ESR STUDIES ON THE EFFECT OF IONIC-RADII ON DISPLACEMENT OF MN2+ BOUND TO A SOLUBLE BETA-GALACTOSIDE-BINDING HEPATIC LECTIN

Binding of divalent metal ions to hepatic soluble beta-galactoside binding lectin was studied using electron spin resonance (ESR) spectroscopy. The Mn2+ bound to hepatic lectin could be displaced by Mg2+, Cu2+ Ni2+ and Ca2+ but not by Sr2+. As the ionic radii of Mg2+ (0.65 Angstrom), Cu2+ (0.73 Angstrom) and Ni2+ (0.72 Angstrom) are appreciably smaller than Ca2+ (0.99 Angstrom), it appears that the Mn2+ binding site is more accessible to Mg2+, Cu2+ and Ni2+ as compared to Ca2+, the ionic radius of Mn2+ being 0.80 Angstrom, Sr2+ dth an ionic radius of 1.13 is thus unable to displace bound Mn2+. Surprisingly, the presence of specific sugars like alpha-lactose, or alpha-D-galactose facilitated the displacement of bound Mn2+ by metal ions whereas non-specific sugars, i.e. alpha-D-glucose, beta-D-fructose and alpha-D-ribose had no effect. It appears that minor perturbations in the saccharide binding site significantly affect the ability of the metal binding site to ligate bivalent metals.