Protolytic properties and biological activity of spinorphin and its butyric acid derivatives in aqueous solution

The present work describes the protolytic properties and in vitro biological activity of spinorphin (Leu-Val-Val-Tyr-Pro-Trp-Thr) and three spinorphin derivatives containing butyric acid residue: Butyryl-Lys-Lys-Leu-Leu-Val-Tyr-Pro-Trp-Thr, Butyryl-Lys-Lys-Leu-Val-Val-Tyr-Pro-Trp-Thr (butyric acid bound to the α-amino group of lysine), Lys(Butyryl)-Lys-Leu-Val-Val-Tyr-Pro-Trp-Thr (butyric acid bound to the ε-amino group of lysine) in an aqueous solution. The overall protonation constants and the stepwise dissociation constants of the ligands studied were calculated by the potentiometric method. The percentage of each species formed was estimated from the species distribution curves as a function of pH. The biological activity of all tested compounds was characterized in vitro, in the neutral red uptake and Griess assay tests in RAW264.7 macrophage cell line. The three protonation constants for spinorphin and four for its derivatives suggest that metal ions may bind to these peptides and form complexes by coordination with the functional groups of the respective amino acid residues. In vitro biological activity tests suggest that two peptides deserve attention for their potential anti-inflammatory role.