[Propionylcholinesterases from the brain of Mollusca Interaction with substrates and inhibitors]

Abstract

The kinetics of hydrolysis of choline esters under the action of propyonylcholinesterases (acylcholine--acylhydrolase, EC 3.1.1.8) from the brain of some Gastropoda--Lymnaea stagnalis, Murex frunculus and Rapana thomasiana were studied. It was shown that the propyonylcholinesterases under study differ from typical cholinesterases of vertebrates. Their catalytic action is characterized by a high rate of hydrolysis of propyonylcholine (PCh) and butyrylcholine (BCh), inhibition of activity by high concentrations of the substrates and the inability to catalyze the hydrolysis of benzoyl choline and acetyl-beta-methylcholine (AMCh) with the exception of a low rate of AMCh hydrolysis induced by propyonylcholinesterase of L. stagnalis. The correlation of the hydrolysis rates of PCh, BCh and acetylcholine (ACh) is different. A comparison of the kinetic parameters of Km, V and V/Km for the enzymatic hydrolysis of substrates allowed to establish differences between various propyonylcholinesterases coupled with the values of the kinetic constants for initial and final steps of hydrolysis. Propyonylcholinesterase from M. trunculus is a "proper" propyonylcholinesterase, since PCh is the best substrate for it, both at the stage of the enzyme-substrate complex formation and upon its catalytic conversion. The most preferable substrate for the L. stagnalis enzyme at the stage of the enzyme-substrate complex formation is ACh, that for the R. thomasiana enzyme--BCh. The Kss values for the substrate inhibition of propyonylcholinesterase activity were determined. The enzyme from M. trunculus is characterized by the lowest Kss values for BCh, PCh and ACh. The propyonylcholinesterase activity is inhibited by eserine and organophosphorus inhibitors (OPI). The values of bimolecular constants (kappa II) of the rate of interaction with the cationic OPI, methylsulfomethylate (O-ethyl-S-(beta-ethylmercaptoethyl) methylthiophosphonate (Gd-42) for all propyonylcholinesterases greatly exceed those for the corresponding cation-free analog of Gd-7; however, the kappa IIGd-42/kappa IIGd-7 ratio is different. The selectivity towards the acyl radical structure and the split-off moiety of the OPI molecule is also different, i. e. the highest kappa II for L. stagnalis propyonylcholinesterase was determined with Gd-42, for the M. trunculus enzyme--with methylsulfomethylate O,O-diethyl-S-(beta-ethylmercaptoethyl) thiophosphate, for the R. thomasiana enzyme--with iodomethylate O,O-diethyl-S-(beta-cyclohexyldimethylaminoethyl) thiophosphate. Data on OPI suggest that the propyonylcholinesterases under study differ in some features in the structure of anionic and esterase moieties of their active surface.