Kinetic studies on the autoxidation of cysteine catalyzed by copper complexes: Catecholamines stimulate the autoxidation

Catecholamines including epinephrine, norepinephrine, dopamine, and dihydro xyphenylalanine accelerated copper-catalyzed oxidation of cysteine under ae robic conditions, while they had no effect under anaerobic conditions. The efficiency of the catecholamines was arranged in the decreasing order as fo llows; epinephrine is approximately equal to norepinephrine greater than or equal to dopamine > dihydroxyphenylalanine. On the contrary, phenolic amin es such as tyramine and tyrosine neither accelerate nor inhibit the autoxid ation. The reaction can be elucidated by a "sequentical mechanism" which is composed of Eqs. I, II, III, IV, V, and VI: Cu(II)-L + CyS- reversible arrow L-Cu(II)(CyS-) (I) L-Cu(II)(CyS-) --> Cu(I)-L + .CyS (II) Cu(I)-L + CyS- reversible arrow Cu(I)(CyS-) + L (IIa) Cu(I)(CyS-)+ O-2 reversible arrow (CyS-)Ci(I)-O-2 (III) (CyS-)Cu(I)-O-2 + CyS- + 2H(+) --> Cu(II)(CyS-) + .CyS + H2O2 (IV) (CyS-)Cu(I)-O-2 + Cat(-) + 2H(+) --> Cu(II)(CyS-) + .Cat + H2O2 (V) CyS- + .Cat --> Cat(-) + .CyS (VI) where L is glycylglycine used as the buffer, and CyS- and Cat(-) denote cys teinate and catecholamine ions, respectively. The reactions (IV), (V), or b oth are considered to be rate-determining. The optimum pH for the autoxidat ion was observed at pH 7.5-7.6, and below pH 7.2 reaction (V) does not part icipate in the autoxidation system.