A xylogalacturonan subunit present in the modified hairy regions of apple pectin

The high molecular weight fraction (fraction A) of the modified hair): regions (MHR) from apple cell walls was treated by rhamnogalacturonase (RGase) after saponification and after deacetylation by rhamnogalacturonan acetylesterase (RGAEase). Three fractions could be recognized by size-exclusion chromatography: rhamnogalacturonan oligomers, residual stubs of the rhamnogalacturonan backbone rich in arabinan side-chains, and a fraction rich in xylose and galacturonic acid. The ''xylogalacturonans'' obtained after saponification and after enzymic deacetylation had rather similar sugar compositions (xylose:galacturonic acid ratios of 0.4-0.9) and molecular weights. After saponification, the xylogalacturonan was eluted as a single peak on anion-exchange chromatography whereas three peaks were obtained when the MHR was deacetylated by RGAEase, indicating variations in the degrees of methylation, One of the methyl-esterified fled xylogalacturonan fractions was characterized by NMR spectroscopy: the xylose residues were beta-(1-->3)-linked to some of the galacturonic acid residues within a rather high molecular weight xylo-(1-->4)-alpha-galacturonan and a degree of methylation of 39 was calculated, The methyl esters were found to be equally divided between the substituted and unsubstituted galacturonosyl residues.