How do calcium ions induce free radical oxidation of hydroxy-1,4-naphthoquinone? Ca2+ stabilizes the naphthosemiquinone anion-radical of echinochrome A.
A surprising effect is the direct action of Ca(2+) on redox reactions of ortho-quinoid compounds. The effect of Ca(2+) on oxidation of the sea urchin pigment 6-ethyl-2,3,5,7,8-pentahydroxy-1,4-naphthoquinone (echinochrome A) has been studied by electron paramagnetic resonance (EPR) spectroscopy, by UV/VIS absorbance spectroscopy, and by measurement of oxygen consumption. Echinochrome A per se reacted with dioxygen only in an alkaline solution; 2,3-semiquinone anion-radical of echinochrome A and superoxide anion-radical were the intermediates of the oxidation. Addition of calcium ions sharply increased the rate of echinochrome A autooxidation at alkaline pH and provoked oxidation at neutral pH. To explain this phenomenon we have focused on changes of the acid-base properties of echinochrome A in the presence of calcium and on stabilization of 2,3-semiquinone anion-radical of echinochrome A by Ca(2+). Dissociation constants (pK(a1), pK(a2), and pK(a3)) of echinochrome A determined by potentiometric titration were 5.20, 6.78, and >10 in calcium-free solution and 5.00, 6.10, and 7.15 in the presence of Ca(2+). We have found that Ca(2+) forms an insoluble adduct with the 2,3-semiquinone anion-radical. Thus, the effect of redox-inert calcium on the free radical reactions could be explained (i) by additional deprotonation of echinochrome A and (ii) by formation of a Ca(2+)-naphtho-2,3-semiquinone complex (calcium semiquinonate). Additionally, we have shown that the dried red spines from Strongylocentrotus intermedius possess paramagnetic properties; the EPR signal of the natural spines was similar to that of calcium semiquinonate obtained in our artificial chemical system.
Department of Biochemistry, Institute of Experimental Cardiology, Cardiology Research Center, 121552 Moscow, Russia. avleb@mail.ru
Alexander V
AV
Lebedev
Marina V
MV
Ivanova
Enno K
EK
Ruuge
D000818:Animals; D000838:Anions; D016227:Benzoquinones / Q000737:chemistry*; D002118:Calcium / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D006863:Hydrogen-Ion Concentration; D007477:Ions; D007700:Kinetics; D008280:Magnetics; D008956:Models, Chemical; D009285:Naphthoquinones / Q000737:chemistry* / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D010101:Oxygen Consumption; D012617:Sea Urchins; D013053:Spectrophotometry; D013997:Time Factors
D000838:Anions; D016227:Benzoquinones; D005609:Free Radicals; D007477:Ions; D009285:Naphthoquinones; C025232:semiquinone radicals; C007573:echinochrome A; D010100:Oxygen; D002118:Calcium