Interaction of natural polyhydroxy-1,4-naphthoquinones with superoxide anion-radical

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.