PROTEIN SOLUBILITY OF RAW AND COOKED BEANS (PHASEOLUS-VULGARIS) - ROLE OF THE BASIC RESIDUES

The protein solubility of raw and cooked beans (Phaseolus vulgaris) was tested in the pH range 1.0-13.0. The protein solubility of the raw beans in water increased, up to 90-100%, on both sides of pH 4.0, while in 0.5 M NaCl it was higher than in water in the pH range 2.0-7.0. A reduction to 15-20% in protein extraction was observed, both in water and in NaCl, after bean cooking up to pH 10.0, where solubilization sharply occurred. Protein solubilization of cooked beans in water, as analyzed over 24 h at increasing pH (3.3-12.0), showed that the higher the pH, the higher the rate of protein solubilization, but a marked increase at pH 12.0, and in 8 M urea, was detected. Protein solubilization of the raw beans in water increased by lysine acetylation and succinylation or by arginine modification with phenylglyoxal. Only succinylation had a positive effect on the protein solubilization of cooked beans. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and ultracentrifugation of the protein water-extracted at pH 6.5 showed a higher protein dissociation in cooked than in raw beans and further dissociation in cooked bean proteins at pH 12.0. The involvement of the basic residues in the association-dissociation of raw and cooked bean proteins is suggested.