Fine structural features of oyster glycogen: mode of multiple branching

Abstract

The fine structural features of oyster glycogen, especially its mode of multiple branching, was investigated by repeated enzymic treatment with β-amylase and pullulanase, followed by the precise analysis of the α-1,4-linked glucosyl unit-chains by high performance anion exchange chromatography (HPAEC). The purified glycogen (average mol. wt 8.5 × 10 5, CL 11) obtained by DMSO-extraction from fresh oysters ( Crassostrea gigas) collected in February (a time when the oysters are edible) showed a distribution of α-1,4- d-glucosyl unit-chains, with degrees of polymerization (dp) in the range 2–35 (dp 6, dominant), as measured by HPAEC after complete enzymic debranching. The oyster glycogen was subjected to stepwise degradations with β-amylase and pullulanase, and this procedure was repeated until complete hydrolysis was achieved (extent and degradation of 98% after five treatments). The yield of the limit dextrin formed at each trimming step and quantitative analysis of the unit-chain distributions indicated that the oyster glycogen has a highly branched structure (A:B-chain, 0.7:1), involving five or six times interlinkings of the chains (B-chains). Assuming that B1 chain carrying only A-chains, attaches by α-1,6-bonds to another B-chain (B2 chain), which in turn attaches to a B3-chain, and so on, the molar ratios of the unit-chains (A, B1, B2-) of the dextrins during successive enzymic trimming showed that the ratio of A:B1:B2:B3:B4:B5-chain was 34:25:11:5:5:1, confirming the multiple ramified molecule. In connection with the digestion of oyster glycogen in the mammalian digestive tract, the glycogen was hydrolyzed by salivary and pancreatic α-amylase, and several branched maltosaccharides in the digestion product were fractionated, and their structures determined using HPAEC.