Binding of Ca(II) by the 11S fraction of soybean proteins.

Binding of Ca(Il) by the l1S fraction of soybean proteins was determined by equilibrium analysis
at pH 7.8 and 5.5. The binding was negligible at pH 5.5. At pH 7.8 binding was
increased by the addition of 0.5 M NaC!. Prior treatment with EDTA also reduced the
binding. Analysis of the binding data with the Scatchard equation suggested that the probable
binding site on the protein molecules was the imidazole group. Addition of CaII did not
cause association or dissociation of the protein. Heat coagulation was increased by the
addition of Ca(II). The protein was almost quantitatively precipitated at 1.0 x 1O-2 M Ca(II).
This precipitation was decreased by the addition of NaCl. Binding studies with the
unfractionated soybean proteins indicated that Ca(ll) was bound by the proteins as well as the
phytate impurities. When these were removed Ca(II) appeared to be bound by the proteins at
the imidazole group.