Human serum albumin (HSA) is a promising drug delivery carrier. Although covalent modification of Cys34 is a well-established method, it is desirable to develop a novel covalent modification method that targets residues other than cysteine to introduce multiple functions into a single HSA molecule. We developed a tyrosine-selective modification of HSA. Three tyrosine selective modification methods, hemin-catalyzed, horseradish peroxidase (HRP)-catalyzed, and laccase-catalyzed reactions were performed, and the modification efficiencies and modification sites of the modified HSAs obtained by these methods were evaluated and compared. We found that the laccase-catalyzed method could efficiently modify the tyrosine residue of HSA under mild reaction conditions without inducing oxidative side reactions. An average of 2.2 molecules of functional groups could be introduced to a single molecule of HSA by the laccase method. Binding site analysis using mass spectrometry suggested Y84, Y138, and Y401 as the main modification sites. Furthermore, we evaluated binding to ibuprofen and found that, unlike the conventional lysine residue modification, the inhibition of drug binding was minimal. These results suggest that tyrosine-residue selective chemical modification is a promising method for covalent drug attachment to HSA.
Keywords: drug binding; human serum albumin; laccase; modification site; tyrosine click.