Protein binding of small molecules: IV. Relation between binding of phenolsulfophthalein dyes and other ligands with a high affinity for human serum albumin

Abstract

1. 1. In the present study, binding of phenolsulfophthalein (phenol red) by human serum albumin is compared with binding of bromphenol blue and a variety of other high affinity ligands. 2. 2. Phenol red and bromphenol blue are bound with a high affinity by serum albumin at 5 common sites. The association constants of these sites differ widely and are approx. 100–1000-fold smaller for phenol red than for bromphenol blue. 3. 3. 1-Anilino-8-naphthalene sulfonate (ANS), dodecylsulfate and dodecyl-sulfonate displace phenol red competitively from the high affinity sites of serum albumin. Dodecylsulfate and dodecylsulfonate are less effective inhibitors of dye binding than 1-anilino-8-naphthalene sulfonate which competes with phenol red at 4–5 sites. On the other hand bilirubin inhibits phenol red binding in more than stoichiometric amounts, while l-thyroxine does not affect dye binding. 4. 4. Serum albumin defatted by charcoal treatment binds more phenol red than native serum albumin. However, palmitate and oleate have only a modest inhibitory effect on phenol red binding, the fatty acids not being effective at binding levels below 4. 5. 5. The results indicate that common binding sites exist for phenolsulfophthalein dyes, 1-anilino-8-naphthalene sulfonate and bilirubin, whereas fatty acids and l-thyroxine predominantly are bound at other locations on the albumin molecule.