Chemical modification and NMR studies on a mushroom lectin Ischnoderma resinosum agglutinin (IRA)

Abstract

Chemical modification and NMR studies on a beta-galactosyl-specific lectin which was isolated from the fruiting bodies of a mushroom, Ischnoderma resinosum, has been carried out in order to investigate the amino acid residues involved in its sugar-binding sites. Modification of amino groups with succinic anhydride greatly affected the hemagglutinating activity. Inhibitory sugar lactulose could prevent the loss of the activity. Modification of carboxyl groups with glycine ethyl ester led to a 75% loss of the activity, the presence of inhibitory sugar being protective against the modification. Treatment with cyclohexane-1,2-dione for modification of arginine residues was accompanied by a complete loss of the activity. The arginine residues modification could also be protected by the inhibitory sugar. N-Bromosuccinimide treatment for modification of tryptophan residues caused a loss of the activity, although the inhibitory sugar exhibited no protective effect against this treatment. Modification of thiol groups with 5,5'-dithiobis(2-nitrobenzoic acid) resulted in a 50% loss of the activity. Modification of histidine residues with ethoxyformic anhydride led to a complete loss of the activity. The loss of the activity could be protected by the inhibitory sugar. Treatment with N-acetylimidazole for modification of tyrosine residues was accompanied by a loss of the activity. This modification was completely prevented in the presence of the inhibitory sugar. The activity of the tyrosine-modified lectin was recovered by the treatment with hydroxylamine. Furthermore, in the NOESY spectrum of the mixture of IRA and its inhibitory sugar, methyl beta-galactoside, an NOE cross peak between H-3 and/or 5 of the p-hydroxyphenyl group of a tyrosine in the lectin, and H-5 of the galactoside could be observed. These results indicate that a tyrosine residue is involved in the carbohydrate-binding site of the lectin. In addition, line broadening and down-field shifts of the galactoside-protons were observed in the presence of the lectin.