The effects of manganese and copper in vitro and in vivo on peroxidase catalytic cycles.

Here we present the results of in vitro and in vivo studies of the influence of Mn2+ and Cu2+ on the peroxidative and oxidative catalytic functions of class III peroxidase. Complex peroxidase catalysis by intermediates generated in the reaction was analyzed by utilizing the activating effect of Mn2+ and the inhibitory effect of Cu2+ on the oxidative reaction in vitro. p-Coumaric acid was used as an enzyme substrate in the peroxidative reaction and as a cofactor in the oxidative reaction. In order to correlate the observed in vitro effects with the in vivo situation, we exposed maize plants to excess concentrations of Mn2+ and Cu2+ in the hydroponic solutions. Copper severely arrested plant growth, while manganese exerted no significant effect. The effects on peroxidase activity and isoforms profile of root soluble and cell wall bound fractions were studied. Inhibition of the peroxidase oxidative function by copper was reversible, localized in the cell wall, and accompanied by disappearance of some and appearance of new cationic isoforms. Copper-mediated changes were suppressed by the presence of manganese, although Mn2+ treatment per se did not affect the activity of the peroxidase enzyme. The results on the peroxidase activity in maize roots grown with excess Mn2+ and Cu2+ point to the coupling between the oxidative cycle, root growth and different peroxidase isoforms.