Alkylating derivative of hexadecamethonium protects muscle synaptic acetylcholinesterase against inhibition.

The action of the alkylating derivative of hexadecamethonium on frog neuromuscular transmission was studied with the help of intracellular microelectrodes. Treatment of frog m. cutaneous pectoris-n. pectoralis preparations with the alkylating derivative of hexadecamethonium (0.5 microM) for 30 min led to an irreversible decrease in the amplitude of the end-plate potentials by 2.5-fold without a change of their latency period or quantal content. Such treatment led also to a considerable reduction of the anticholinesterase effects of neostigmine and of the organophosphorus irreversible inhibitor, armine. Thus, when applied to intact nerve-muscle preparations, neostigmine (2 microM) or armine (1 microM) increased the amplitude of end-plate potentials by 80-90%, and the rise time and half-decay time by about 2- to 3-fold. However, after the nerve-muscle preparations were pretreated with the alkylating derivative of hexadecamethonium (0.5 microM, for 30 min), the amplitude of end-plate potentials increased by 20-25%, rise time by 15-20% and half-decay time by 40-50% only. Investigation of muscle acetylcholinesterase activity, using the Ellman technique, showed that the alkylating derivative of hexadecamethonium diminished the sensitivity of the muscle acetylcholinesterase to inhibition without exerting its own inhibitory action.

[The effect of pH and reversible inhibitors on decarbamylation of acetylcholinesterase]. / Vliianie pH i obratimykh ingibitorov na dekarbamilirovanie atsetilkholinésterazy.

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.

[Interaction of membrane-bound and solubilized erythrocyte acetylcholinesterase with carbamates]. / Vzaimodeistvie membranosviazannoi i soliubilizirovannoi atsetilkholinésterazy éritrotsitov s karbamatami.

Physostigmine and neostigmine methylsulphate are shown to be the most strong inhibitors of acetylcholine esterase of human erythrocytes. The action of baigon is less pronounced and pyrimor is characterized as the weakest inhibitor. No differences are found between the membrane-bound and solubilized acetylcholine esterase relative to their ability to be inhibited by these carbamates. The preliminary treatment of acetylcholine esterase with carbamates protects the enzyme from the subsequent inhibition by the organophosphoric inhibitor. A higher concentration (1.6-2.1 times) of physostigmine and pyrimor and lower (1.7-1.9 times) one of baigon and neostigmine methylsulphate are needed for protection of the soluble enzyme than of the membrane-bound enzyme.

[Intracellular distribution of phosphoorganic cholinesterase inhibitors in rat brain]. / Vnutrikletochnoe raspredelenie fosfororganicheskikh ingibitorov kholinésterazy v mozge krys.

Content of organophosphorous inhibitors (with the structure RO/CH3/P/O/SC2H4SC2H5) of cholinesterase as well as their hydrophobic properties (distribution coefficient in hexan/water system) were studied in subcellular fractions of rat brain. Relative content of organophosphorous inhibitors was distinctly decreased in supermicrosomal fraction with increase of hydrophobic properties of the fraction. Nuclear and mitochondrial fractions contained the more hydrophobic substances in relatively higher amount. When homogenate of supermicrosomal fraction was incubated at 37 degrees, own brain cholinesterase was not depressed by organophosphorous inhibitors, containing in the fraction at low concentration. The phenomenon exhibits that content of free organophosphorous inhibitors is distinctly lower in the subcellular fractions studied than amount of the inhibitors, extracted with chloroform.

[Effect of environment on allosteric properties of acetylcholinesterase]. / Vliianie uslovii sredy na allostericheskie svoistva atsetilkholinèsterazy.

In the presence of organophosphorus inhibitors (OPI) AChE inhibition is initiated at a lower concentration of ACh; the plot reaction rate versus substrate concentration shows two maxima with a distinct minimum between them. It was shown that extremely mild conditions (short-term heating up to 50 degrees C; acidic or alkaline pH shift by 0.5 units; high concentrations of bivalent cations; erythrocyte storage) which do not affect substrate inhibition, remove this effect. The data obtained suggest that OPI effect is not directed to the site of AChE responsible for enzyme inhibition by ACh excess ("substrate inhibition site"), but to some other area. This results in a change in the conformation of the substrate inhibition site and a pronounced inhibition of the AChE activity takes place at lower substrate concentration.

[Binding of phosphoorganic cholinesterase inhibitors in rat brain tissue]. / Osobennosti sviazyvaniia fosfororganicheskikh ingibitorov kholinésterazy v tkani mozga krys

Hydrophobity (coefficient in distribution in the hexane water system) and the content of cholinesterase organophosphorous inhibitors (OPI) of the structure Ro (CH3) P (O) SC2H4 SC2H5 were studied in the rat brain. When the O-alkyl radical is increased hydrophobity rises and the relative content of free OPI in the brain extracted by chloroform decreases. With an increase in R from the ethyl to butyl one the ability to the additional inhibition of the brain own cholinesterase lowers due to incubation of homogenate at 37 degrees C, that evidences for an essential drop in the studied series of the free OPI fraction relative to the free OPI extracted by chloroform.

[The distribution of cholinesterase charged phosphorganic inhibitors and their uncharged analogues in tissues].

The article deals with the hydrophobic character (distribution coefficient in the systems hexane - water and chloroform - water) and certain peculiarities of distributing three cation-containing phosphoroganic inhibitors of cholinesterase and their uncharged analogues in the organisms. The distribution coefficients in the charged and uncharged compounds in the system hexane - water differ inconsiderably, whereas in the system chloroform-water by the thousands and millions times. In rabbits with intravenous administration the content of all inhibitors in blood decreases rapidly, the uncharged compounds accumulate selectively in the lungs and the charged ones are distributed evenly in the tissues.

[Forms of deposition of phosphoorganic cholinesterase inhibitors in the brain]. / Formy deponirovaniia fosfororganicheskikh ingibitorov kholinésterazy v mozgu

The content and hydrophobic properties (distribution coefficient in hexan : water) of organophosphorus inhibitors OPT of the following structure--R-O (CH3)/P/O/S C2H4SC2H5 have been studied in rat brain. On enlargement of the O-alkyl radical from ethyl to isopropyl and pinacolin hydrophobecity increases from 1 to 12 and 39, while the relative content of the chloroform extractable free OPT in brain, under conditions of uniform distribution, decreases from 11--18% to 3.2%. On incubation of the homogenate at 37 degrees C the further inhibition of the specific cholinesterase of the brain indicates the presence of an absolutely free form of OPT, the amount of which is not dependent on the degree of its hydrophobicity.