Labeling and chemistry of grapefruit pectic substances

The labeling of a number of polysaccharides found in grapefruit (Citrus paradisi) was achieved by feeding labeled myo-inositol to ripening grapefruit through their cut fruit stem, and allowing 4 days for the metabolism of label. The pectic polysaccharides were isolated by successive extraction of the labeled grapefruit with 80% ethanol, chloroform-methanol (1:1) and finally with 0.2 M Na₂ EDTA to solubilize pectic polysaccharides. The incorporation of label from myo-inositol into galacturonosyl, arabinosyl, xylosyl and galactosyl residues of pectic polysaccharides via myo-inositol oxidation pathway was demonstrated. Ion exchange chromatography of these labeled pectic polysaccharides using DE-52 cellulose resulted in the elution of eight totally or partially resolved polysaccharides with increasing salt concentration. The results suggest that, like other plant tissues, the myo-inositol oxidation pathway is also operative in ripening grapefruit and this metabolic pathway could be successfully utilized to achieve labeling of a number of pectic polysaccharides.