Mn2+ and Ca2+ binding to the lima bean lectins.

We have purified the two lectins from lima beans (Phaseolus lunatus) and studied the binding of Ca2+ and Mn2+ ions to each. The 120,000-dalton tetrameric lima bean lectin (LBL4) and the 240,000-dalton octamer (LBL8) were purified by chromatography on Ultrogel and sulfopropyl Sephadex. Using equilibrium dialysis, we have found that demetallized LBL4 (apo-lbl4) will bind either 2 Mn2+ or 4 Ca2+ ions. The Ca2+ binding is strongly cooperative while the Mn2+ binding is not. If apo-LBL4 is presaturated with Mn2+ (Mn2+-LBL4), it will bind only 2 Ca2+ ions in a noncooperative manner. Apo-LBL4 presaturated with Ca2+ (Ca2+-LBL4) does not bind Mn2+. The metal stoichiometry for LBL8 is double that for LBL4 under all of the above conditions.