Cell-wall-associated oxidases from the lignifying xylem of angiosperms and gymnosperms: 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 alcohol and p-coumaryl alcohol (SA > CA > pCA). The enzyme from ash shows a marked 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 preference between the spruce and ash enzymes may relate to their required functions during lignification, in that the hardwood enzyme would be supplied mainly SA and the softwood enzyme would be supplied mainly CA. The spruce enzyme also displayed a marked preference for the oxidation of sinapyl alcohol over sinapyl aldehyde even when the two compounds were presented 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 peroxidase activity and 240 mu M H2O2. However, the initial dimeric and trimeric products of the peroxidase- and oxidase-catalysed reactions detected by liquid chromatography-mass spectrometry were the same and present in similar proportions. This indicates that the oxidation of CA by oxidase or by peroxidase proceeds via the same intermediates and occurs by a similar mechanism. Insoluble dehydrogenation polymers (DHPs) of CA were formed in similar yields by spruce extracts in the absence (oxidase activity) or presence (peroxidase activity) of H2O2. The peroxidase-catalysed DHPs and the oxidase-catalysed DHPs gave Fourier transform infra-red spectra with maxima that were characteristic of DHPs of CA. However, differences in the comparative intensities of some maxima suggest that the oxidase-catalysed DHPs were less condensed than the peroxidase-catalysed polymers. These findings are discussed with respect to the possible contribution of oxidases to lignin structure in developing wood.