Role of Peroxidase in Lignification of Tobacco Cells : I. Oxidation of Nicotinamide Adenine Dinucleotide and Formation of Hydrogen Peroxide by Cell Wall Peroxidases

The two peroxidase isoenzyme groups (G(I) and G(III)) localized in the cell walls of tobacco (Nicotiana tabacum L.) tissues were compared with respect to their capacity for NADH-dependent H(2)O(2) formation. Peroxidases of the G(III) group are slightly more active than those of the G(I) group when both are assayed under optimal conditions. This difference is probably not of major regulatory importance. NADH-dependent formation of H(2)O(2) required the presence of Mn(2+) and a phenol as cofactors. The addition of H(2)O(2) to the reaction mixture accelerated subsequent NADH-dependent H(2)O(2) formation. In the presence of both cofactors or Mn(2+) alone, catalase oxidized NADH. However, if the cofactors were absent or if only dichlorophenol was present, catalase inhibited NADH oxidation. No H(2)O(2) accumulation occurred in the presence of catalase. Superoxide dismutase inhibited NADH oxidation quite significantly indicating the involvement of the superoxide radical in the peroxidase reaction. These results are interpreted to mean that the reactions whereby tobacco cell wall peroxidases catalyze NADH-dependent H(2)O(2) formation are similar to those proposed for horseradish peroxidase (Halliwell 1978 Planta 140: 81-88).