A rat brain fraction and different purified peroxidases catalyzing the formation of dopaminochrome from dopamine

Dopaminochrome formation is catalyzed by commercially available purified pe roxidases (EC 1.11.1.7) such as horseradish, lacto- and myelo-peroxidase us ing dopamine, hydrogen peroxide or promethazine sulfoxide as substrates. A rat brain fraction (RBF) catalyzes a similar reaction and its catalytic pow er increases after preincubation with hydrogen peroxide/ascorbic acid. The activity of both the purified enzymes and the RBF preparation is inhibited by carnosine and characterized by excess substrate inhibition. The enzymes recognize different substrates but show the highest affinity for dopamine. The RBF fraction is strongly buffered against oxidation by compounds such a s glutathione and by bioreductive enzymes such as DT-diaphorase (EC 1.6.99. 2) which can use as a substrate menadione or dopaminochrome. The rat brain dopamine peroxidizing activity appeared to be mostly bound to the synaptoso mal fraction. The reaction catalyzed by the purified peroxidases was follow ed by electron spin resonance spectroscopy and, unlike that catalyzed by RB F, was shown to produce the signal of a transient dopamine-o-semiquinone ra dical. (C) 1999 Elsevier Science B.V. All rights reserved.