Binding studies of bile acids using the native fluorescence of the tryptophan residue of bax protein

Ursodeoxycholic acid (UDCA) and its taurine-conjugate, tauroursodeoxycholic acid (TUDCA), play a unique role in modulating the apoptotic threshold in cells. The mechanism is thought to involve, in part, inhibition of translocation for Bax from the cytosol to mitochondria. Here, we attempted to use the native fluorescence of the tryptophan residues of Bax to determine whether bile acids bind directly to recombinant Bax protein. The results showed that UDCA had no effect on the tryptophan fluorescence of Bax. Similarly, there was no evidence of direct binding between Bax protein and the more hydrophobic bile acid, deoxycholic acid (DCA). In contrast, the fluorescence change detected for Bax solution titrated against TUDCA in dimethylsulfoxide was greater than that observed with solvent alone. In conclusion, data from fluorescence spectroscopy does not support a direct interaction of UDCA or DCA with Bax protein, whereas it suggests that there may be some potential interaction with TUDCA.