Journal Article

A Long-lived <i>o</i>-Semiquinone Radical Anion is Formed from <i>N</i>-β-alanyl-5-S-glutathionyl-3,4-dihydroxyphenylalanine (5-S-GAD), an Insect-derived Antibacterial Substance

Nobuko Akiyama, Ikuo Nakanishi, Kei Ohkubo, Kazue Satoh, Koichiro Tsuchiya, Takeshi Nishikawa, Shunichi Fukuzumi, Nobuo Ikota, Toshihiko Ozawa, Masafumi Tsujimoto and Shunji Natori

in The Journal of Biochemistry

Published on behalf of The Japanese Biochemical Society

Volume 142, issue 1, pages 41-48
Published in print July 2007 | ISSN: 0021-924X
Published online July 2007 | e-ISSN: 1756-2651 | DOI: http://dx.doi.org/10.1093/jb/mvm101
A Long-lived o-Semiquinone Radical Anion is Formed from N-β-alanyl-5-S-glutathionyl-3,4-dihydroxyphenylalanine (5-S-GAD), an Insect-derived Antibacterial Substance

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N-β-Alanyl-5-S-glutathionyl-3,4-dihydroxyphenylalanine (5-S-GAD), an insect-derived antibacterial peptide, generates hydrogen peroxide (H2O2) that exerts antitumour activity. We have investigated the precise mechanism of H2O2 production from 5-S-GAD by autoxidation aiming to understand its action toward tumour cells. Using the electron spin resonance (ESR) technique, we detected a strong signal due to radical formation from 5-S-GAD. Surprisingly, the ESR signal of the radical derived from 5-S-GAD appeared after incubation for 30 min at 37°C in the buffer at pH 7.4; the signal was persistently detected for 10 h in the absence of catalytic metal ions. The computer simulation of the observed ESR spectrum together with the theoretical calculation of the spin density of the radical species indicates that an o-semiquinone radical anion was formed from 5-S-GAD. We demonstrated that H2O2 is produced via the formation of superoxide anion ([math]) by the electron-transfer reduction of molecular oxygen by the 5-S-GAD anion, which is in equilibrium with 5-S-GAD in the aqueous solution. The radical formation and the subsequent H2O2 production were inhibited by superoxide dismutase (SOD), when the antitumour activity of 5-S-GAD was inhibited by SOD. Thus, the formation of the o-semiquinone radical anion would be necessary for the antitumour activity of 5-S-GAD as an intermediate in the production of cytotoxic H2O2.

Keywords: electron spin resonance; hydrogen peroxide; o-semiquinone radical anion; superoxide dismutase; 5-S-GAD

Journal Article.  4593 words.  Illustrated.

Subjects: Biochemistry

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