STUDIES ON RHAMNOGALACTURONASE IN FRUIT

Abstract

Rhamnogalacturonase (RGase) is a new fungal enzyme which degrades the highly branched regions of apple fruit cell wall pectin by cleaving the glycosyl linkage between rhamnosyl and galacturonosyl residues (Schols et al., 1990. Carhohydr. Res. 206:105.). This enzyme, if present in fruit, could play a significant role in fruit softening. Partial purification of RGase was accomplished from a fungal enzyme preparation (Pectinex Ultra SP-L, NOVO Ferment) produced from Aspergillus niger. The crude enzyme hydrolyzed chelator-soluble pectin from red ripe tomato fruit. Methylation linkage analysis of the product suggested that an increase in terminal-rhamnosyl residues accompanied pectin hydrolysis, indicative of RGase activity. Cross-linked alginate, hydroxyapatite, and DEAE-Sephadex chromatography were used to partially purify RGase. Polygalacturonase was efficiently removed using the alginate column. Crude pectin obtained from mature-green tomato fruit cell wall by extracting with 0.5 M imidazole buffer (pH 7) and 50 mM Na-carbonate was incubated with pure polygalacturonase and the residue hydrolyzed with 0.1 N trifluoroacetic acid. This modified pectin was used as a substrate to investigate the presence of RGase in tomato and other fruit.