Arch. Toxikol. 27, 233-241 (1971) 9 by Springer-Verlag 1971

Inhibition of Acetylcholinesterase in Different Parts of the Brain of Mice by Isopropyl Methylphosphonofluoridate in vitro and in vivo J. BAJO~ Purkyn~i Medical Research Institute, Hradee Kr~lov6, Czechoslovakia Received October 19, 1970

Abstract. AChE activity in four parts of the mouse brain, i.e. in the pens and medulla oblongata, mesencephalon, diencephalon and basal ganglia, was inhibited in vitro by isopropyl methylphosphonofluoridate. The inhibition constants, n (Hill coefficient) and Is0 were determined. The values of both constants, n and Isc, were the same for all studied parts of the brain (n ~ 1.7 and Is0 ~ 5.0 • 10-1~ M). In experiments in rive, mice were administered the poison in doses ranging from 1.4 to 28.6 mol • 10-7/kg, i.e., 0.02 to 0.40 mg/kg. AChE activity was then measured and the degree of inhibition was correlated with the dose of organophosphate given. AChE in the basal ganglia was the most resistant. The highest degree of inhibition was observed in the ponto-medullar portion. This selective inhibition lends support to a concept of tiffs particular portion of the brain as having special importance in the toxidynamics of poisoning. The comparison of AChE inhibition in vitro and in r i v e suggests that only about 1% of an injected dose is involved inhibiting AChE in the brain. Key.Words: Organophosphate Poisoning - - Acetylcholinesterase I n h i b i t i o n - Brain Parts. Zusammen/assur~j. Die Aktivit~t der Acetylcholinesterase in den vier Partien des MEusegehirns (Pens et Medulla 0blongata, Mcseneephalon, Diencephalon und Ganglia basalia) wurde in vitro mit Isopropyl-methylphosphorofluoridate gehemmt. Die Hemmungskonstanten, n (Hill Koeffizient) und I50 warden bestimmt. Die Werte der beiden Konstanten waren fiir alle untersuchten Gehirnpartien gleieh ( n ~ l , 7 und I50=5,0 • 10-1~ Die Versuche in vivo wurden an den M~usen durchgefiihrt. Die Tiere wurdcn mit dieser Verbindung in Desert yon 1,4 bis 28,6 mol • 10-~/kg, d.h. 0.02 bis 0,40 mg/kg vergiftet. Die Aeetylcholinesterase yon Ganglia basalia erwies die grSBte Resistenz. Die hSchste Hemmung wurde in der ponto-medullar Partie beobachtet. Die selektive Hemmung best~tigte die Bedeutung dieser Pattie in der Toxidynamik der Vergiftung. Beim Vergleich der Acetylcholinesterase-Hemmung in r i v e mit der in vitro wurde festgestellt, dab nur etwa 1% der injizierten Dosis die Acetylcholinesterase im Gehirn hemmen kann. ~chliisselw~rter: 0rganophosphatvergiftung-- Acetylcholinesterase-Hemmung - Gehirnpartien.

234

J. Bajgar:

Isopropyl methylphosphonofluoridate (IMPF) belongs to the group of organophosphates with high toxicity. The basis of its action is the inhibition of cholinesterases in the central and peripheral nervous system. The distribution of I M P F and its inhibiting effects on cholinesterases have been previously studied. Using radioisotopic techniques, it was shown t h a t only a small portion of injected I M P F can subsequently be found in the brain (Christen et al., 1969; Polak and Cohen, 1969; 1970a, b). However, in such experiments it is difficult to differentiate the toxic compound from its metabolic products. I t is known from inhibition experiments t h a t I M P F is an irreversible inhibitor of eholinesterases, and its affinity to acetylcholinesterase (ACHE, EC 3.1.1.7) in vitro is higher than to cholinesterase (Aldridge, 1953; Jandorf et al., 1955; Jandorf, 1956; tIeilbronn, 1962; PatoSka, 1969a, b). I n certain cases it is possible after organophosphate administration in vivo to estimate its concentration in some organs on the basis of known values of AChE inhibition (Novgorodskaya et al., 1968; Bajgar, 1969). I t is impossible to obtain actual concentrations of I M P F in given compartments, but concentrations can be estimated from the amount of remaining active ACHE. A minimum of AChE activity in certain organs and especially in the brain is an important factor in determining whether an organism poisoned by organophosphates lives or dies (Hobbiger, 1951 ; Green, 1958; Bajgar et al., 1970; Herink et al., 1970). However, the AChE activity and inhibition in the brain after organophosphate intoxication is not uniform (Koelle, 1950; Koelle and Steiner, 1956; Bajgar, 1969; Bajgar and Urban, 1970). Presently it is not known whether the selective effect of I M P F is caused by the different affinity of I M P F to AChE in different parts of the brain, or if there is different penetration in the various areas.

