Esterase-like activity of human serum albumin. VIII. Reaction with amino acid p-nitrophenyl esters.

The reactions of human serum albumin (HSA) with optically active amino acid p-nitrophenyl esters (substrate, S) were examined kinetically at 25 degrees C. The rate data were analyzed in terms of a mechanism involving 1:1 complexing (S.HSA) between S and HSA. The dissociation constant (Ks in M) and the catalytic rate constant (k2 in s-1) of S.HSA were determined. Among ten substrates examined, the reactions with N-carbobenzoxy-D(L)-alanine p-nitrophenyl esters (N-CBZ-D(L)-AlaNP) were most accelerated by HSA. Results of the reaction in the presence of excess N-CBZ-D(L)-AlaNP over HSA indicated the existence of one strong reactive site on HSA. The effects of the reversible binding of the site-specific drug and the chemical modification by site-specific reagents on the HSA activity showed that the reactive site towards N-CBZ-D(L)-AlaNP is the R site located near tyrosine-411 residue of HSA.