Studies on the interaction between bovine serum albumin and natural and synthetic polyribonucleotidesII. Study of the interaction by means of density gradient ultracentrifugation

Abstract

The technique of density gradient ultracentrifugation in a sucrose medium has been used to study the conditions governing complex formation between bovine serum albumin and various natural and synthetic polynucleotides. Soluble complex formation between native bovine serum albumin and polynucleotide occurs in 0.0015 M citrate buffer (pH 5.5, I 0.005). If the concentration of citrate is kept constant, and the pH is raised to 6.5 complex formation is abolished. If the pH is maintained at 5.5, but the ionic strength is raised to 0.165 (0.05 M citrate buffer) complex formation is again prevented. Low concentrations of Mg 2+ will also prevent complex formation. No native bovine serum albumin-polynucleotide complexes were found in phosphate-saline buffer (pH 7.4, I 0.184). Study of heated mixtures of bovine serum albumin and polynucleotide prepared in phosphate-saline buffer showed no evidence of complex formation. Complex formation in the heated mixtures was noted in 0.0015 M citrate buffer (pH 5.5). These complexes were easily dissociated by raising the ionic strength and pH of the heated mixtures to 0.184 and 7.4 respectively. Following dissociation of the heated complexes, no protein with the sedimentation properties of native bovine serum albumin could be detected. It is concluded that stabilization of polynucleotide-bovine serum albumin complexes, both native and heated, is due mainly to electrostatic forces; if hydrogen-bonding does occur, it appears to play a minor role in bovine serum albumin-polynucleotide interaction.