I n this paper, the affinity of I M P F to AChE in different parts of the brain of mice in vitro is described and compared with the affinity to AChE in vivo following I M P F intoxication. Material and Methods

Preparation of Brain Homogenates White male mice (Mezno) weighing 15-17 g were killed by bleeding the carotid artery. The brains were removed and four different parts prepared: Pons and medulla oblongata, mesencephalon, diencephalon and a portion of the basal ganglia. Each part of the brain was homogenized (Ultra-Turrax, Janke and Kunkel, Germany) and 0.2 M tris-HCl buffer at pH 7.6 added to make a 10% homogenate. Enzyme Assay AChE activity was measured by using a modification (Bajgar and PatoSka, 1969) of the Ellman method (Ellman et al., 1961) in 0.2 M tris-ttCl buffer at pit 7.6,

Antieholinesterase Action of Isopropyl Methylphosphonofhioridate

235

with aeetylthiocholine iodide (Lachema Brao, Czechoslovakia) as substrate and 5,5'-dithio-bis-(2-nitrobenzoic) acid (Serva, Heidelberg, Germany) as ehromogen. AChE activity was expressed as zJ E/min caused by 1 mg of wet weight tissue (412 nm, Vitatron, Sei. Instr., Holland).

A C h E Inhibition in vitro The homogenates of the various parts of the brain from 6 mice were incubated with different concentrations of IMPF in 0.2 M tris-HCl buffer at pH 7.6 and 25~ C. After 2 hours, the AChE activity was measured. The AChE activity of an uninhibited reaction was taken as 100%; activities of AChE after IMPF treatment were expressed in terms of percent inhibition.

A C h E Inhibition in vivo White male mice were randomly divided iuto 9 groups with 6 animaJs in each group. The control group received i.m. injections of saline whereas the study groups were given IMPF i.m. in doses of 0.02, 0.05, 0.10, 0.14, 0.17, 0.20, 0.30 and 0.40 mg/kg, i.e. 1.43, 3.57, 7.14, 10.0, 12.1, 14.3, 21.4 and 28.6 mol • 10-7/kg. The animals were killed after 2 hours and homogenates of the brain pai~ were prepared. The AChE activity in the control group was considered to be 100 % ; AChE activities in the experimental groups were expressed in percent inhibition.

Statistical Evaluation The inhibition constants /50 were calculated in probit-logarithmical transformation programmed for a MINSK 22 computer (PatoSka and Tulach, 1969). The values of n (Hills' coefficient) were calculated by the graphical method of Taketa and Pogell, 1965. The homogeneity of experimental groups in vivo was tested by Bartletts' test. The differences between groups were calculated by regression analysis with use of MINSK 22 computer programs (Hra~ka and Tulach, 1966). Results

A C h E Inhibition in vitro F r o m probit-logarithmieal t r a n s f o r m a t i o n of p e r c e n t AChE i n h i b i t i o n correlated with I M P F c o n c e n t r a t i o n , i t is a p p a r e n t t h a t i n h i b i t i o n c o n s t a n t s of the i n t e r a c t i o n of I M P F with AChE are the same for all p a r t s of the b r a i n s t u d i e d (Fig. 1). The value of I50 for all b r a i n p a r t s is a b o u t 5 • 10-l~ M; the value of the Hill coefficient (n) for all studied p a r t s is a b o u t 1.7 (Table 1).

