Interaction of Natural Polyhydroxy-1,4-Naphthoquinones with Superoxide Anion-Radical

A. V. Lebedev^*, M. V. Ivanova, and N. I. Krasnovid

Institute of Experimental Cardiology, Cardiology Research Center, 3-ya Cherepkovskaya ul. 15a, Moscow, 112552 Russia; fax: (7-095) 414-6699; E-mail: avleb@mail.ru

^* To whom correspondence should be addressed.

Received September 18, 1998; Revision received May 25, 1999

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The interaction of natural polyhydroxy-1,4-naphthoquinones (PHNQ) with superoxide anion-radical (O₂) was studied by UV--visible spectrophotometry. 3-Acetyl-2,6,7-trihydroxynaphthazarin (spinochrome C), 2,3,7-trihydroxynaphthazarin (spinochrome D), 2,3,6,7-tetrahydroxynaphthazarin (spinochrome E), 6-ethyl-2,3,7-trihydroxynaphthazarin (echinochrome A), 6-ethyl-2,3,7-trimethoxynaphthazarin (trimethoxyechinochrome A), and 2,3-dihydroxy-6,7-dimethylnaphthazarin (A618) were tested. Xanthine and xanthine oxidase were used to generate **O₂. The interaction with O₂ led to significant time-dependent changes in the spectra of echinochrome A and spinochromes D and E. There was a weak influence of O₂ on the spinochrome C spectrum and no change in the trimethoxyechinochrome A spectrum. The spectra that were transforming during the time of the reaction contained a pronounced isobestic point. This indicates that a single reaction product is being formed. We suggest that 1,2,3,4-tetraketones are formed from 2,3,5,8-tetrahydroxy-1,4-naphthoquinones (echinochrome A and spinochromes D and E) via O₂-induced oxidation of their OH-groups in the 2nd and 3rd positions. Reaction constants were determined by a competitive method using nitro blue tetrazolium (NBT). The reaction constants were about 10⁴-10⁵ M^-1·sec^-1. They decreased in the sequence: echinochrome A > spinochrome D > spinochrome C > NBT > trimethoxyechinochrome A. Thus, we conclude that some of the natural PHNQ containing hydroxyl groups in the 2nd and 3rd positions may act as powerful superoxide anion-radical scavengers.

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KEY WORDS: hydroxynaphthazarins, echinochrome A, spinochromes, weak acid properties, reaction constants, autoxidation, xanthine--xanthine oxidase

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