A method of determining inhibitory constants during reversible inhibition of enzymatic hydrolysis of a substrate

Determination of inhibitory constants and antienzymic activity of reversible retardants of different type of action by the generalized inhibitory constant K sigma is estimated, results being presented. To determine more precisely the values of inhibitory constants of the competitive (Ki), noncompetitive (K'i) inhibition and of the generalized K sigma it is suggested to conduct linearization of the experimental data in coordinates: 1/K sigma, s; 1/[S], where K sigma, s is a total inhibitory constant whose value depends on the substrate [S] concentration and to make calculations by the least-square method with the allowance for principal intervals of the Km values and the maximal rate of the enzymic reaction V. Comparative calculations are made by a computer using the BASIC/RT-60 language programme through the example of the acetyl choline acetyl hydrolase (EC 3.1.1.7) interaction with the quaternary phosphonium salts--reversible retardants of this enzyme.     

Selective N-heterylazimine inhibition of reactions catalyzed by rat liver glutathione transferase]

Three reactions (nucleophile substitution, thiolysis and N-deoxygenation) catalyzed by rat liver glutathione transferase have been studied using several N-heterylazimine inhibitors. The inhibitors are sharply different in their effectiveness in the transferase reactions. Their efficiency depends on their structure. The mechanism which underlies the found regularities is suggested.     

The effect of pH and reversible inhibitors on decarbamylation of acetylcholinesterase

Decarbamylation rate of membrane-bound methyl- and dimethyl-carbamylated acetylcholinesterase of human erythrocytes and bovine brain is reliably 1.1-1.6 times lower than that of the soluble enzyme. Such reversible inhibitors as tacrine (of non-competition action), ambenonium (mixed action) and galanthamine (competitive type of action) decelerate the decarbamylation rate of acetylcholinesterase. At pH 6 tacrine inhibits the reduction rate of soluble acetylcholinesterase activity of human erythrocytes more intensively than that of membrane-bound acetylcholinesterase. No differences in decarbamylation rate were found for the both forms of the enzyme at pH 8. Tacrine, a non-competitive inhibitor in concentrations below the inhibition constant (Ki = 1.4 x 10(-7) M) exerts the most intensive effect on the decarbamylation rate of methyl- and dimethylcarbamylated acetylcholinesterase of the mouse brain, while ambenonium and galanthamine in concentrations much (tens times) exceeding their Ki (3.1 x 10(-10) M and 4.4 x 10(-7) M, respectively) provide a decrease of the decarbamylation rate.     

A theoretical conformational analysis of several substrates of cholinesterase having a cyclic ammonium group structure]

Conformational possibilities of pirrolidine analogues of acetylcholine beta-(N-methyl pirrolidinium)-ethyl ester of acetic acid and beta-(N-ethyl pirrolidinium)-ethyl ester of acetic acid and beta-(N-ethyl pirrolidinium)-ethyl ester of acetic acid were investigated by the method of atomic potentials. The conformational energy was considered as a sum of non-bonded and electrostatical interactions, torsional energy and distortions of bond angles. It has been shown that the replacement of the nitrogen methyl group to ethyl group results in decrease of the average barrier height between two gauche conformations of the O--C--C--N fragment. Comparison of conformational properties of some cholinesterase substrates permit to draw a suggestion that the barrier height influences the rate of the enzymatic hydrolysis.     

Guanidine Derivatives: Conformation, Capability for Chelation, Study as Reversible Inhibitors of Cholinesterases of Different Origin

Study of spatial structure of biologically active guanidine derivatives by the method of molecular mechanics has shown that in an anticoccidial drug, 1,3-bis ( p-chlorobenzylidenamino)guanidine (Cl-BAG) the most preferable are convolute conformations, in which the chlorine atoms that are distant in the valent chain are approached to each other at a distance of 3.7 . This indicates predisposition of the optimal conformations to form chelate complexes with ions of metals, which is confirmed by comparative spectrophotometric studies of the second derivative of differential UV-spectra of Cl-BAG in the presence and absence of calcium ions. Its derivative without chlorine (BAG) is unable to bind Ca²⁺ and has been shown to have no anticoccidial action, which associates the biological potency with the presence of calcium-binding ability of the compounds. The capability of Cl-BAG for chelation depends essentially on nature of the chelated metal ion. The antienzyme testing of inhibiting action of the guanidine derivatives toward cholinesterases of human erythrocytes, horse blood serum, mink brain and serum, optic ganglia of the Pacific squid Todarodes pacificus has revealed difference between the enzymes due to possibility of redistribution of the positive charge between the guanidinium fragment and amino groups and a change of the degree of charge delocalization.     

On Mechanism of Interaction of Organophosphorus Inhibitors with Cholinesterases of Different Origin

A study is carried out as a development of A.P. Brestkin's concept of mechanism of irreversible inhibition of cholinesterases (ChE) by organophosphorus inhibitors (OPI) with taking into account reversibility of the first stage of this reaction, which has made it possible to determine individual constants of separate stages of the process. For the first time, a comparative study is performed on horse blood serum BuChE, human erythrocyte AChE, and ChE of optical ganglia of Pacific squid Todarodes pacificus. Besides, the OPI set is enlarged essentially due to use of some highly specific inhibitors of each of the enzymes. To evaluate the cholinesterase activity, chromogenic indophenol esters are used as substrates. For each of the studied ChE, differences in sensitivity to the studied OPI are realized only in values of the kinetic constant of formation of the enzyme-inhibitor complex (k ₅), whereas the rate constants of dissociation of this complex to initial components (ChE and OPI) (k _–5) and of process of its transformation into phosphorylated ChE (k ₆) are close to each other by the values, values of these constants k _–5 and k ₆ for different enzymes also being similar. Some statements about the molecular mechanism of the cholinesterase catalysis are formulated. It is suggested that the revealed elements of similarity of different ChE are realized in the work of the catalytic machine of active centers of the enzymes.