Binding of indole compounds with modified albumins: Characterization of the indole binding site

Abstract

The binding of indole compounds (skatole, acetyl- l-tryptophan, l-tryptophan, indolepropionate) with modified bovine plasma albumin at the primary site has been studied. Guanidination and acetamidination of the ε-amino groups (80 and 95% reacted, respectively) or carboxymethylation of the imidazole groups (90% reacted) reduces but does not destroy binding of skatole and acetyl- l-tryptophan. Guanidination of 30% of the ε-amino groups increases the association constants of skatole, acetyl- l-tryptophan, and l-tryptophan. Acetylation of 25% of the ε-amino groups by acetic anhydride has little effect on the association of skatole and indolepropionate but completely blocks association of l-tryptophan and acetyl- l-tryptophan. Other evidence indicates that the group responsible for blocking the binding of indole compounds with the α-amino and α-acetamidyl groups present (i.e. l-tryptophan and acetyl- l-tryptophan) is an ε-amino residue of lysine in an apolar environment at the binding site. The indole ring part of the binding site remained intact (as evidenced by skatole binding) in all modifications of albumin except preparations extensively acetylated with acetic anhydride.