The cholinesterases: a discussion of some unanswered questions

Inhibition effects of benactyzine and drofenine on human serum butyrylcholinesterase

Benactyzine and drofenine are widely used anticholinergic drugs. Benactyzine is used to treat organophosphate poisoning and drofenine acts on smooth muscle to stop muscle spasms. Both of these drugs are esters. After they enter the bloodstream, they will interact with butyrylcholinesterase (BChE; acylcholine acyl hydrolase: EC 3.1.1.8), which has an ability to hydrolyze a wide variety of esters. Therefore, the kinetic analysis of their inhibitory effects on human serum BChE was examined using butyrylthiocholine as substrate. Both drugs were competitive inhibitors of BChE and the Ki values of benactyzine and drofenine were calculated to be 0.010 +/- 0.001 and 0.003 +/- 0.000 mM, respectively, using the Systat (version 5.03, 1991) nonlinear regression analysis software package. According to these parameters, drofenine is a more potent competitive inhibitor of BChE than benactyzine.

Inhibition kinetics of human serum butyrylcholinesterase by Cd2+, Zn2+ and Al3+: comparison of the effects of metal ions on cholinesterases

Butyrylcholinesterase (BChE, EC 3.1.1.8) has been purified about 6600-fold from human serum with a procedure including ammonium sulfate fractionation (55-70%) with acid step at pH 4.5 and procainamide-Sepharose 4B affinity chromatography. The purified enzyme exhibited negative cooperativity with respect to butyrylthiocholine (BTCh) binding at pH 7.5. Ks was found to be 0.128 +/- 0.012 mM. Inhibition kinetics of the enzyme by Cd2+, Zn2+ and Al3+ were studied in detail. The 1/v vs 1/[BTCh] plots in the absence (control plot) and in the presence of different concentrations of cations intersected above 1/[BTCh]-axis. The data were analyzed by means of a nonlinear curve fitting program. The results demonstrated that all of the three cations are the linear mixed-type inhibitors of BChE. Ca2+ and Mg2+ had no effect on the enzyme activity in the experimental conditions. But when the enzyme was inhibited by 0.5 mM Cd2+ or Zn2+, Ca2+ and Mg2+ partially reactivated the inhibited allosteric form of BChE. Results were compared with data obtained from brain BChE purified from sheep.

Cholinesterase inhibitors induce growth cone collapse and inhibit neurite extension in primary cultured chick neurons

Cholinesterase inhibitors are commonly prescribed medicines in neurological and anesthesiological clinical practices at relatively high doses. Recent studies report that cholinesterases, originally described as neurotransmitter degradation enzymes, are expressed in the developing central nervous systems. They are hypothesized to play an important role during the establishment of neuronal cytoarchitecture. In this study, the effects of several clinically utilized cholinesterase inhibitors on one potential aspect of neuronal development were examined using a primary culture system of chick central and peripheral neurons. Three cholinesterase inhibitors, physostigmine, neostigmine, and edrophonium, suppressed the activity of the leading edges of the extending axons (the nerve growth cones) dose dependently. Filopoda and lamellipodia of nerve growth cones were collapsed by the exposure to the cholinesterase inhibitors. This action was quick, mostly within 5 min, and partly irreversible. The lowest concentration that could induce statistically significant growth cone collapse was 10(-5) M for physostigmine, 10(-4) M for neostigmine, and 10(-2) for edrophonium, respectively. The ED50 values for the growth cone collapsing activity were approximately, 10(-3) M for physostigmine. 10(-2.5) M for neostigmine, and 10(-2) M for edrophonium both in dorsal root ganglion culture and in retinal culture. Even though the result of this in vitro study cannot be applied directly to the in vivo situation, physicians should consider the potential detrimental effects of cholinesterase inhibitors to growing and regenerating nervous tissues.

Butyrylcholinesterase: an enzyme antidote for cocaine intoxication

Cocaine-associated toxicity is the result of effects on the cardiovascular and central nervous systems. Since the primary route of cocaine inactivation is enzymatic degradation by butyrylcholinesterase (BChE), we sought to determine if the administration of purified human enzyme would ameliorate the lethal effects of cocaine. While the cardiovascular, autonomic or central nervous systems were unaffected by BChE, the enzyme reduced the adverse effects of cocaine including hypertension, hyperactivity and convulsions. BChE decreased both the brain and blood levels of cocaine and shifted the metabolites towards the production of the inactive product ecgonine methyl ester and away from the physiologically active metabolites, norcocaine and benzoylecgonine. We conclude that BChE would appear to be an ideal antidote in the treatment of cocaine intoxication and has potential therapeutic application.

Surface and Spectroscopic Properties of Acetylcholinesterase Monolayer at the Air/Water Interface

The behavior of the enzyme acetylcholinesterase was studied at the air/water interface. Surface pressure-area (pi-A) isotherms and UV-vis spectra recorded at different surface pressures were determined for different salt concentrations in the subphase. The ionic strength of the subphase does not influence the physical properties in consideration; however, the pH of the subphase has a great effect on its surface and optical properties. A subphase at pH 6.5 has shown that the enzyme is highly stable, based on the pi-A compression/decompression isotherms. No changes in the area per molecule were observed when the surface pressure was maintained constant at 16 mN/m for a period of 120 min. The long-term stability of acetylcholinesterase at the air/water interface was demonstrated for pH 6.5 and a salt concentration of 10(-2) M (KCl). The absorption spectra of the monolayer, measured directly at the air/water interface, are considered good evidence of the organization of the enzyme molecules. Copyright 1997 Academic Press. Copyright 1997Academic Press

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.