A C h E Inhibition in vivo AChE activities i n all p a r t s of the b r a i n were decreased i n p r o p o r t i o n to the i n j e c t e d doses of I M P F . Small doses of I M P F altered AChE a c t i v i t y b y 1 0 - 2 0 % , b u t n o clinical s y m p t o m s of o r g a n o p h o s p h a t e poisoning were observed (Table 2). A p p l i c a t i o n of doses causing t h e first s y m p t o m s of i n t o x i c a t i o n (i.e. a l t e r a t i o n of v e n t i l a t i o n , etc.) resulted i n AChE activities b e t w e e n 50 a n d 70% of controls. W i t h

236

J. Bajgar: 99

I

~

!

}"

[

I

2

5

40

20

90 50 3O

50

[ I ] * 'IO-~IOM Fig. l. AChE inhibition in different parts of mouse brain caused by LIVfPFin vitro, expressed in probit-logarithmical transformation, o pons and medulla oblongata, 9 mesencephalon, /, diencephalon, 9 basal ganglia

Table 1. Some inhibition constants o/interaction o / I M P F with A C h E in the mouse brain in vitro (n and I~o) and in vivo (IaoD). The re~alts listed represent means with their confidence limits Part of the brain

n+

I50 (M) ( • 10-1~

Ib0I) (mol/kg) ( X 10-6)

Pons and metulla oblongata

1.70 (1.66-1.75)

5.0 (4.1-5.8)

1.00 (0.91-1.11)

Mesencephalon

1.70 (1.66-1.75)

5.0 (4.1-5.8)

1.15 (1.01-1.33)

Diencephalon

1.70 (1.66-1.75)

5.0 (4.1-5.8)

1.11 (0.95-1.38)

Basal ganglia

1.70 (1.66-1.75)

5.0 (4.1-5.8)

1.30 (1.11-1.52)

n + = Hill coefficient. This coefficient indicates the apparent number of inhibitor molecules bound to the molecule of the enzyme (Monod et al., 1963; Taketa and Pogell, 1965).

doses leading t o convulsions a n d d e a t h , a r a p i d decrease of A C h E a c t i v i t y was observed. T h e c o n s t a n t s IboD were c a l c u l a t e d f r o m t h e p r o b i t - l o g a r i t h m i c a l t r a n s f o r m a t i o n of e x p e r i m e n t a l d a t a . Since t h e p e r c e n t a g e s of A C h E i n h i b i t i o n b y t h e highest a n d lowest doses adm i n i s t e r e d r e p r e s e n t those of a " s t e a d y s t a t e " , o n l y a p o r t i o n of t h e e x p e r i m e n t a l l y d e t e r m i n e d p o i n t s were used for calculating t h e constants.

Antieholinesterase Action of Isopropyl Methylphosphonofluoridate

A :~

~

237

~ 4o 20 90

5o

"~

70 70 40 I

,

r

40

20

30

dose( ,10-YmoLIA:q)

Fig. 2. AChE inhibition in the mouse brain (pons and medulla oblongat~) by I M P F in vivo. A without transformation, B in probit-logarithmical transformation.

For the calculation of transformation and IsoD constants, only the black points were used Table 2, 8ome clinical symptoms in mice poisoned by diHeren$ doses o] I M P F Dose

First

Convul-

(mol • 10-~/kg)

symptoms

sions

0/6

0/6

0/6

3/6 6/6 6/6 6/6 6/6

0/6 3/6 6/6 6/6 6/6

0/6 0/6 316 6/6 6]6

1.4 3.6 7.1 10.0 12.1 14.3 21.4 28.6

Death

No great difference in IsoD constant values for the individual brain parts was noted, and complete AChE inhibition in the various parts of the brain was never observed. Only in the pens and medulla oblongata after application of the highest doses used, did the value of AChE inhibition attain 95 to 99.9% (Fig. 2). Following application of lethal doses of IMPF, only 75-90 percent inhibition was observed in the other parts of the brain. The basal ganglia portion was the most resistant (Fig. 3). The results of AChE inhibition in vitro and in vivo are summarized in Table 1. 17

