Characterization of excitation spectra for Tb3+ luminescence from nucleic acids: calcium binding environs in icosahedral viruses

Terbium fluorescence studies of cisplatin binding to GH3/B6 pituitary tumor cells

Terbium (Tb3+) fluorescence was used to investigate the interaction of cisplatin with GH3/B6 pituitary tumor cells. The binding of cisplatin to GH3/B6 cells quenched the fluorescence intensity of bound Tb3+. The IC50 for cisplatin inhibition of Tb3+-GH3/B6 fluorescence was determined to be 190 microM. Cisplatin was found to non-competitively inhibit the cellular binding of Tb3+, causing a dramatic decrease in the maximum number of high-affinity Tb3+ binding sites (by 33%), without markedly affecting their binding affinity. The half-life for the cellular binding of cisplatin was calculated to be 2.7 min. It was suggested that the plasma membrane of GH3/B6 cells contain a specific protein receptor for binding cisplatin.

Terbium binding to neoplastic GH3 pituitary cells

The fluorescent properties of terbium (Tb3+) were used to study the calcium (Ca2+) binding sites on GH3 pituitary tumor cells. The fluorescence emission of Tb3+ was enhanced with the binding of GH3 cells, accompanied by a red shift in its excitation maximum to resemble the excitation peak of the native cell fluorescence. The Tb3+ fluorescence enhancement increased with increasing concentrations of GH3 cell protein. Scatchard plots revealed at least two classes of Tb3+ binding sites on GH3 cells. The low and high affinity binding sites have apparent dissociation constants equal to 0.56 mM and 11 microM, respectively. The high affinity Tb3+ binding was displaced by Ca2+, but the more abundant low affinity site was not sensitive to Ca2+. The data suggest that GH3 cells possess a specific Ca2+ binding receptor on their plasma membrane.