Redox intermediates of plant and mammalian peroxidases:: A comparative transient-kinetic study of their reactivity toward indole derivatives

A comparative study on the reactivity of five indole derivatives (tryptamine, N-acetyltryptamine, tryptophan, melatonin, and serotonin), with the redox intermediates compound I (k(2)) and compound II (k(3)) of the plant enzyme horseradish peroxidase (HRP) and the two mammalian enzymes lactoperoxidase (LPO) and myelo-peroxidase (MPO), was performed using the sequential-mixing stopped-flow technique. The calculated bimolecular rate constants (k(2), k(3)) revealed substantial differences regarding the oxidizability of the substrates by redox intermediates at pH 7.0 and 25degreesC. With HRP it was shown that k(2) and k(3) are mainly determined by the reduction potential (Edegrees') of the substrate with k(2) being 7-45 times higher than k(3). Compound I of mammalian peroxidases was a much better oxidant than HRP compound I with the consequence that the influence of the indole structure on k(2) of LPO and HPO was small varying by a factor of only 88 and 38, respectively, which is in strong contrast to a factor of 160,000 determined for k(2) of HRP. Interestingly, the k(3) values for all three enzymes were very similar. Oxidation of substrates by mammalian peroxidase compound II is stronly constrained by the nature of the substrate. The k(3) values for the five indoles varied by a factor of 3,570 (LPO) and 200,000 (MPO), suggesting that the reduction potential of compound II of mammalian peroxidase is less positive than that of compound I, which is in contrast to the plant enzyme. (C) 2002 Elsevier Science.