DIFFERENT POPULATIONS OF PECTIC HAIRY REGIONS OCCUR IN APPLE CELL-WALLS

Alcohol insoluble solids from apple were extracted in sequence by buff er at 20 degrees C and at 70 degrees C, EDTA/oxalate, and mild alkali, yielding four populations of pectins. These pectins and the insoluble residue were characterized by their sugar composition, degree of este rification (methyl ester and O-acetyl groups), molecular weight distri bution, and degradability by the combination of endopolygalacturonase (PG) and pectin esterase (PE) and by rhamnogalacturonase (RGase) after chemical saponification. After PG/PE treatment, the remaining high mo lecular weight material representing the pectic hairy regions was isol ated and characterized. Clear differences were found in the sugar comp osition of the fractions obtained, while only small variations were ob served in the sugar linkage composition. The pectic hairy regions were further degraded by RGase and the digests separated into high molecul ar weight and oligomeric degradation products. These ''RGase oligomers '' consisted of between 4 and 9 sugar units with a backbone of alterna ting rhamnose and galacturonic acid residues, partly substituted with galactose linked to C-4 of the rhamnose moiety. Both the absolute amou nt of RGase oligosaccharides released as well as the degree of galacto se-substitution of the oligomers increased when severer extraction con ditions were used. Relatively more RGase oligomers were released from the low molecular weight hairy regions as compared to the high molecul ar weight fraction. Typical high molecular weight fragments isolated f rom the RGase digests of various hairy regions included residual segme nts of the rhamnogalacturonan backbone rich in arabinose and a polymer presumably enriched in xylogalacturonan segments.