Peroxidatic oxidation of catecholamines. A kinetic electron spin resonance investigation using the spin stabilization approach.

Using spin stabilization, ESR measurements have been made of o-semiquinone production from the horseradish peroxidase-H2O2 oxidation of catecholamine substrates. The termination rate constant for semiquinones stabilized with Zn2+ at pH 5 is about 10(4) times smaller than for uncomplexed semiquinones at neutral pH. Stabilization allows steady state concentrations of semiquinones to be obtained. The duration of the steady state is dependent upon the concentrations of enzyme, hydrogen peroxide, and catecholamine substrate. The relative reactivity of the substrates 3,4-dihydroxyphenylalanine, norepinephrine, and dopamine at pH 5 is 1:8:40. The effects of phenol and ascorbate were studied and shown to be consistent with scavenging of phenoxyl radicals by catecholamine and semiquinone radicals by ascorbate, respectively.