The reaction rates of NO with horseradish peroxidase compounds I and II

In this study the reactions between nitric oxide (NO) and horseradish peroxidase (HRP) compounds I and II were investigated. The reaction between compound I and NO has biphasic kinetics with a clearly dominant initial fast phase and an apparent second-order rate constant of (7.0 +/- 0.3) x 10(5) M(-1) s(-1) for the fast phase. The reaction of compound II and NO was found to have an apparent second-order rate constant of k(app) = (1.3 +/- 0.1) x 10(6) M(-1) s(-1) or (7.4 +/- 0.7) x 10(5) M(-1) s(-1) when measured at 409 nm (the isosbestic point between HRP and HRP-NO) and 419 nm (lambda(max) of compound II and HRP-NO), respectively. Interestingly, the reaction of compound II with NO is unusually high relative to that of compound I, which is usually the much faster reaction. Since horseradish peroxidase is prototypical of mammalian peroxidases with respect to the oxidation of small substrates, these results may have important implications regarding the lifetime and biochemistry of NO in vivo after inflammation where both NO and H(2)O(2) generation are increased several fold.