Studies on “Nonspecific” Binding

The binding of flourescein to bovine serum albumin was studied by equilibrium dialysis under various conditions. At pH 8.0 and 5°C there are three binding sites with association constants of 2.8 × 104 M−1. Specific labelling of the binding sites using fluorescein‐isothiocyanate showed that two of the binding sites are immediately adjacent. The labelled peptides were isolated and the sequence around one of the binding regions was determined revealing the presence of three lysines in this eight amino acid sequence. The binding is mainly casused by hydrophobic interactions and to lesser extent by electrostatic forces. The electrostatic contribution is much less than expected compared with other substances which are bound to bovine serum albumin. This is probably related to the fact that fluorescein is a nonphysiological compound and is “nonspecifically” bound to bovine serum albumin. Studies on binding properties of various peptic fragments of bovine serum albumin were performed. They showed that even slight fragmentation caused a drastic decrease in the ability to bind fluorescein.