Ferroxidase activity in a laccase-like multicopper oxidase from Liriodendron tulipifera

Ferroxidase activity was detected in a laccase-like multicopper oxidase (LMCO) produced in transgenic tobacco cells expressing an LMCO cDNA (Ltlacc2.2) cloned from yellow-poplar (Liriodendron tulipifera). This marks the first report of ferroxidase activity associated with a plant laccase and suggests that some members of this plant enzyme family may have physiological functions based on activities other than their more widely recognized phenoloxidase activity. Recent work with LMCOs from bacteria, yeast and mammals has shown that metal oxidase activities in these enzymes can be important for their primary physiological functions, With respect to ferroxidase activity in certain plant LMCOs, it is proposed that the high levels of LMCO expression in plant vascular tissues may reflect the need for high-efficiency iron uptake pumps in tissues that undergo lignification during normal development. Such iron uptake pumps would function to minimize levels of free iron so that reactive oxygen species do not reach toxic levels when H2O2 is generated for peroxidase-mediated monolignol coupling during lignin deposition.