OSCILLATORY LOW-LEVEL CHEMILUMINESCENCE FROM A NONEQUILIBRIUM β-NICOTINAMIDE ADENINE DINUCLEOTIDE-PEROXIDASE SYSTEM: EXPERIMENTAL OBSERVATIONS and COMPUTER SIMULATIONS

Effects of tyramine and 4-aminophenol on the oscillating peroxidase-oxidase reaction

The peroxidase-oxidase oscillator, a model of biological oscillations, is usually studied in conjunction with the effector molecule, 2,4-dichlorophenol. In this account, we present evidence of the effects of a naturally occurring phenol, tyramine, on the reaction, and also those of the structurally similar 4-aminophenol. Whereas 2,4-dichlorophenol gives rise to sustained oscillations at 40 μM, it was discovered that tyramine promotes damped oscillations at a concentration of 120 μM. Oxidation of NADH was completely inhibited by 4-aminophenol and ascorbate. In separate experiments, the peroxidase-catalyzed ring coupling of tyramine and 4-aminophenol was observed, which in the case of tyramine, may provide an explanation for the damping of oscillations.

Kinetic studies of the oscillatory dynamics in the peroxidase-oxidase reaction catalyzed by four different peroxidases

Oscillatory kinetics in the peroxidase-oxidase reaction catalyzed by structurally different peroxidases were investigated using NADH as a substrate. For horseradish peroxidase, lactoperoxidase, and soybean peroxidase the oscillatory waveforms of their dominating enzyme intermediates, ferric peroxidase and compound III, are similar. Coprinus peroxidase, on the other hand, has ferrous peroxidase and compound III as the dominating intermediates. The oscillatory waveform of its compound III differs from the waveforms of compound III of the three other peroxidases. Also, the phase plot of the signal for compound III versus the oxygen concentration for Coprinus peroxidase differs from the corresponding phase plots obtained using other peroxidases. A detailed model of the reaction mechanism is proposed, which is able to simulate these different kinds of behaviour. Substituting NADH with dihydroxyfumaric acid as a substrate, oscillations in the oxygen concentration were observed for about 1.5 h when a concentrated solution of this substrate was continuously fed to a solution containing horseradish peroxidase. This is the first demonstration of sustained oscillations with this substrate.

Oscillations in the peroxidase-oxidase reaction: a comparison of different peroxidases

The nonlinear behavior of the peroxidase-oxidase reaction was studied using structurally different peroxidases. For the first time sustained oscillations with peroxidases other than horseradish peroxidase in a single-enzyme system were observed. All peroxidases that showed significant oxidase activity were able to generate sustained oscillations. When adjusting the overall reaction rate, either of the two modifiers 2,4-dichlorophenol or Methylene blue could be omitted from the reaction. Due to the observation of different enzyme intermediates when using different peroxidases, we conclude that the mechanisms responsible for oscillatory kinetics may vary from one peroxidase to the other.

Visible chemiluminescence associated with the reaction between methemoglobin or oxyhemoglobin with hydrogen peroxide

Visible chemiluminescence is emitted in the irreversible deactivation of hemoglobin or methemoglobin with excess H2O2. The emission takes place in two phases. The most intense one lasts a few seconds and is followed by a second phase of lower intensity that remains for longer periods. This second phase presents chaotic or sustained oscillations. Free radicals are implicated in the luminescent process since the emission can be reduced by free radical scavengers such as 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) or ascorbic acid. These additives lead to a delay in reaching the maximum intensity, which can be related to their consumption, implying substantial recycling of the hemoprotein. Chemiluminescence is also observed in the oxidation of hemin by H2O2, suggesting a role for the heme group in the processes leading to the excited state production. The lower intensity observed in the presence of hemin can be related to the contribution of the globin chains.