Esterase-like activity of human serum albumin: Structure-activity relationships for the reactions with phenyl acetates and p-nitrophenyl esters.

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Abstract
To study the reactivity of human serum albumin (HSA) with ester-type drugs and to characterize the site of the esterase-like activity, the Michaelis constants (Ks) and the catalytic rate constants (kcat) were determined for the reactions of phenyl acetates and p-nitrophenyl esters with HSA at 25.degree.. A linear relationship between log kcat and Hammett .sigma.-values was found for phenyl acetates at pH 9.9; its slope was +1.52. Aspirin may react with HSA by the same mechanism. The effects of aromatic substituents on the KS values were small. The KS values for p-nitrophenyl esters at pH 7.0 were correlated with Hansch''s .pi. [hydrophobic substituent constant] and Taft''s ES [steric substituent constant] values as follows; log KS = -0.578.pi. - 0.184 ES - 3.566 (r [correlation coefficient] = 0.963). The hydrophobic interaction was predominant in the binding of the substrates to HSA. The log kcat-pH profile obtained for p-nitrophenyl acetate indicates the participation of a single catalytic group, pKa [negative log of the Ka] = 9.5, in this reaction.