Lemon albedo cell walls contain distinct populations of pectic hairy regions

Abstract

Alcohol insoluble solids from lemon albedo were extracted to obtain a chelating agent soluble pectin fraction (ChSS), a diluted sodium hydroxide soluble pectin fraction (ASS) which was separated into a water soluble and water insoluble part (ASSws and ASSwi), and an insoluble residue (Residue). ChSS, ASSws, ASSwi and Residue represented 64.3%, 10.4%, 0.4% and 8.1% of the galacturonic acid residues present in the lemon albedo, respectively. The pectin fractions and the residue were characterised by their sugar composition, degrees of methylation and acetylation, and molecular weight distribution. After extensive digestion with polygalacturonase and pectinesterase, the remaining high molecular weight fragments, representing the so-called pectic hairy regions (HR), were isolated and further characterised. After cold alkaline treatment (de-esterification), HR shown to be accessible to a further degradation by rhamnogalacturonan hydrolase (RGase) and the digests were analysed by high-performance size-exclusion chromatography (HPSEC) and high-performance anion-exchange chromatography (HPAEC). The various HR fractions differed in their degradation pattern: typically 2–4% of all glycosidic linkages were hydrolysed. The amount of RGase oligomers formed from the various fractions decreased in the order these were extracted while the degree of galactose substitution of the oligomers increased. Indications were found for the presence of small quantities of xylogalacturonan fragments within the pectic hairy regions of lemon albedo. On the basis of these results the structure of lemon pectin is compared with the structure of pectins extracted from other sources.