ARE HISTIDINE RINGS THE MAIN POTENTIAL SITES OF THE INTERACTION BETWEEN PROTEINS AND THE FLUORESCENT MG2-INDO1( INDICATOR MAG)

The investigation of the physicochemical properties of Mag-indol, a fl uorescent probe used for intracellular magnesium measurements, has sho wn that, in a biological environment, the deprotonated form of this pr obe is in simultaneous equilibrium with a protonated form, a protein a nd a magnesium-bound form. To understand the origin of the interaction of Mag-indol with proteins, we have studied the effect of pH on the b inding of Mag-indol with bovine serum albumin (BSA). This interaction occurs for pH less than or equal to 8.5. This indicates that interacti on should take place with the protonated amino acids histidine, lysine and arginine. The investigation of the interaction of Mag-indol with these amino acids, or the corresponding homologous polypeptides, has s hown that interactions are only observed between Mag-indol and histidi ne and Mag-indol and polyhistidine. Furthermore, the interaction with histidine induces a blue shift of only 7 nm of the fluorescence spectr um of Mag-indol, whereas the interaction with polyhistidine induces a blue shift of 28 nm of the fluorescence spectrum of Mag-indol. The lat ter value, similar to that observed for the binding of Mag-indol to BS A, indicates that binding occurs through an interaction mediated by th e histidine ring.