High molecular weight glucan of the culinary medicinal mushroom Agaricus bisporus is an alpha-glucan that forms complexes with low molecular weight galactan

Abstract

An alpha-glucan was isolated from the culinary medicinal mushroom A. bisporus by hot water extraction, ethanol precipitation and DEAE-cellulose chromatography. The resulting material showed a single HMW peak excluded from a Sephadex G50 column that could completely be degraded by alpha-amylase treatment. After heating in 1% SDS a small additional peak of low MW eluted from the G50 column. The monosaccharide composition of the main peak was evaluated by HPLC, and was found to consist of a majority of glucose (97.6%), and a minor proportion of galactose (2.4%). Methylation analysis and degradation by alpha-amylase indicated the presence of an alpha-glucan with a main chain consisting of (1(R)4)-linked units, substituted at O-6 by alpha-D-glucopyranose single-units in the relation 1:8. Mono- (13C-, 1H-NMR) and bidimensional [1H (obs.),13C-HSQC] spectroscopy analysis confirmed the alpha-configuration of the Glcp residues by low frequency resonances of C-1 at delta 100.6, 100.2, and 98.8 ppm and H-1 high field ones at delta 5.06, 5.11, and 4.74 ppm. The DEPT-13C-NMR allowed assigning the non-substituted and O-substituted -CH(2) signals at delta 60.3/60.8 and 66.2 ppm, respectively. Other assignments were attributed to C-2, C-3, C-4, C-5 and C-6 of the non-reducing ends at delta 71.8; 72.8; 70.0; 71.3 and 60.3/60.8 ppm, respectively. The minor proportion of galactose that was demonstrated was probably derived from a complex between the alpha-glucan and a low molecular weight galactan.

Figure 1

DEAE-cellulose chromatography of high MW glucan of A. bisporus before and after incubation with α-amylase.

Figure 2

Dionex anion exchange HPLC profile of high MW glucan of A. bisporus after incubation with 300 U mL^-1 α-amylase for 1 h at 50 °C. The glucan was then loaded onto the column and eluted as described in the Experimental.

Figure 3

¹³C-NMR spectrum of α-glucan in dimethyl sulfoxide-d₆ at 70 °C (chemical shifts are expressed in δ ppm).

Figure 4

DEPT spectrum of α-glucan in dimethyl sulfoxide-d₆ at 70 °C (chemical shifts are expressed in δ ppm).

Figure 5

HSQC spectrum of α-glucan in dimethyl sulfoxide-d₆ at 70 °C (chemical shifts are expressed in δ ppm).

Figure 6

Structure of α-glucan isolated from Agaricus bisporus.