Kinetic prerequisites of oximes as effective reactivators of organophosphate-inhibited acetylcholinesterase: a theoretical approach

Figures & data

Figure 1.  Chemical structures of important oximes.

Figure 2.  Reaction scheme for the reactivation of organophosphate-inhibited acetylcholinesterase (AChE) by oximes. The respective concentrations are: [EP] the phosphylated AChE, [OX] the reactivator, [EPOX] the Michaelis-type phosphyl-AChE-oxime-complex, [E] the active enzyme and [POX] the phosphylated oxime. K_D is equal to the ratio [EP] × [OX]/[EPOX] and describes the dissociation constant which is inversely proportional to the affinity of the oxime to [EP], and k_r is the rate constant for the displacement of the phosphyl residue from [EPOX], indicating the reactivity of the oxime.

Table 1.  Highest and lowest reactivation rate constants of oximes with OP-inhibited human AChE.

Oxime	OP	kr (min^−1)	KD (µM)
Pralidoxime	MFPßCh^#	0.002^a
Methoxime	n-Butylsarin	5.91^b
Pralidoxime	Methamidophos		2.1^c
Pralidoxime	MFPhCh^§		3837^d

AChE, acetylcholinesterase; affinity (K_D) and reactivity (k_r) constants; OP, organophosphorus compounds.

^a,dfrom¹⁸, ^bfrom¹⁹, ^cfrom¹⁶.

^#Methylfluoro-ß-phosphonylcholine.

^§Methylfluorophosphonylhomocholine.

Table 2.  Reactivation rate constants of selected oximes and OP.

Oxime	OP	kr (min^−1)	KD (µM)
Obidoxime	Tabun	0.04	97.3
Obidoxime	Cyclosarin	0.395	945.6
Pralidoxime	Cyclosarin	0.182	3159
HI-6	Cyclosarin	1.3	47.2

Affinity (K_D) and reactivity (k_r) constants; OP, organophosphorus compounds.

Data are from¹⁸.

Figure 3.  Calculated acetylcholinesterase (AChE) activities of tabun- (A), cyclosarin- (B), paraoxon-ethyl- (C), and methamidophos-inhibited AChE (D) after reactivation by obidoxime (OBI, 10 µM), 2-PAM (100 µM), HI-6 (50 µM), and MMB-4 (100 µM). Based on experimental reactivation rate constants¹⁶, the AChE activities were calculated using the equation AChE_t% = 100 × (1-exp^{−kobs × t}).

Figure 4.  Double-logarithmic plot showing the relationship between reactivation half-time (t_1/2, min) and oxime reactivity (k_r in min⁻¹). The t_1/2 was calculated using equation (1) assuming five different dissociation constants (K_D), i.e. 1 µM, 10 µM, 100 µM, 1000 µM, and 3000 µM for two different oxime concentrations, i.e. 10 µM (A) and 100 µM (B). The dashed line indicates a reactivation t_1/2 of 5 min. In addition, reactivation half-times of selected oximes and organophosphorus compounds were included using measured reactivation rate constants (cf. Table 2): Tabun and obidoxime (x), cyclosarin and pralidoxime (+), obidoxime (*), and HI-6 (▪).

Figure 5.  Double-logarithmic plot showing the relationship between oxime concentration (µM) and dissociation constant (K_D in µM). Oxime concentrations necessary to obtain 40% reactivation of inhibited acetylcholinesterase within 10 min were calculated according to¹⁷ for three different oxime reactivity constants (k_r) of 0.1, 1 and 4 min⁻¹. The hatched area resembles the range of clinically used oxime concentrations. In addition, necessary oxime concentrations of selected oximes and organophosphorus compounds were included using measured reactivation rate constants (cf. Table 2): Cyclosarin and pralidoxime (+), obidoxime (*), and HI-6 (▪).

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