Inhibition of butyrylcholinesterase

The dual inhibition of acetylcholinesterase and butyrylcholinesterase may lead to broader efficacy. As acetylcholinesterase activity decreases with disease progression, the acetylcholinesterase-selective agents may lose their effect, while the dual inhibitors may still be effective due to the added inhibition of butyrylcholinesterase. However, this has not been demonstrated clinically. 

Some cholinesterase inhibitors also inhibit butyrylcholinesterase (pseudocholinesterase). Flowever, inhibition of butyrylcholinesterase plays little role in the action of indirect-acting cholinomimetic drugs because this enzyme is not important in the physiologic termination of synaptic acetylcholine action. Some quaternary cholinesterase inhibitors also have a modest direct action as well, eg, neostigmine, which activates neuromuscular nicotinic cholinoceptors directly in addition to blocking cholinesterase. 

Peripheral inhibition of butyrylcholinesterase can cause gastrointestinal side effects... 

P6. Pavlic, M., The inhibition of butyrylcholinesterase by methanesulfonyl fluoride. Biochim. Biophys. Acta 198, 389-391 (1970). 

Walsh R., Martin E. Darvesh S. (2011b). Synergistic inhibition of butyrylcholinesterase by galantamine and citalopram. Biochim. Biophys. Acta, 1810,1230-1235. 

Hahn T, Ruhnke M, Luppa H. 1991. Inhibition of acetylcholinesterase and butyrylcholinesterase by the organophosphorus insecticide methyl parathion in the central nervous system of the golden hamster i Mesocricetus aumtus). Acta Histochem (Jena) 91 13-19. 

Consistent decreases in plasma cholinesterase may not have been observed in rats and dogs because they were treated with lower doses of diisopropyl methylphosphonate. In general, depression of plasma cholinesterase, also known as pseudocholinesterase or butyrylcholinesterase, is considered a marker of exposure rather than an adverse effect. Depression of cholinesterase activity in red blood cells (acetylcholinesterase) is a neurological effect thought to parallel the inhibition of brain acetylcholinesterase activity. It is considered an adverse effect. Acetylcholinesterase is found mainly in nervous tissue and erythrocytes. Diisopropyl methylphosphonate was not found to inhibit RBC... 

Although this study (Hart 1980) did not identify an effect level, the NOAEL is below the LOEL found in all studies examining the toxicity of diisopropyl methylphosphonate. The LOEL for diisopropyl methylphosphonate is 262 mg/kg/day for male mink and 330 mg/kg/day for female mink (Bucci et al. 1997), doses at which statistically significant decreases in plasma cholinesterase (butyrylcholinesterase) but not RBC cholinesterase (acetylcholinesterase) activity were observed (Bucci et al. 1997). In general, a decrease in plasma cholinesterase activity is considered to be a marker of exposure rather than a marker of adverse effect, while a decrease in RBC acetylcholinesterase activity is a neurological effect thought to parallel the inhibition of brain acetylcholinesterase activity and is thus considered an adverse effect. Diisopropyl methylphosphonate was not found to inhibit red blood cell cholinesterase at doses at which plasma cholinesterase was significantly inhibited. No effects were observed in males at 45 mg/kg/day (Bucci et al. 1997) or at 63 mg/kg/day (Bucci et al. 1994), and no effects were observed in females at 82 mg/kg/day (Bucci et al. 1994), or at 57 mg/kg/day (Bucci et al. 1997). 

The inhibition of two cholinesterase activities in blood can also be used to confirm exposure to certain organophosphate ester compounds. Red blood cell acetylcholinesterase is the same cholinesterase found in the gray matter of the central nervous system and motor endplates of sympathetic ganglia. Synonyms for this enzyme include specific cholinesterase, true cholinesterase, and E-type cholinesterase. Plasma cholinesterase is a distinct enzyme found in intestinal mucosa, liver, plasma, and white matter of the central nervous system. Synonyms for this enzyme include nonspecific cholinesterase, pseudocholinesterase, butyrylcholinesterase, and S-type cholinesterase (Evans 1986). Nonspecific cholinesterase is thought to be a very poor indicator of neurotoxic effects. 

Donepezil (Aricept) is a piperidine derivative with specificity for inhibition of acetylcholinesterase rather than butyrylcholinesterase. 

Inhibition of the two principal human cholinesterases, acetylcholinesterase and pseudocholinesterase, may not always result in visible neurological effects (Sundlof et al. 1984). Acetylcholinesterase, also referred to as true cholinesterase, red blood cell cholinesterase, or erythrocyte cholinesterase is found in erythrocytes, lymphocytes, and at nerve synapses (Goldfrank et al. 1990). Inhibition of erythrocyte or lymphocyte acetylcholinesterase is theoretically a reflection of the degree of synaptic cholinesterase inhibition in nervous tissue, and therefore a more accurate indicator than pseudocholinesterase activity of inhibited nervous tissue acetylcholinesterase (Fitzgerald and Costa 1993 Sundlof et al. 1984). Pseudocholinesterase (also referred to as cholinesterase, butyrylcholinesterase, serum cholinesterase, or plasma cholinesterase) is found in the plasma, serum, pancreas, brain, and liver and is an indicator of exposure to a cholinesterase inhibitor. 

