Biosynthesis of Xyloglucan in Suspension-cultured Soybean Cells. Evidence that the Enzyme System of Xyloglucan Synthesis does not Containβ-1,4-Glucan 4-β-D-Glucosyltransferase Activity (EC 2.4.1.12)

摘要

Xyloglucan 4-β-D-glucosyltransferase, an enzyme responsible for the formation of the xyloglucan backbone, in a particulate preparation of soybean cells has been compared withβ-1,4-glucan 4-β-D-glucosyltransferase of the same origin. The following observations indicate that the enzyme system of xyloglucan synthesis does not containβ-1,4-glucan 4-β-D-glucosyltransferase activity, although both enzymes transfer the glucosyl residue from UDP-glucose to form theβ-1,4-glucosidic linkage: 1. The incorporation of 14Cglucose into xyloglucan depended on the presence of UDP-xylose in the incubation mixture. 2. No measurable amount of radioactivity was incorporated from UDP-14Cxylose into the cello-oligosaccharides, although the incorporation of 14Cxylose into xyloglucan depended on the presence of UDP-glucose in the incubation mixture (Hayashi and Matsuda 1981b). 3. The activity of xyloglucan 4-β-D-glucosyltransferase was stimulated more strongly by Mn2+than by Mg2+, whereas Mg2+was the most active stimulator for the activity ofβ-1,4-glucan 4-β-D-glucosyltransferase. 4. An addition of GDP-glucose (100μM) to the incubation mixture inhibited the activity of xyloglucan 4-β-D-glucosyltransferase by 17, whereas the activity ofβ-1,4-glucan 4-β-D-glucosyltransferase was inhibited 56under the same conditions. 5.Irpexexo-cellulase did not hydrolyze the xyloglucan synthesized in vitro. 6. Theβ-1,4-glucan synthesized in vitro was not a branched xyloglucan because it gave no 2,3-di-O-methyl glucose derivative on methylation analysis. 7. Pulse-chase experiments indicated that theβ-1,4-glucan was not transformed into the xyloglucan.The subcellular distribution of the xyloglucan synthase, however, was similar to that of theβ-1,4-glucan synthase (Golgi-located 1,4-β-D-glucan 4-β-D-glucosyltransferase). Thus, it appears that the latter enzyme is located at a site close to xyloglucan synthase and is set aside for the assembly of these polysaccharides into the p