[Special role of phosphate in the stability of the protein to the destruction by polyelectrolyte].

X-ray analysis shows the presence of specific anion-binding sites in proteins for sulfate, citrate and phosphate ions, but the functional role of these anions is not always clear. Thus, it is unknown which of the two types, mono- or divalent phosphate, plays an important role in the stability of proteins to stress effects on cells. In the present work, the influence of phosphate, sulfate, and chloride salt on the stability of lactate dehydrogenase (LDH) to its destruction by poly(styrenesulphonate) (PSS) was investigated by the methods of steady-state kinetics and the own protein fluorescence. The analysis was based on the differences between the influence of phosphate and sulfate ions on the process at two pH values, 6.2 and 7.0, at which the ratio of mono- and divalent phosphate changed, whereas sulfate remained in the divalent form. It was shown that the difference between the influence of phosphate and sulfate ions increased with increasing pH, which indicates that divalent phosphate ions much more effectively stabilized LDH compared to sulfate and monovalent phosphate. The differences in the effect of sulfate and chloride salts on the protein corresponded to differences in ionic strength of their solutions. The study of the own fluorescence of LDH in the complex with PSS showed that the rate of fluorescence quenching and the amplitude of the fast stage significantly decreased with increasing concentration of divalent phosphate in solution, as compared to the same effect in the presence of sulfate anions. The conclusion was made that, from two anion-binding sites in the LDH molecule, the intersubunit center is most important in stabilizing the protein to the destruction by polyelectrolyte, and from two phosphate anions, hydrophosphate HPO4(-2) plays the stabilizing role.