Cell-wall-associated oxidases from the lignifying xylem of angiosperms andgymnosperms: Monolignol oxidation

Cell-wall-associated oxidases extracted from the lignifying xylem of Sitka spruce and ash oxidise sinapyl alcohol at a greater rate than coniferyl alc ohol and p-coumaryl alcohol (SA > CA > pCA). The enzyme from ash shows a ma rked preference, on a specific activity basis, for the oxidation of SA over CA and PCA (SA much greater than CA greater than or equal to pCA) and has a particularly low affinity for the oxidation of coniferyl alcohol compared to the enzyme from spruce (SA > CA > pCA). This difference in monolignol p reference between the spruce and ash enzymes may relate to their required f unctions during lignification, in that the hardwood enzyme would be supplie d mainly SA and the softwood enzyme would be supplied mainly CA. The spruce enzyme also displayed a marked preference for the oxidation of s inapyl alcohol over sinapyl aldehyde even when the two compounds were prese nted in a mixture. Purified cell walls from the lignifying xylem of spruce could oxidise CA by the action of bound oxidase activity and dissolved oxygen (similar to 240 mu M) but CA oxidation was increased many fold by the action of the bound p eroxidase activity and 240 mu M H2O2. However, the initial dimeric and trim eric products of the peroxidase- and oxidase-catalysed reactions detected b y liquid chromatography-mass spectrometry were the same and present in simi lar proportions. This indicates that the oxidation of CA by oxidase or by p eroxidase proceeds via the same intermediates and occurs by a similar mecha nism. Insoluble dehydrogenation polymers (DHPs) of CA were formed in similar yiel ds by spruce extracts in the absence (oxidase activity) or presence (peroxi dase activity) of H2O2. The peroxidase-catalysed DHPs and the oxidase-catal ysed DHPs gave Fourier transform infra-red spectra with maxima that were ch aracteristic of DHPs of CA. However, differences in the comparative intensi ties of some maxima suggest that the oxidase-catalysed DHPs were less conde nsed than the peroxidase-catalysed polymers. These findings are discussed w ith respect to the possible contribution of oxidases to lignin structure in developing wood.