Polymerization of Soybean Proteins and Spinach RuBisCO by FXIIIa with Respect to the Labeling of Reactive Glutaminyl Residues

Calcium-activated human placental factor XIII was assayed for polymerization on some food proteins. High molecular weight polymers were visualized on SDS-PAGE. Polymerization of soybean proteins, β-conglycin, a trimer [α,α',β], and glycinin, a hexamer of A-B dimer, depends on protein concentration, time, and the presence of DTT, although no disulfide bridge is known for β-conglycin. FXIIIa can also polymerize spinach RuBisCO, especially the L subunit. These proteins, β-conglycin, glycinin, and RuBisCO, were then assayed for incorporation of monodansylcadaverine to identify the subunit(s) containing reactive glutaminyl residues. Although all subunits can be polymerized, when purified, only the α of β-conglycin, A of glycinin, and S of RuBisCO subunits were labeled on the whole proteins. These findings could be explained by the better accessibility of the reactive glutaminyl residues from the isolated subunits to FXIIIa