Substrate specificities of enzymes involved in monolignol biosynthesis are recognized to control differentiation of guaiacyl and syringyl lignin, but peroxidases are regarded to have rather less substrate specificity and their specificity related to lignification has not been fully elucidated. We have investigated the substrate specificity of peroxidases (EC126.96.36.199) in poplar (Populus alba L.) and Japanese cedar (Cryptomeria japonica D. Don) with respect to the dehydrogenative polymerization of monolignols. Peroxidases were fractionated into three groups, namely soluble peroxidases, ionically bound peroxidases (IPO), and covalently bound peroxidases. Populus IPO was found to have the largest preference for sinapyl alcohol among all of peroxidases from both woody plant species, and only this peroxidase could produce the dehydrogenative polymer (DHP) from sinapyl alcohol. On the other hand, all peroxidases from both Cryptomeria and Populus easily produced the DHP from coniferyl alcohol. The results support that the participation of substrate-specific peroxidases is an important factor regulating the accumulation of syringyl and guaiacyl lignins in angiosperms and gymnosperms. Electrophoresis of IPO revealed the existence of syringyl-specific isoperoxidase only in Populus IPO.
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