Role of Peroxidase in Lignification of Tobacco Cells : II. Regulation by Phenolic Compounds.

Coniferyl alcohol is the primary substrate for peroxidase-mediated lignification, a process which depends on the generation of H(2)O(2) by NADH oxidation. We measured the concentrations of various phenols (synthetic and natural) at which maximal enhancement of NADH oxidation occurs. Coniferyl alcohol was found to stimulate NADH oxidation at a much lower concentration (0.01 mm) than other natural or synthetic phenols (1-100 mm). In addition, coniferyl alcohol prevented the conversion of active peroxidase into the inactive intermediate compound III-which is usually formed in the presence of NADH-at equally low concentrations. This conversion was found to be a prerequisite for stimulation of NADH-oxidation, but it was not necessarily connected to stimulation.The oxidation of NADH and coniferyl alcohol (or guaiacol) can occur simultaneously, but there is a strong competitive interaction between these two substrates. At pH 5, the presence of NADH at concentrations 30 to 60 times lower than the phenols completely prevents their oxidation. The results are discussed in relation to the role of cell wall peroxidases in conversion of coniferyl alcohol to lignin and in formation of H(2)O(2).