CONIFERYL ALCOHOL OXIDASE OPERATES THROUGH A BOUND FREE-RADICAL INTERMEDIATE

Oxidase activity was enriched in extracts of the developing, lignifyin g xylem of Sitka spruce, Leyland cypress, Lawson cypress and Wych elm obtained by a procedure that selects cell-wall-associated glycoprotein s. All of the xylem extracts were able to oxidise the monolignol, coni feryl alcohol, and had a greater affinity for coniferyl alcohol than A BTS. The oxidases were strongly inhibited by Cu-chelators and had inhi bition profiles broadly similar to catechol oxidase-type polyphenol ox idases. Analysis of the oxidation products of coniferyl alcohol genera ted in the presence of the spin trap POBN, by electron paramagnetic re sonance spectroscopy, showed that a spin adduct of parameters a(N) = 1 5.65-15.74 G and a(H) = 2.73-2.78 G had been generated which can be mo st readily assigned to a POBN adduct of the beta-carbon-centred free r adical of coniferyl alcohol. Electron paramagnetic resonance spectra o f the extracts from the four species contained signals representative of Cu2+ and a free radical species of g value similar to 2.0036. The i ntensity of the free radical signal was diminished by addition of coni feryl alcohol but was restored upon aeration; The Cu2+ signals were si milarly altered by this treatment, but diminution was less marked. Cyc les of depletion then recovery of the free radical signal could be obt ained by addition of aliquots of coniferyl alcohol followed by aeratio n which strongly suggests that the free radical is directly involved i n the oxidation mechanism. We propose that coniferyl alcohol oxidase o perates via a bound free radical which is re-oxidised via a charge rel ay mechanism involving bound Cu2+ ions and molecular oxygen. (C) 1998 Elsevier Science Ltd. All rights reserved.