Partial characterization of xylogalacturonans from cell walls of ripe watermelon fruit: inhibition of endopolygalacturonase activity by xylosylation

Abstract

In our studies of pectins from cotton suspension culture cell walls, we consistently find a 50% methylesterified homogalacturonan (HG) region, resistant to endopolygalacturonase (EPG) digestion, associated with the rhamnose-rich pectic region commonly known as RGI. Since saponification does not enable the EPG to digest this HG and it contains approximately one Xyl residue per three GalA residues, it is likely that its resistance to digestion is caused by the xylose. Only small amounts of the Xyl-substituted HG can be obtained from the cotton cell walls. Large amounts of Xyl-substituted HG are present in the cell walls of watermelon, and we have used it to characterize the effects of xylosylation on the activity of EPG and to produce fragments of the xylogalacturonan for characterization. The action of EPG on the xylogalacturonan solubilized from watermelon walls by 0.1 M NaOH, which contains about one Xyl for every seven GalA residues, produces fragments ranging in the length of their HG backbone from one up to at least 26 residues. Apart from the expected GalA1–3 oligomers that one would obtain from the limit digest of pectic acid, all of the fragments contained one Xyl per three or four GalA. Thus, it appears that the Xyl residues are clustered within the substrate(s) and do provide protection from digestion by EPG.