Effectiveness of phenoxyl radicals generated by peroxidase/H2O2-catalyzed oxidation of caffeate, ferulate, and p-coumarate in cooxidation of ascorbate and NADH

The rate of ascorbate and nicotinamide adenine dinucleotide plus hydrogen (NADH) cooxidation (i.e., their nonenzymic oxidation by peroxidase/H2O2-generated phenoxyl radicals of three hydroxycinnamates: caffeate, ferulate and p-coumarate) was studied in vitro. The reactions initiated by different sources of peroxidase (EC 1.11.1.7) [isolates from soybean (Glycine max L.) seed coat, maize (Zea mays L.) root-cell wall, and commercial horseradish peroxidase] were monitored. Native electrophoresis of samples and specific staining for peroxidase activity revealed various isoforms in each of the three enzyme sources. The peroxidase sources differed both in the rate of H2O2-dependent hydroxycinnamate oxidation and in the order of affinity for the phenolic substrates. The three hydroxycinnamates did not differ in their ability to cooxidize ascorbate, whereas NADH cooxidation was affected by substitution of the phenolic ring. Thus, p-coumarate was more efficient than caffeate in NADH cooxidation, with ferulate not being effective at all. Metal ions (Zn2+ and Al3+) inhibited the reaction of peroxidase with p-coumarate and affected the cooxidation rate of ascorbate and the peroxidase reaction in the same manner with all substrates used. However, inhibition of p-coumarate oxidation by metal ions did not affect NADH cooxidation rate. We propose that both the ascorbate and NADH cooxidation systems can function as mechanisms to scavenge H2O2 and regenerate phenolics in different cellular compartments, thus contributing to protection from oxidative damage.