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.
Lebedev
Alexander V
AV
Department of Biochemistry, Institute of Experimental Cardiology, Cardiology Research Center, 121552 Moscow, Russia. avleb@mail.ru
Ivanova
Marina V
MV
Ruuge
Enno K
EK
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