Arch, Toxikol., Bd. 27

238

J. Bajgar:

55

I-B

%.,

40 20 dose ( #O-7moL/kq )

/Igo

30

Fig. 3. AChE inhibition in the mouse brain (basal ganglia) by IMPF in rive. A without transformation, B in probit-logarithmical transformation. For the transformation and I~oD calculations, only the black points were used

Discussion F r o m the above results it m a y be concluded t h a t the affinity of I M P F to A C h E in vitro is uniform in the parts of the brain studied. The values we obtained for the I50 of AChE in the various parts of mice brain are in good agreement with the results of Pato~ka (1969a, b) who determined these constants for AChE in whole brain of mice (/5o 9.8 • 10 -1~ M), rats (/5o = 6.6 • 10-l~ M) and guinea-pigs (/5o = 9.6 • 10-UM). The Hill coefficient value indicates t h a t two molecules of I M P F are bound to one molecule of brain ACHE. For phosphorylation of A C h E in the whole brain of mice, the same constant (1.6) was found. F o r A C h E of the brain of the rat and guinea-pig, the values of this coefficient were found to be 2.3 and 1.55 respectively (Pato6ka 1969a, b). Comparison of A C h E inhibition in vitro ~nd in vivo showed t h a t a v e r y small p a r t (about 1%) of an injected dose of I M P F actually inhibited ACHE. I n agreement with this are the findings of Polak and Cohen (1969, 1970a, b) and Christen et al. (1969) who demonstrated t h a t a great part of IM32pF is bound to non-specific plasma esterases, and t h a t only a very small a m o u n t of radioactivity is observed in the brain after IM32pF injections. The disappearance of I M P F in the organism can result from the binding of this compound to other nonspecific proteins or b y the degradation of I M P F by arylesterases or alkylphosphoftuoridases.

Anticholinesterase Action of Isopropyl iV[ethylphosphonofluoridate

239

The penetration of I M P F into the brain is probably limited by its concentration gradient, which is determined by the dose of I M P F administered and by the amount of blood circulating to the brain. I n the case of small injected doses of IMPF, the amount transferred b y the circulation is not sufficient to inhibit the AChE in the brain. With increase in the I M P F concentration, the AChE inhibition in the brain is very rapid, and it correlates in one part of the brain (pens and medulla) with symptoms of poisoning. Complete inhibition of AChE at the time of death was not found in a n y of the parts of the brain studied. The highest degree of inhibition was observed in the pens and medulla oblongata The lowest degree of inhibition was found in t h a t p a r t of the brain with the highest normal activity (basal ganglia). This finding agrees with t h a t of Hobbiger (1951), who observed t h a t AChE inhibition is dependent not only on inhibitor concentration but on AChE activity as well. The relationship between AChE activity and inhibition in vitro and in vivo has been described for other organophosphorus compounds by Schaumann (1960a, b, c) and Schaumann and Schiller (1960). Quantities of free inhibitor were found after organophosphate intoxication in the organs studied. I t s amount was dependent on the dose as well as on the nature of the compound. For compound 217-A0, a considerably higher concentration of free inhibitor was detected. This amount was lower for D F P ; and for E-600, no free inhibitor was observed (Sehaumann, 1960a; Schaumann and Schiller, 1960). I M P F is more similar to DFP. By analogy, it might have been predicted t h a t the amount of free I M P F would be small. As a matter of fact, Jakl (1966) determined the amount of free inhibitor present following 2 • LDs0 doses in mice. He found free inhibitor in the brain of pinacolylmethyl phosphonofluoridate-poisoned mice, but was unable to detect any in the case of IMPF. The negligible amount of free I M P F detected by J a k l (1966) is most likely the result of spontaneous reactivation. I t m a y be concluded t h a t I M P F has a selective effect on AChE in certain of the parts of the brain we studied. The selectivity is not caused b y different affinity of I M P F to AChE in the various parts of the brain. More likely it is caused by different degrees of penetration of I M P F into the various areas studied, i.e. b y dissimilar blood circulation to these parts, and (or) b y different AChE concentration in these parts. Also, the possibility that circulatory changes take place during the course of the poisoning cannot be excluded, although the time dynamics of AChE inhibition in the brain of mice poisoned by I M P F (Jakl, 1966) suggest t h a t the reaction is one of first kinetic order. The inhibition of AChE in the ponto-medul!ary area is probably very important for the toxicodynamics of I M P F poisoning since the 17"

