STUDIES ON APPLE PROTOPECTIN-VI - EXTRACTION OF PECTINS FROM APPLE CELL-WALLS WITH RHAMNOGALACTURONASE

Apple cell walls were extracted by treatment with rhamnogalacturonase, an enzyme that cleaves the rhamnogalacturonic backbone in the rhamnose-rich regions. It solubilised 13.6% of the cell walls after 24 h. The extracted material was composed of galacturonic acid (42%) and neutral sugars (46%), especially arabinose, and had a high rhamnose: galacturonic acid ratio; it was also rich in methyl and acetyl groups (58 and 24 mg/g, respectively). The material was separated into two fractions by anion-exchange chromatography on DEAE Sepharose CL-6B. The neutral, non-retained fraction was composed mostly of arabinose and had a low hydrodynamic volume as shown by its elution pattern on Sephacryl S500. The retained fraction was enriched in galacturonic acid and had a higher hydrodynamic volume. Methylation analysis of the side-chains showed mostly arabinans and type I arabinogalactans. Some terminal rhamnose could be detected, and was relatively more abundant in the non-retained fraction. Admixture with pectinlyase or polygalacturonase plus pectinmethylesterase during treatment of the apple cell walls resulted in degradation of the high-molecular-weight fractions, and with arabinanases and galactanase in degradation of the low-molecular-weight fraction of the rhamnogalacturonase extract.