title: Do alkylating agents modify the histidine residue of the desensitized butyrylcholinesterase?
abstract: Tosylphenylalanine chloromethyl ketone (TPCK) and tosyllysine chloromethyl ketone (TLCK) are irreversible modifiers of histidine which is located in the catalytic triad of chymotrypsin and trypsin, respectively. The effects of TPCK and TLCK on the histidine in the catalytic triad of the desensitized butyrylcholinesterase (BChE), prepared from human serum by heating at 45 degrees C for 24 h, were investigated in detail. It is found that these reagents do not modify, but reversibly inhibit the desensitized enzyme as a function of time. Just as it is for the native enzyme, TPCK is a hyperbolic mixed-type inhibitor of the desensitized BChE with Ki, alpha and beta values of 0.017 +/- 0.003 mM, 3.942 +/- 1.125 and 0.524 +/- 0.070, respectively. However, TLCK is the pure competitive inhibitor of the desensitized BChE with a Ki value of 0.008 +/- 0.000 mM, while it is hyperbolic mixed-type inhibitor of the native form. These findings show that the conformation of the active site cavity of desensitized BChE is different from that of the native enzyme.
authors.affiliation: Hacettepe University, Faculty of Medicine, Department of Biochemistry, 06100 Ankara, Turkey. ncokugras@superonline.com
authors.forename: A Neşe
authors.initials: AN
authors.lastname: Cokuğraş
authors.forename: Doğan
authors.initials: D
authors.lastname: Cengiz
authors.forename: E Ferhan
authors.initials: EF
authors.lastname: Tezcan
mesh_terms: D000477:Alkylating Agents / Q000737:chemistry*; D001665:Binding Sites; D002091:Butyrylcholinesterase / Q000097:blood / Q000737:chemistry*; D002800:Cholinesterase Inhibitors / Q000737:chemistry*; D003956:Dialysis; D006639:Histidine / Q000737:chemistry*; D006801:Humans; D007700:Kinetics; D014107:Tosyllysine Chloromethyl Ketone / Q000737:chemistry*; D014108:Tosylphenylalanyl Chloromethyl Ketone / Q000737:chemistry*
chemical_list: D000477:Alkylating Agents; D002800:Cholinesterase Inhibitors; D014107:Tosyllysine Chloromethyl Ketone; D014108:Tosylphenylalanyl Chloromethyl Ketone; D006639:Histidine; D002091:Butyrylcholinesterase