Calcium- and magnesium-dependent aggregation of legume seed storage proteins

The solubility characteristics and sedimentation behavior of total or indiv idual globulins from legume seeds [Lupinus albus L., Pisum sativum L., and Glycine max (L.) Merr.] were investigated. The typical insolubility of glob ulins detected during their extraction seems to be due to the presence of a low molecular weight factor(s) in the seed extract. The solubility of the purified globulins decreases with increasing concentrations of calcium and/ or magnesium, but not of other cations, showing minimum values at concentra tions that vary with the particular globulin considered. Ultracentrifugatio n analyses revealed that the Ca2+- and/or Mg2+-induced insolubilization of the globulins involves the formation of high-order aggregates of molecules of the same or of different globulins. These macromolecular structures are dissociated under conditions of high ionic strength, suggesting the involve ment of electrostatic interactions in the aggregation process. The degree o f association relies heavily on the amount of Ca2+- and/or Mg2+ available, on the presence of chelating agents for these divalent cations, and on the ionic strength of the surrounding medium. The possible physiological signif icance of the findings is discussed.