Comparative Studies of Carbohydrate-Binding Proteins from Xenopus laevis Skin and Eggs

Salt and detergent extracts of acetone-dried powder of Xenopus laevis skin and eggs were fractionated on sugar-Sepharose columns, to which lactose, melibiose, galactose, rhamnose and mannose had been covalently linked, by successive elution with chelating reagent and specific sugars, resulting in separation of the different Ca2+ -dependent and Ca2+ -independent carbohydrate-binding proteins. The skin of X. laevis contains a salt-extractable Ca2+ -dependent lactose-binding lectin of 30 kilodalton (kDa) and the eggs a similar lectin of 43kDa, but they both lack Ca2+ -dependent galactose-binding lectins, The 30 kDa lactose-binding lectin which agglutinates human A erythrocytes was isolated by successive affinity chromatography on two linked sugar-Sepharose columns, i.e., a galactose-Sepharose-lactose-Sepharose (GL) column system. Since the 30 kDa lectin was not recovered in the Ca2+ -dependent lactose-binding protein fraction from the GL column system under the dithiothreitol (DTT)-free conditions, it was concluded that the lectin requires the presence of DTT and calcium for binding to the lactose-Sepharose column.