Tryptophan residues play an important role in the extraordinarily high affinity binding interaction of UCN-01 to human alpha-1-acid glycoprotein

To investigate the factors that contribute to the exceptionally high affinity binding of UCN-01 to human alpha1-acid glycoprotein (hAGP).Interactions between UCN-01, UCN-02, and staurosporine with native and chemically modified hAGPs were examined using ultracentrifugation and spectroscopic analysis.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.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.