The stoichiometry of the topa quinone biogenesis reaction in phenylethylami
ne oxidase from Arthrobacter globiformis (AGAO) has been determined. We hav
e shown that the 6e(-) oxidation of tyrosine to topa quinone (TPQ) consumes
2 mol of O-2 and produces 1 mol of H2O2/mol of TPQ formed. The rate of H2O
2 production is first-order (k(obs) = 1.0 +/- 0.2 min(-1)), a rate only sli
ghtly lower than the rate of TPQ formation directly determined previously (
k(obs) = 1.5 +/- 0.2 min(-1)). This gives the following net reaction stoich
iometry for TPQ biogenesis: E-Tyr + 2O(2) --> E-TPQ + H2O2. This stoichiome
try is in agreement with recently proposed mechanisms for T-PQ biogenesis,
and rules out several possible alternatives.