Kuznetsova LP, Nikol skaya EB, Sochilina EE, Inhibition of human blood acetylcholinesterase and butyrylcholinesterase by some alkaloids,/EvolBiochem Physiol 38 35-39, 2002. 

Donepezil HC1, a piperidine, is a highly selective inhibitor of the enzyme AChE [3,4] that is chemically unique from other AChE inhibitors [5, 6]. In vitro and preclinical studies have demonstrated that donepezil is approximately 1200 times more selective for AChE in the brain than for butyrylcholinesterase (BuChE) in the periphery [3, 4, 7]. Phase II and III studies conducted in the United States have shown that donepezil (5 or 10 mg once daily) produces statistically significant improvements in cognition and global function in patients with AD [8-10]. Its clinical efficacy and minimal side-effect profile are thought to be related to its specific inhibition of AChE in the areas of the brain affected by the cholinergic deficit that typifies this disease [3, 4, 7],... 

J.A. Doom, M. Schall, D.A. Gage, T.T. Talley, C.M. Thompson and R.J. Richardson, Identification of butyrylcholinesterase adducts after inhibition with Isomalathion using mass spectrometry difference in mechanism between (1R) and (1S)-Stereoisomers, Toxicol. Appl Pharmacol., 176, 73-80 (2001). 

Fasciculins inhibit AChE from mammals, electric fish, and some snake venoms with Ki values in the pico- to nanomolar range in contrast, avian, insect, and some other snake venom AChEs are relatively resistant, and high micromolar concentrations are required to inhibit mammalian butyrylcholinesterases (BuChE) (Marchot et al, 1993). Dissociation constants of Fasl and Fas3 are two-fold and 60-fold lower, respectively, than that of Fas2 for synaptosomal rat brain. 

Exposure to a toxic dose of OP results in inhibition of acetylcholinesterase and butyrylcholinesterase activities. The most common method to measure OP exposure is to assay acetylcholinesterase and butyrylcholinesterase activities in blood using a spectrophotometric method (EUman et al, 1961 Wilson et al, 2005 Worek et al, 1999). The drawbacks of activity assays are that they do not identily the OP. They show that the poison is a cholinesterase inhibitor but do not distinguish between nerve agents, OP pesticides, carbamate pesticides, and tightly bound, noncovalent inhibitors like tacrine and other anti-Alzheimer drugs. In addition, low-dose exposure, which inhibits less than 20% of the cholinesterase, carmot be determined by measuring acetylcholinesterase and butyrylcholinesterase activity because individual variability in activity levels is higher than the percent inhibition. 

Both intensity and length of exposure play important roles in determining the extent of inhibition of NTE in lymphocytes 50% of preexposed values of NTE activity were obtained when measured 3 or 4 weeks after the beginning of DEF exposure. However, there is no direct evidence of a correlation between a high level of lymphocyte NTE inhibition and development of neuropathy in humans. Blood acetylcholinesterase and plasma butyrylcholinesterase levels remained unchanged during the study period. There is no available weight-of-the-evidence summary assessment for DEF as a developmental or reproductive toxin. 

Compound (258) was reported to be 190 times more potent than physostigmine against acetylcholinesterase, and it was 60 times more selective for inhibition of acetylcholinesterase than for butyrylcholinesterase (323). 

Boter, H.L. and Van Dijk, C., Stereospecificity of hydrolytic enzymes on reaction with asymmetric organophosphorus compounds. The inhibition of acetylcholinesterase and butyrylcholinesterase by enantiomeric forms of sarin, Biochem. Pharmacol, 18, 2403, 1969. 

Saxena, A. et al. Edrophonium as a reactivator of organophosphate-inhibited human butyrylcholinesterase, presented at the Second Singapore International Symposium on Protection Against Toxic (SISPAT) Substances, Singapore, 4-7 December 2000. 

FIGURE 8.3 Comparative reactivation kinetics of soman-inhibited human butyrylcholinesterase single mutant G117H ( ) and double mutant G117H/E197Q ( ). Note that the recovery rate of the double mutant is very fast (with reaction rates of 77,000 and 128,000/min for the PsCs and PsCr isomers of soman, respectively), whereas the single mutant does not recover measurably. The insert shows that reactivation of the double mutant... 

I Donepezil. DonepeziF is a piperidine cholinesterase inhibitor with specificity for inhibition of acetylcholinesterase as compared to butyrylcholinesterase. This specificity is claimed to result in fewer peripheral side effects (such as nausea, vomiting, and diarrhea) than with nonspecific cholinesterase inhibitors such as tacrine. ... 

Activation at high substrate concentrations not only explains the failure of butyrylcholinesterase to follow simple Michaelis-Menton kinetics, but also explains the enigma of substrate inhibition of the enzyme using either benzoylcholine (A21, T7) or acetyl- or butyryl-salicylcholine as substrates. The proposal made by Hastings is analogous to that of Myers (M24, M25) for the inhibition of acetylcholinesterase by excess substrate, in this case acetylcholine. 

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