Ca2+ -specific fluorescence changes in N-dansylaminoethyl-labelled oncomodulin

Rat oncomodulin (isoelectric point (pI) = 3.9) and homologous carp parvalbumin (pI = 3.95) was labelled at Cys-18 which is located in the AB loop, with the fluorescent reagent N-dansylaziridine. The fluorescence spectrum of the labelled parvalbumin was insensitive to both Mg2+ and Ca2+ binding. In contrast the dansyl fluorescence emission spectrum of labelled oncomodulin was quenched approximately 20% by Ca2+ in the presence of physiological concentrations of Mg2+. Approximately 90% of the Ca2+-induced fluorescence quenching was complete at a Ca2+/dansyl-S-aminoethyloncomodulin ratio of approximately 1. The labelled oncomodulin was insensitive to Mg2+ binding. The present study indicates that oncomodulin, unlike parvalbumin, undergoes a large Ca2+-specific perturbation that can be felt over the entire molecule. In addition this Ca2+-specific perturbation appears to be complete upon occupation of only one metal-binding site.