Effect of group II metal cations on catecholate oxidation.
The unexpected effects of Ca(2+) on the free-radical chain reactions of dopamine, norepinephrine, isoproterenol, and pyrocatechol oxidation are studied using oxygen consumption measurements, EPR-spectroscopy, UV/VIS spectrophotometry, and by potentiometric titration. It is found that the formation of Ca(2+)-catecholate complexes is accompanied by an increase in the dissociation constants (K(ai) ) of their phenolic hydroxyls. At pH>pK(ai) and in the presence of alkaline-earth metal cations, the rate of catecholate oxidation increases (Ca(2+), Mg(2+)> Sr(2+), Ba(2+)), whereas on addition of Zn ions the rate decreases. The effects of Group II metal cations on catecholate autoxidation are concomitant with a transient increase of the EPR signal for metal-semiquinonate complexes. Therefore, the effects of Ca(2+) and other alkaline-earth metal cations on catecholate autoxidation can be defined as 1) additional deprotonation of catechol OH-groups involved in the formation of M(2+)-catecholate complexes, the latter exceeding catechols in the susceptibility to dioxygen-induced oxidation and 2) formation of relatively stable free-radical intermediates responsible for chain propagation.
Lebedev
Alexander V
AV
Department of Biochemistry, Institute of Experimental Cardiology, Cardiology Research Center, 3rdCherepkovskaya Street 15A, 121552 Moscow, Russia. avleb@cardio.ru
Ivanova
Marina V
MV
Timoshin
Alexander A
AA
Ruuge
Enno K
EK
D002396:Catechols / Q000737:chemistry*; D002413:Cations, Divalent; D004298:Dopamine / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007545:Isoproterenol / Q000737:chemistry; D008673:Metals, Alkaline Earth / Q000737:chemistry*; D009638:Norepinephrine / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
D002396:Catechols; D002413:Cations, Divalent; D005609:Free Radicals; D008673:Metals, Alkaline Earth; D015032:Zinc; D007545:Isoproterenol; C034221:catechol; D010100:Oxygen; D004298:Dopamine; D009638:Norepinephrine