Novel O-D-galacturonoyl esters in the pectic polysaccharides of suspension-cultured plant cells

Abstract

Driselase digestion of uronate-6-14C-labeled primary walls of cultured spinach (Spinacia oleracea L.) cells yielded about 18 novel uronate-containing compounds, most of which could be hydrolyzed by cold dilute alkali to yield oligo-[14C]galacturonides. One typical Driselase digestion product (compound 17) yielded alpha-(1-->4)-D-[14C]galacturonotriose(GalA3) upon very mild treatment with alkali (50% yield of GalA3 in 7.2 min at pH 11 and 25 degrees C). One of the three galacturonate residues in compound 17 was reducible to a galactose residue with sodium borohydride, indicating that that GalA residue was esterified, via its--COOH group, to a putative alcohol. Compound 17 had a higher mobility than GalA3 on paper chromatography, indicating that the putative alcohol was relatively nonpolar. The putative alcohol could not have been methanol because Driselase readily hydrolyzed mono-, di-, and trimethyl esters of GalA3 to yield free galacturonic acid. Another Driselase digestion product (compound 12) was a derivative of GalA3 that apparently possessed two nonpolar esterified substituents: one about as labile as in compound 17, and the other approximately 10 times more stable. Compounds 12 and 17 could not labeled by in vivo feeding of [U-14C]cinnamate, suggesting that they were not phenolic conjugates. Similar but chromatographically distinguishable uronate-14C-labeled esters were obtained by Driselase digestion of walls of cultured carrot (Daucus carota L.), Paul's Scarlet rose (Rosa sp.), and tall fescue (Festuca arundinacea Schreber) cells. In spinach, the novel compounds constituted about 5% of the total galacturonate residues of the cell wall. The observations suggest that pectic polysaccharides are linked, via O-D-galacturonoyl ester bonds, to relatively hydrophobic constituents of the primary cell wall. Their possible role in wall architecture is discussed.