Cell binding and mitogenic properties of the lima bean lectins.

Two lectins, a tetramer designated LBL4 and an octamer LBL8 designated have been purified from the lima bean Phaseolus lunatus. The tetramer appears to be nonmitogenic for human lymphocytes and is a weak mitogen for bovine cells. The octamer and a chemically cross-linked form of the tetramer are good mitogens. The lima bean lectin binds to only certain sub-populations of human lymphocytes. The primary class which does not bind appears to be a sub-population of T-lymphocytes. Comparisons of cell binding with other lectins which bind to 2-acetamido-2-deoxy-Dgalactose have been carried out. Quantitative analysis of the binding to human erythrocytes is co-operative but binding to lymphocytes is non-co-operative. These results show that there may not be a direct correlation between mitogenic stimulation and cooperative binding to membrane receptors.

Metal ion binding to concanavalin A.

Metal ion activation of saccharide binding has been studied for concanavalin A near pH 7.0. Although two metal ions, a transition metal ion and a Ca2+ ion, can bind, both are not required. Ca2+ alone, Mn2+ alone, or Ca2+ with other transition metal ions can activate this lectin. Only one Ca2+ ion per subunit or only one Mn2+ per subunit is sufficient. Metal ion binding was studied by magnetic resonance techniques and direct binding assays. Saccharide binding activity was monitored by following the fluorescence of 4-methylumbelliferyl a-D-mannopyranoside. When Ca2+ binds to demetalized concanavalin A, the transition metal ion site is hindered. When Mn2+ alone binds to demetalized concanavalin A, saccharide binding activity is induced. A subsequent conformational change, not necessary for carbohydrate binding activity, covers the Mn2+.