Esterase-like activity of human serum albumin. V. Reaction with 2,4-dinitrophenyl diethyl phosphate.

Abstract
The reaction of 2,4-dinitrophenyl diethyl phosphate (DDP) with human serum albumin (HSA) was investigated kinetically at various pHs and 25.degree. C. The pH profile of the catalytic rate constant indicated the involvement of an ionizable group with pKa 7.5 in the reaction. Ethoxycarbonylation of about two histidine residues (imidazole groups) per mol of HSA by diethylpyrocarbonate inactivated the reaction of DDP with HSA by about 70%, suggesting the existence of more than two reactive histidine residues towards DDP. Among these residues, one has higher activity than the others and it appears to be located near the tyrosine-411 residue (R site), because the reaction with DDP was inhibited most strongly by drugs which bind to the R site of HSA.

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