Interaction of 1,1′-bi(4-anilino)naphthalene-5,5′-disulfonic acid with α-crystallin

The hydrophobic sites in cu-crystallin were evaluated using a fluorescent probe 1,1'-bi(4-anilino)naphthalenesulfonic acid (bis-ANS). Approximately one binding site/subunit of alpha-crystallin at 25 degrees C was estimated by equilibrium binding and Scatchard analysis (K-d = 1.1 mu M). Eased on fluorescence titration, the dissociation constant was 0.95 mu M The number of bis-ANS binding sites nearly doubled upon heat treatment of the protein at 60 degrees C. Likewise, the exposure of alpha-crystallin to 2-3 at urea resulted in increased binding of bis-ANS. Above 3 nn urea there was a rapid loss in the fluorescence indicating the loss of interaction between bis-ANS and protein. The alpha-crystallin refolded from 6 nr urea showed tryptophan fluorescence emission similar to the native alpha-crystallin However, the refolded alpha-crystallin showed a 60% increase in bis-ANS binding, suggesting distinct changes on the protein surface resulting from exposure to urea similar to the changes occurring due to heat treatment. The fluorescence of tryptophan in native alpha-crystallin was quenched by the addition of bis-ANS. The quenching was inversely related to the amount of bis-ANS bound to alpha-crystallin Additionally, the binding of bis-ANS reduced the chaperone-like activity of the protein. Photolysis of bis-ANS-alpha-crystallin complex resulted in incorporation of the probe to both A-and E-subunits, indicating that both subunits in native alpha-crystallin contribute to the surface hydrophobicity of the protein.