Spectrophotometric study of the interaction of xanthomegnin with serum albumin.

The interaction of bovine serum albumin (BSA) and xanthomegnin, an uncoupler of oxidative phosphorylation was studied spectrophotometrically. BSA caused a marked red shift of the absorption maxima of xanthomegnin from 406 to 555 nm but heating the reaction mixture abolished the BSA-induced spectral change. Spectrophotometric titrations of xanthomegnin with increasing concentrations of BSA gave a molar ratio of unity for these two components with an association constant of 1.4 X 10(6) M-1. The absorption spectrum of xanthomegain was affected by pH changes of the system. The pK was found to be 8.4 and was significantly shifted to a lower pH (pH 5.4) in the presence of BSA. The BSA-induced spectral change was abolished by sodium dodecyl-sulfate or 1 anilino-8 napthalene sulfonic acid but not by L-tryptophan. These results would suggest that the characteristic spectral changes of xanthomeginin are caused by reversible binding to a strong, hydrophobic anion binding site of BSA. In many respects the binding resembles that of bilirubin with this same protein. In the latter reaction, the red shift was much smaller than the reaction of BSA and xanthomegnin.