Fusidic acid sodium salt (SQ-16360) is a sodium salt form of fusidic acid, a bacteriostatic antibiotic derived from the fungus Fusidium coccineum and used as a topical medication to treat skin infections.

Binding assays: Binding assays are performed as previously described (Allenby et al., 1993, 1994). Briefly, labeled and unlabeled retinoids are added to nucleosol or cytosolic fractions in ethanol so that the total amount of ethanol added is constant in all tubes and did not exceed 2% of the incubation volume. The receptor preparations are incubated with retinoids at 4°C for 4–6 hr. Sephadex PD-10 desalting columns are used to separate bound radioligand from free radioligand after equilib- rium is achieved. For competitive binding assays, varying concentrations of unlabeled competing ligand are incubated with the appropriate nucleosol or cytosol in the presence of a fixed concentration of [3H]tRA (sp. act. 49.3 Ci/mmol) or [3H]9-cis RA (sp. act. 24.0 Ci/mmol). Final concentrations of [3H] tRA and [3H]9-cis RA for nuclear receptor binding assays are 5 nM. Final concentrations of [3H] tRA for CRABP binding assays is 30 nM. The IC50s are calculated as described above (DeLean et al., 1978). For saturation kinetics, increasing concentrations of radiolabeled ligand ([3H]tRA sp. act. 49.3 Ci/mmol, [3H]TTNPB sp. act. 5.5 Ci/ mmol) are added to the nucleosol of the appropriate receptor subtype in the presence (nonspecific binding) or absence (total binding) of a 100-fold molar excess of the corresponding unlabeled retinoid. Specific binding is defined as the total binding minus nonspecific binding. Saturation kinetics are calculated as previously described (Scatchard, 1949; Grippo and Gudas, 1987; Levin et al., 1992).