Geldanamycin (NSC 122750)

The titration experiments are performed using the MSC system. In each experiment, 16 aliquots of 15 μL of geldanamycin (300 μM in 1% DMSO) are injected into 1.3 mL of protein (31 μM in 20 mMTris-HCl, pH 7.5, 1 mMEDTA) at 25 °C, and the resulting data are fit after subtracting the heats of dilution. Heats of dilution are determined in separate experiments from addition of geldanamycin into buffer and buffer into protein. No evidence for binding of DMSO in the nucleotide binding site is observed. Titration data are fit using a nonlinear least-squares curve-fitting algorithm with three floating variables: stoichiometry, binding constant (K_b) 1/K_d), and change of enthalpy of interaction (ΔH°). Dissociation constants estimated for geldanamycin binding to intact yeast Hsp90 is 1.22 μM, and for binding to Hsp90 N-terminal domain is 0.78 μM. No meaningful heat is observed with binding to the C-terminal fragment.

Exponentially growing cells are treated with Geldanamycin and at various times DNA synthesis is assessed by incorporation of bromodeoxyuridine (BrdUrd) and flow cytometric analysis. No marked difference in total cell number is noted during this time course for treated and untreated cultures. BrdUrd (10 μM) is incorporated over a 4-h incubation period at 37 °C and cells are harvested and fixed in 70% ethanol. After denaturation of the DNA with 2 N HC1, cells are incubated with an anti-BrdUrd mouse monoclonal antibody followed by a fluorescein isothiocyanate (FITC)-linked goat anti-mouse IgG. Cells are stained for 30 minutes at room temperature with propidium iodide and analysed by flow cytometry using a Coulter EPICS Profile Analyzer.