Multi-target neuroprotective effects of herbal medicines for Alzheimer's disease.

ETHNOPHARMACOLOGICAL RELEVANCE: Alzheimer's disease is the most common form of dementia, but its treatment options remain few and ineffective. To find new therapeutic strategies, natural products have gained interest due to their neuroprotective potential, being able to target different pathological hallmarks associated with this disorder. Several plant species are traditionally used due to their empirical neuroprotective effects and it is worth to explore their mechanism of action. AIM OF THE STUDY: This study intended to explore the neuroprotective potential of seven traditional medicinal plants, namely Scutellaria baicalensis, Ginkgo biloba, Hypericum perforatum, Curcuma longa, Lavandula angustifolia, Trigonella foenum-graecum and Rosmarinus officinalis. The safety assessment with reference to pesticides residues was also aimed. MATERIALS AND METHODS: Decoctions prepared from these species were chemically characterized by HPLC-DAD and screened for their ability to scavenge four different free radicals (DPPH•, ABTS•+, O2•‒ and •NO) and to inhibit enzymes related to neurodegeneration (cholinesterases and glycogen synthase kinase-3ß). Cell viability through MTT assay was also evaluated in two different brain cell lines, namely non-tumorigenic D3 human brain endothelial cells (hCMEC/D3) and NSC-34 motor neurons. Furthermore, and using GC, 21 pesticides residues were screened. RESULTS: Regarding chemical composition, chromatographic analysis revealed the presence of several flavonoids, phenolic acids, curcuminoids, phenolic diterpenoids, one alkaloid and one naphthodianthrone in the seven decoctions. All extracts were able to scavenge free radicals and were moderate glycogen synthase kinase-3ß inhibitors; however, they displayed weak to moderate acetylcholinesterase and butyrylcholinesterase inhibition. G. biloba and L. angustifolia decoctions were the less cytotoxic to hCMEC/D3 and NSC-34 cell lines. No pesticides residues were detected. CONCLUSIONS: The results extend the knowledge on the potential use of plant extracts to combat multifactorial disorders, giving new insights into therapeutic avenues for Alzheimer's disease.

Protective effect of thymoquinone, the main component of Nigella Sativa, against diazinon cardio-toxicity in rats.

Several studies have shown that oxidative stress and cell damage can occur at very early stages of diazinon (DZN) exposure. The present study was designed to determine the beneficial effect of thymoquinone (Thy), the main component of Nigella sativa (black seed or black cumin), against DZN cardio-toxicity in rats. In the present experimental study, 48 male Wistar rats were randomly divided into six groups: control (corn oil gavages), DZN gavages (20 mg/kg/day), Thy gavages (10 mg/kg/day) and Thy + DVN gavages (2.5, 5 and 10 mg/kg/day). Treatments were continued for 28 days, then the animals were anesthetized by ether and superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), lactate dehydrogenize (LDH) and glutathione peroxide (GPX) activity was evaluated. In addition, glutathione (GSH) and malondialdehyde (MDA) the heart tissue and creatinephosphokinase-MB (CPK-MB) and troponin (TPI) levels and cholinesterase activity in the blood were evaluated. DZN-induced oxidative damage and elevated the levels of the cardiac markers CK-MB, TPI, MDA and LDH and decreased SOD, CAT and cholinesterase activity and GSH level compared with the control group. Treatment with Thy reduced DZN cardio-toxicity and cholinesterase activity. The success of Thy supplementation against DZN toxicity can be attributed to the antioxidant effects of its constituents. Administration of Thy as a natural antioxidant decreased DZN cardio-toxicity and improved cholinesterase activity in rats through the mechanism of free radical scavenging.

Chemical profiling of edible seaweed (Ochrophyta) extracts and assessment of their in vitro effects on cell-free enzyme systems and on the viability of glutamate-injured SH-SY5Y cells.

Neurodegenerative processes involve numerous and closely related events that ultimately culminate in neuronal cell injury. The aim of this study was (i) to assess, for the first time, the neuroprotective potential of acetone extracts of six edible species of Ochrophyta, by evaluating their cholinesterase and lipoxygenase inhibitory activity in cell-free assays, as well as their capacity to attenuate glutamate-induced toxicity in neuronal (SH-SY5Y) cells, and (ii) to try to relate the chemical composition of the extracts with their biological activity, evaluating also the effect of the main compounds thereof. In spite of a modest cholinesterase inhibition, a dose-dependent response towards lipoxygenase was found for all macroalgae extracts. At non-cytotoxic concentrations, the extracts from Fucus serratus Linnaeus and Saccharina latissima (Linnaeus) C.E. Lane, C. Mayes, Druehl & G.W. Saunders were able to improve the viability of glutamate-insulted SH-SY5Y cells. These results encourage further studies for a more detailed understanding of the mechanisms beyond the documented biological activities, and point to the potential interest of the selected seaweed species and their extracts as promising candidates for in vivo studies.

