title: Evidence that the conformational stability of 'aged' organophosphate-inhibited cholinesterase is altered.
abstract: In order to determine whether a structural modification at the active center of cholinesterase may alter the conformational stability of the enzyme we compared the urea-induced unfolding of the tetrameric form of non-inhibited and irreversibly inhibited human plasma cholinesterase (acylcholine acylhydrolase, EC 3.1.1.8). We studied enzyme inhibited by methanesulfonyl fluoride, diisopropylfluorophosphonate (DFP) and racemic soman. DFP- and soman-inhibited cholinesterases are converted spontaneously into non-reactivable forms called 'aged' enzymes through a process involving dealkylation of the bound organophosphate residue. The unfolding was followed by transverse urea-gradient polyacrylamide electrophoresis at various temperatures ranging from 0 to 60 degrees C. Unfolding of cholinesterase appears to be a complex process. The denaturation patterns showed that partially unfolded states are thermodynamically unstable, but that several intermediates are involved; the lifetime of these depends on the temperature at which electrophoreses are carried out. Cholinesterase inhibited by methanesulfonyl fluoride behaved like the non-inhibited enzyme. On the other hand, small but significant differences in stability between non-inhibited and aged enzymes were observed. Whatever the temperature, the urea concentration at the mid-point of transition was always greater for aged enzyme than for the non-inhibited enzyme. In addition, aged enzymes showed more complex denaturation patterns at the lower temperatures (under 20 degrees C). These findings suggest that the overall stability of aged-cholinesterases is slightly increased as compared with the stability of non-inhibited or methanesulfonyl fluoride-inhibited enzymes. The denaturation pattern obtained at 0 degree C for soman-inhibited cholinesterase under non-aging conditions (inhibition at 0 degree C, pH 10.7) was similar to that of non-inhibited enzyme at this temperature, although splitting in two of the denaturation curve over the transition zone reflects the heterogeneity of soman-inhibited enzyme. The slight difference in denaturation behavior between these species may be due to stereoisomerism in soman. The differences in electrophoretic behavior and apparent stability observed between non-inhibited and aged enzymes were interpreted as the result of a conformational change induced by the dealkylation reaction of enzyme-inhibitor conjugates.
mesh_terms: D002091:Butyrylcholinesterase / Q000097:blood; D002800:Cholinesterase Inhibitors / Q000494:pharmacology*; D002802:Cholinesterases / Q000097:blood; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007531:Isoflurophate / Q000494:pharmacology; D046911:Macromolecular Substances; D011487:Protein Conformation / Q000187:drug effects; D011489:Protein Denaturation; D012999:Soman / Q000494:pharmacology; D013450:Sulfones / Q000494:pharmacology; D013696:Temperature; D013816:Thermodynamics; D014508:Urea / Q000494:pharmacology
chemical_list: D002800:Cholinesterase Inhibitors; D046911:Macromolecular Substances; D013450:Sulfones; D007531:Isoflurophate; D014508:Urea; D012999:Soman; C005109:methanesulfonyl fluoride; D002091:Butyrylcholinesterase; D002802:Cholinesterases