Partial Reversible Inhibition of Enzymes and Its Metabolic and Pharmaco-Toxicological Implications.

Partial reversible inhibition of enzymes, also called hyperbolic inhibition, is an uncommon mechanism of reversible inhibition, resulting from a productive enzyme-inhibitor complex. This type of inhibition can involve competitive, mixed, non-competitive and uncompetitive inhibitors. While full reversible inhibitors show linear plots for reciprocal enzyme initial velocity versus inhibitor concentration, partial inhibitors produce hyperbolic plots. Similarly, dose-response curves show residual fractional activity of enzymes at high doses. This article reviews the theory and methods of analysis and discusses the significance of this type of reversible enzyme inhibition in metabolic processes, and its implications in pharmacology and toxicology.

Activation/Inhibition of Cholinesterases by Excess Substrate: Interpretation of the Phenomenological b Factor in Steady-State Rate Equation.

Cholinesterases (ChEs) display a non-michaelian behavior with positively charged substrates. In the steady-state rate equation, the b factor describes this behavior: if b > 1 there is substrate activation, if b < 1 there is substrate inhibition. The mechanistic significance of the b factor was investigated to determine whether this behavior depends on acylation, deacylation or on both steps. Kinetics of human acetyl- (AChE) and butyryl-cholinesterase (BChE) were performed under steady-state conditions and using a time-course of complete substrate hydrolysis. For the hydrolysis of short acyl(thio)esters, where acylation and deacylation are partly rate-limiting, steady-state kinetic analysis could not decide which step determines b. However, the study of the hydrolysis of an arylacylamide, 3-(acetamido)-N,N,N-trimethylanilinium (ATMA), where acetylation is rate-limiting, showed that b depends on the acylation step. The magnitude of b and opposite b values between AChE and BChE for the hydrolysis of acetyl(thio)- versus benzoyl-(thio) esters, then indicated that the productive adjustment of substrates in the active center at high concentration depends on motions of both the Ω and the acyl-binding loops. Benzoylcholine was shown to be a poor substrate of AChE, and steady-state kinetics showed a sudden inhibition at high concentration, likely due to the non-dissociation of hydrolysis products. The poor catalytic hydrolysis of this bulky ester by AChE illustrates the importance of the fine adjustment of substrate acyl moiety in the acyl-binding pocket. Molecular modeling and QM/MM simulations should definitively provide evidence for this statement.

Steady-state kinetic analysis of human cholinesterases over wide concentration ranges of competing substrates.

Substrate competition for human acetylcholinesterase (AChE) and human butyrylcholinesterase (BChE) was studies under steady-state conditions using wide range of substrate concentrations. Competing couples of substates were acetyl-(thio)esters. Phenyl acetate (PhA) was the reporter substrate and competitor were either acetylcholine (ACh) or acetylthiocholine (ATC). The common point between investigated substrates is that the acyl moiety is acetate, i.e. same deacylation rate constant for reporter and competitor substrate. Steady-state kinetics of cholinesterase-catalyzed hydrolysis of PhA in the presence of ACh or ATC revealed 3 phases of inhibition as concentration of competitor increased: a) competitive inhibition, b) partially mixed inhibition, c) partially uncompetitive inhibition for AChE and partially uncompetitive activation for BChE. This sequence reflects binding of competitor in the active centrer at low concentration and on the peripheral anionic site (PAS) at high concentration. In particular, it showed that binding of a competing ligand on PAS may affect the catalytic behavior of AChE and BChE in an opposite way, i.e. inhibition of AChE and activation of BChE, regardless the nature of the reporter substrate. For both enzymes, progress curves for hydrolysis of PhA at very low concentration (≪Km) in the presence of increasing concentration of ATC showed that: a) the competing substrate and the reporter substrate are hydrolyzed at the same time, b) complete hydrolysis of PhA cannot be reached above 1 mM competing substrate. This likely results from accumulation of hydrolysis products (P) of competing substrate and/or accumulation of acetylated enzyme·P complex that inhibit hydrolysis of the reporter substrate.

A new sensitive spectrofluorimetric method for measurement of activity and kinetic study of cholinesterases.

A new spectrofluorimetric method more sensitive than the Ellman method was developed for determination of both acetylcholinesterase and butyrylcholinesterase activity and for kinetic analysis of these enzymes and their mutants. Two selected mutants of human butyrylcholinesterase (E197Q and E197G) were included in this work. As for the Ellman's method, substrates are thiocholine esters, but the chromogenic reagent, DTNB (dithio-bisnitro benzoic acid) is replaced by a fluorogenic probe, "Calbiochem Probe IV", (3-(7-Hydroxy-2-oxo-2H-chromen-3-ylcarbamoyl)acrylic acid methylester). Compared to the classical Ellman's method, the sensitivity of this new spectrofluorimetric assay is 2 orders of magnitude higher. The method allows measurement of activity in media containing <10-11 M of cholinesterase active sites at low substrate concentrations, either under first order conditions, [S] << Km, or under conditions where kinetics obeys the Michaelis-Menten model, i.e. at [S] < 1 mM for wild-type enzymes. The method adapted to titration plate reader assays is suitable for clinical and toxicological routine analyses, for high throughput screening of novel cholinesterase mutants and screening of inhibitor libraries of pharmacological interest.

Time-course of human cholinesterases-catalyzed competing substrate kinetics.

Competing substrate kinetic analysis of human butyrylcholinesterase (BChE) and acetylcholinesterase (AChE) from the time-course of enzyme-catalyzed substrate hydrolysis, using spectrophotometric assays is described. This study is based on the use of a chromogenic reporter "visible" substrate (substrate A), whose complete hydrolysis time course is retarded by a competing "invisible" substrate (substrate B). For BChE, four visible substrates were used, two thiocholine esters, benzoylthiocholine and butyrylthiocholine, and two aryl-acylamides, o-nitro trifluoro acetaminide and 3-(acetamido)-N,N,N-trimethylanilinium. Three different competing invisible substrates were used, phenyl acetate, acetylcholine and butyrylcholine. For AChE, two visible substrates were used, acetylthiocholine and 3-(acetamido)-N,N,N-trimethylanilinium. For AChE, acetylcholine was competing with visible substrates. The ratio (R) of bimolecular rate constants, kcat/Km, for all couples of substrates, invisible/visible (B/A) covered all possible limit situations, R ≪ 1, R ≈ 1 and R ≫ 1. The kinetic approach, based on the method developed by Golicnik and Masson allowed determination of binding and catalytic parameters of cholinesterases for both visible and invisible substrates. This analysis was applied to michaelian and non-michaelian catalytic behaviors (activation and inhibition by excess substrate). Reevaluation of catalytic parameters obtained for acetylcholine and butyrylcholine more than 50 years ago was made. The method is fast, reliable, and particularly suitable for poorly soluble substrates and for substrates B when no direct spectrophotometric assays exist. Moreover, replacing substrate B by a reversible inhibitor, mechanism of cholinesterase inhibition was possible to study. It is therefore, useful for screening libraries of new substrates and inhibitors, and/or screening of new cholinesterase mutants. This method can be applied to any other enzymes.