Structural characterization of an apple juice arabinogalactan-protein which aggregates following enzymic dearabinosylation

An arabinogalactan-protein (AGP) was isolated from a cider apple juice and contained arabinose and galactose as the main sugars (Ara/Gal mol ratio 0.67), with some uronic acids (1.4%) and protein (1.7%). Its weight-average molecular weight (M(W)) was 80 kDa. Enzymatic dearabinosylation of AGP with a purified alpha-L-arabinofuranosidase released 95% of arabinose and left a galactan-protein GP(1). Enzymatic degalactosylation of GPI(1) with a beta-D-galactosidase released 42% of galactose giving GP(2). Methylation analysis of AGP, GP(1), and GP(2) revealed that AGP is built of inner chains of (1-->3)-linked galactosyl residues 6-substituted by (1-->6)-linked galactan outer chains 3-substituted by terminal arabinofuranosyl units. After dearabinosylation, beta-D-galactosidase hydrolysed specifically 76% of (1-->6)-linked galactan outer chains. Removal of arabinofuranosyl substituents led to partial aggregation of GP(1) upon freeze-drying, and further elimination of (1-->6)-linked galactan outer chains by beta-D-galactosidase did not alter the aggregation phenomenon. The formation of aggregates reveals the possibility that enzymatic degradation of AGPs can lead to haze formation in fruit juices. Copyright (C) 1996 Elsevier Science Limited.