240

J. Bajgar:

cause of d e a t h i n I M P F i n t o x i c a t i o n is ascribed to respiratory failure (de Candole et al., 1953; H e r i n k et al., 1970). The i m p o r t a n c e of the relation b e t w e e n AChE a c t i v i t y a n d the respiratory reflex has also been described b y Metz (1958). The i n h i b i t i o n of AChE i n the region of the respiratory centers m a y well be a d e t e r m i n a n t factor for life or death of the organism poisoned b y I M P F .

Acknowledgements. The author wishes to thank Mrs. M. Zeehovsks and Mrs. R. Thborsks for technical assistance.

References

Aldridge, W.N.: The inhibition of erythrocyte cholinesterase by tri-esters of phosphoric acid. Biochem. J. 54, 442 ~ (1953). Bajgar, J.: Time dynamics of changes of aeetylcholinesterase activity in experimental intoxication by O-isopropyl methylphosphonofluoridate [in Czech]. Sbornik v~d. praci VLVDI~ 44, 57-69 {1969). - - PatoSka, J. : Sensitivity comparison of the three methods for determination of cholinesterase activity [in Czech]. Sbornik v~d. praci VLVDL~ 44, 3-8 (1969). - - Tulach, J., Jakl, A., PatoSka, J.: The differences in anticholinesterase action of some organophosphorns compounds in vivo. Aeta biol. med. germ. {in press). -Urban, R. : Activity of acetylcholinesterase in different portions of rat brain [in Czech]. Suppl. Sborniku v~d. praci LF KU Hradec Krhlov~ 13, 219-227 (1970). Candole, C. A. de, Douglas, W.D., Evens, C.L., Holmes, R., Spencer, K. E. V., Torrance, R. W., Wilson, K. M.: The failure of respiration in death by anticholinesterase poisoning. Brit. J. Pharmacol. 8, 466-475 {1953). Christen, P . J . , Schot, P.K., Cohen, E.M.: Interaction of some eholinesterase inhibitors with aliesterase from rat plasma. Acta physiol, pharmaeol, neerl. 15, 397-398 (1969). Ellman, G. L., Courtney, D. K., Anders, V., Featherstone, R. M. : A new and rapid colorimetrie determination of acetylcholinesterase activity. Biochem. Pharmacol. 7, 88-95 (1961). Green, A. L.: The kinetic basis of organophosphate poisoning and its treatment. Biochem. Pharmacoh 1, 115-128 (1958). Heilbronn, E.: Purification of eholinesterase from horse serum. Bioehim. biophys. Acta (Amst.) 58, 222-230 (1962). Herink, J., Bajgar, J., PatoSka, J. : The influence of arteficial respiration on the rats, poisoned by isopropyl methylphosphonofluoridate [in Czech]. Voj. zdravotn. Listy 39, 191-195 (1970). Hobbiger, F. :Inhibition of cholinesterase by irreversible inhibitors in vitro and in vivo. Brit. J. Pharmacol. 6, 21-30 (1951). Hra~ka, M., Tulach, J.: Contribution to automatization of evaluation of experimental data in experimental medicine. Programmcs for MINSK 22 computer [in Czech]. VLVDI~ Hradec Krs (1966). Jakl, A. : The possibilities of specific antidotal therapy of acute organophosphate poisoning [in Czech]. Kandids disertaSni prs (Candidate dissertation work), VLVDI~ Hradee Kr~lov~ (1966). Jandorf, J. B. : Mode of action of pesticides. Mechanism of reaction of di-n-propyl2,2-diehlorvinylphosphate (DDVP) with esterases. J. Agr. Food Chem. 4, 853858 (1956).

