title: Ionization of tyrosine residues in human serum albumin and in its complexes with bilirubin and laurate.
abstract: Spectrophotometric titration of human serum albumin indicates that ionization of the 18 tyrosine residues takes place between pH 9 and 12.7. A Hill plot indicates that protons dissociate co-operatively from tyrosine residues, in pure albumin between pH 11.0 and 11.4 with a Hill coefficient 1.7, and in the bilirubin-albumin complex between pH 11.2 and 11.7 with a Hill coefficient 1.6. With a stopped-flow technique it is shown that about seven of the tyrosines ionize fast, with rate constants well above 10(2) s-1, when pH is suddenly changed from near neutral to pH 11.76. Further residues ionize slowly, with rate constants around 10(2) s-1 or less. The N-form of albumin (pH 6) contains one more fast ionizing tyrosine than the B-form of albumin (pH 10). Binding of bilirubin or laurate to the albumin molecule (molar ratio 1:1) transforms one to three of the fast ionizing tyrosines to slowly ionizing.
authors.affiliation: Institute of Medical Biochemistry, University of Aarhus, Denmark.
authors.forename: B
authors.initials: B
authors.lastname: Honoré
authors.forename: R
authors.initials: R
authors.lastname: Brodersen
mesh_terms: D001663:Bilirubin / Q000737:chemistry*; D001665:Binding Sites; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007848:Laurates / Q000737:chemistry*; D012709:Serum Albumin / Q000737:chemistry*; D013053:Spectrophotometry; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D007848:Laurates; D012709:Serum Albumin; D014443:Tyrosine; D001663:Bilirubin