The binding of the colored cholephylic anions tetrabromosulfonphthalein (BSP), di-bromosulfonphthalein (DBSP), indocyanine green (ICG) and thymol blue (ThB) to a number of protein preparations including bovine serum albumin, human serum, rat hepatic cytosol and purified rat liver bilitranslocase has been studied by a direct spectrophotometric method. The experimentation provides extinction coefficients, dissociation constants and number of binding sites for the different complexes between dyes and the various proteins. Data obtained by this technique were in excellent agreement with those obtained on the same samples by ultrafiltration. The data presented indicate that the direct spectrophotometry applied to these dyes is simple, rapid and reproducible, making this the approach of choice during the purification of binding proteins when the binding capacity is the only useful criterion to follow the progress of the procedure.