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Jandoff, J. B., Michel, H. O., Schaffer, N. K., Egan, R., Summerson, W. H.: The mechanism of reaction between esterases and phosphorus-containing antiesterases. Disc. Faraday Soe. 20, 134-142 (1955). Koelle, G. B.: The histochemical differentiation of types of cholinesterases and their loealizations in tissues of the cat. J. Pharmacol. exp. Ther. 10O, 158-179 (1950). - - Steiner, E. C. : The cerebral distributions of a tertiary and a quarternary antieholinesterase agent following intravenous and intraventricular injection. J. Pharmacol. exp. Ther. 118, 420~34 (1956). Metz, B. : Brain acetylcholinesterase and a respiratory reflex. Amer. J. Physiol. 1 9 2 , 101-105 (1958). Monod, J., Changenx, J. P., Jacob, F.: Allosteric proteins and cellular control systems. J. molec. Biol. 6, 306-309 (1963). Novgorodskaya, A.M., Rozengart, V . I . , Shcherbak, I. G. : Distribution of the organic phosphorus containing compound cholinesterase inhibitor (LG-63) at various routes of administration into the rats [in Russian]. Vop. med. Khim. 1 4 , 288-294 (1968). PatoSka, J.: Inhibition of tissue cholinesterases of guinea-pig by sarin. Sbornik v~d. praci VLVDI~ 44, 47-50 (1969a). - - Inhibition of some tissue cholinesterases of the mouse and rat by sarin. Sbornik v~d. praci VLVDI~ 44, 51-56 (1969b). - - Tulach, J.: Evaluation of computers in enzyme kinetics, presented at 5th Czechoslovak Biochemical Meeting, Brno 1969, Abstract p. 38 [in Czech]. Polak, R . L . , Cohen, E . M . : The influence of triorthocresylphosphate on the distribution of a=p in the body of the rat after the injection of a2P-sarin. Biochem. Pharmaeol. 18, 813-820 (1969). - - - - The influence of oximes on the distribution of 32p in the body of the rat after injection of 32p-sarin. Biochem. Pharmacol. 19, 865-876 (1970a). - - - - The binding of sarin in the blood plasma of the rat. Biochem. Pharmacol. 19, 877-881 (1970b). Sehaumann, W.: Bestimmung der Cholinesterase-Aktivit~t in vitro und Berechnung der Aktivit~t in vivo nach Vergiftung mit Alkylphosphaten. NaunynSehmiedebergs Arch. exp. path. Pharmak. 239, 81-95 (1960a). - - Beziehungen zwischen den peripheren und zentralen Wirkungen yon Cholinesterase-Hemmern und der Inaktivierung der Cholinesterase. Naunyn-Schmiedebergs Arch. exp. path. Pharmak. 289, 96-113 (1960b). --Vergleich zwischen der Wirksamkeit yon Cholinesterasehemmern in vitro und in vivo. Naunyn-Schmiedebergs Arch. exp. path. Pharmak. 289, 126-130 (1960c). - - Schiller, M.: Inaktivierung yon Alkylphosphaten im Gehirn und im Serum. Naunyn-Schmiedebergs Arch. exp. path. Pharmak. 289, 114-125 (1960). Taketa, K., Pogell, B. M.: Allosterie inhibition of rat liver fructose 1,6-diphosphatase by adenosin 5'-monophosphate. J. biol. Chem. 240, 651-662 (1965). Dr. J i l l Bajgar Medical Research Institute Dukelskh 789 Hradec Kr~lov6 Czechoslovakia