The physicochemical properties of the galactose-specific lectin from Momordica charantia

Abstract

A lectin has been isolated from Momordica charantia seeds and purified to homogeneity by affinity adsorption on a cross-linked arabinogalactan column and subsequent elution with 0.1 M lactose. In sedimentation velocity experiments an s°_20,w value of 6.5 S was obtained for the native lectin and of 2.7 S for its subunits obtained by dissociation with 0.1% sodium dodecyl sulphate in the presence of 0.1% 2-mercaptoethanol. This lectin has a molecular weight of 120 000, is tetrameric in nature with two pairs of non-identical subunits of molecular weights 28 000 and 30 000. It is a glycoprotein containing 10% hexoses (2% glucose and 8% mannose) and 2.5% hexosamine, all glucosamine. It has two binding sites per molecule and has K_a values of 4.1×10⁴ M⁻¹ and 1.8×10⁴ M⁻¹ at 5°C and 25°C respectively for p-nitrophenyl β-D-galactopyranoside with ΔH° and ΔS° values of −28.3 kJ mol⁻¹ and −12.3 J mol⁻¹ K⁻¹ respectively. Hemagglutinating activity of M. charantia lectin is inhibited by galactose and other carbohydrates containing the galactopyranosyl residue. From the chemical modification studies tryptophan and tyrosine residues are found to be important for the carbohydrate binding activity of M. charantia lectin.