title: Tryptophan residues play an important role in the extraordinarily high affinity binding interaction of UCN-01 to human alpha-1-acid glycoprotein.
abstract: OBJECTIVE
  To investigate the factors that contribute to the exceptionally high affinity binding of UCN-01 to human alpha1-acid glycoprotein (hAGP).
  
  
  METHODS
  Interactions between UCN-01, UCN-02, and staurosporine with native and chemically modified hAGPs were examined using ultracentrifugation and spectroscopic analysis.
  
  
  RESULTS
  The binding affinity of staurosporine, as well as UCN-02, to hAGP was lower than that of UCN-01 by 20- and 100-fold respectively. The percentage of UCN-01 that binds to hAGP was low at acidic pH but increased with increasing pH, reaching a maximum at pH 7.4. The binding of UCN-01 to desialylated hAGP was comparable to that of hAGP. No significant difference was found for the binding of UCN-01 to F1*S and A variants of hAGP. Chemical modification of the His, Lys, Trp, and Tyr residues caused a decrease in percentage of bound UCN-01. Trp-modified hAGP showed the largest decrease in binding. Tryptophanyl fluorescence quenching results indicate that Trp residues play a prominent role in the binding of UCN-01 to hAGP.
  
  
  CONCLUSIONS
  A substituent at position C-7 of UCN-01 appeared to influence the binding specificity of the drug, and Trp residues in hAGP play a prominent role in the high affinity binding of UCN-01 to hAGP.
authors.affiliation: Department of Biopharmaceutics, Graduate School of Pharmaceutical Sciences, Kumamoto University, Kumamoto 862-0973, Japan.
authors.forename: Masaaki
authors.initials: M
authors.lastname: Katsuki
authors.forename: Victor Tuan Giam
authors.initials: VT
authors.lastname: Chuang
authors.forename: Koji
authors.initials: K
authors.lastname: Nishi
authors.forename: Ayaka
authors.initials: A
authors.lastname: Suenaga
authors.forename: Masaki
authors.initials: M
authors.lastname: Otagiri
mesh_terms: D000595:Amino Acid Sequence; D000970:Antineoplastic Agents / Q000737:chemistry / Q000378:metabolism; D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D008969:Molecular Sequence Data; D019158:N-Acetylneuraminic Acid / Q000737:chemistry; D009961:Orosomucoid / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D019311:Staurosporine / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism*; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D000970:Antineoplastic Agents; D009961:Orosomucoid; C054852:7-hydroxystaurosporine; D014364:Tryptophan; D019158:N-Acetylneuraminic Acid; D019311:Staurosporine