell and Functional Biology, Faculty of Science, Uniersity of the Ryukyus, Nishihara, Okinawa 903 -0213, Japan
b
Department of Biology, Uniersity of Arkansas at Little Rock, 2801 South Uniersity Drie, Little Rock, AR 72204 -1099, USA
c
Diision of Functional Genomics, Center of Molecular Biosciences, Uniersity of the Ryukyus, Nishihara,
Okinawa 903 -0213, Japan
Abstract
Plant phenolic compounds such as flavonoids and lignin precursors are important constituents of the human diet.
These dietary phytophenolics have been recognized largely as beneficial antioxidants that can scavenge harmful active
oxygen species including O
2
-
,H
2
O
2
,
OH, and
1
O
2
. Here we review our current understanding of the antioxidant
and prooxidant actions of phenolics in plant cells. In plant systems, phytophenolics can act as antioxidants by
donating electrons to guaiacol-type peroxidases (GuPXs) for the detoxification of H
2
O
2
produced under stress
conditions. As a result of such enzymatic as well as non-enzymatic antioxidant reactions, phenoxyl radicals are
formed as the primary oxidized products. Until recently, phenoxyl radicals had been difficult to detect by static
electron spin resonance (ESR) because they rapidly change to non-radical products. Application of Zn exerts
spin-stabilizing effects on phenoxyl radicals that enables us to analyze the formation and decay kinetics of the
radicals. The ESR signals of phenoxyl radicals are eliminated by monodehydroascorbate radical (MDA) reductase,
suggesting that phenoxyl radicals, like the ascorbate radical, are enzymatically recycled to parent phenolics. Thus,
phenolics in plant cells can form an antioxidant system equivalent to that of ascorbate. In contrast to their
antioxidant activity, phytophenolics also have the potential to act as prooxidants under certain conditions. For
example, flavonoids and dihydroxycinnamic acids can nick DNA via the production of radicals in the presence of Cu
and O
2
. Phenoxyl radicals can also initiate lipid peroxidation. Recently, Al, Zn, Ca, Mg and Cd have been found to
stimulate phenoxyl radical-induced lipid peroxidation. We discuss the mechanism of phenoxyl radical prooxidant
activity in terms of lifetime prolongation by spin-stabilizing agents. © 2002 Elsevier Science Ireland Ltd. All rights
reserved.
Keywords: Active oxygens; Antioxidant; Oxidative damage; Phenoxyl radical; Plant polyphenol; Prooxidant
www.elsevier.com/locate/toxicol
* Corresponding author. Tel.: +81-98-895-8550; fax: +81-
98-895-8576.
E-mail address: yamasaki@comb.u-ryukyu.ac.jp (H. Ya-
masaki).
1. Introduction
Plant phenolic compounds such as flavonoids
and lignin precursors are potent antioxidants.
0300-483X/02/$ - see front matter © 2002 Elsevier Science Ireland Ltd. All rights reserved.
PII:S0300-483X(02)00196-8