In Vitro Cysteine Reactivates Organophosphate Insecticide Dichlorvos-Inhibited Human Cholinesterases.

OBJECTIVES: Organophosphate (OP) pesticides inhibit both red blood cell (RBC) and plasma cholinesterases (ChEs). Oximes, especially pralidoxime (2-PAM), are widely used as antidotes to treat OP poisoning. In addition, N-acetylcysteine (NAC) is sometimes used as an adjuvant antidote. The current study aimed to assess the feasibility of using NAC as a single therapeutic agent for OP poisoning in comparison to in vitro 2-PAM. METHODS: This study was carried out at the Razi Drug Research Center of Iran University of Medical Sciences, Tehran, Iran, between April and September 2014. A total of 22 healthy human subjects were recruited and 8 mL citrated blood samples were drawn from each subject. Dichlorvos-inhibited blood samples were separately exposed to low and high doses (final concentrations of 300 and 600 µmol.L-1, respectively) of 2-PAM, NAC and cysteine. Plasma and RBCs were then separated by centrifugation and their ChE activity was measured using spectrophotometry. RESULTS: Although cysteine-and not NAC-increased the ChE activity of both plasma and RBCs over those of dichlorvos, it did not increase them over those of a high dose of 2-PAM. CONCLUSION: These results suggest that the direct reactions of 2-PAM and cysteine with dichlorvos and the reactivation of phosphorylated ChEs occurr via an associative stepwise addition-elimination process. High therapeutic blood concentrations of cysteine are needed for the elevation of ChE activity in plasma and RBCs; however, both this agent and NAC may still be effective in the reactivation of plasma and RBC ChEs.

Cholinesterase and Prolyl Oligopeptidase Inhibitory Activities of Alkaloids from Argemone platyceras (Papaveraceae).

Alzheimer's disease is an age-related, neurodegenerative disorder, characterized by cognitive impairment and restrictions in activities of daily living. This disease is the most common form of dementia with complex multifactorial pathological mechanisms. Many therapeutic approaches have been proposed. Among them, inhibition of acetylcholinesterase, butyrylcholinesterase, and prolyl oligopeptidase can be beneficial targets in the treatment of Alzheimer's disease. Roots, along with aerial parts of Argemone platyceras, were extracted with ethanol and fractionated on an alumina column using light petrol, chloroform and ethanol. Subsequently, repeated preparative thin-layer chromatography led to the isolation of (+)-laudanosine, protopine, (-)-argemonine, allocryptopine, (-)-platycerine, (-)-munitagine, and (-)-norargemonine belonging to pavine, protopine and benzyltetrahydroisoquinoline structural types. Chemical structures of the isolated alkaloids were elucidated by optical rotation, spectroscopic and spectrometric analysis (NMR, MS), and comparison with literature data. (+)-Laudanosine was isolated from A. platyceras for the first time. Isolated compounds were tested for human blood acetylcholinesterase, human plasma butyrylcholinesterase and recombinant prolyl oligopeptidase inhibitory activity. The alkaloids inhibited the enzymes in a dose-dependent manner. The most active compound (-)-munitagine, a pavine alkaloid, inhibited both acetylcholinesterase and prolyl oligopeptidase with IC50 values of 62.3 ± 5.8 µM and 277.0 ± 31.3 µM, respectively.

Protective effects of total alkaloidal extract from Murraya koenigii leaves on experimentally induced dementia.

Dementia is a syndrome of gradual onset and continuous decline of higher cognitive functioning. It is a common disorder in older persons and has become more prevalent today. The fresh leaves of Murraya koenigii are often added to various dishes in Asian countries due to the delicious taste and flavor that they impart. These leaves have also been proven to have health benefits. In the present study, the effect of total alkaloidal extract from M. koenigii leaves (MKA) on cognitive functions and brain cholinesterase activity in mice were determined. In vitro ß-secretase 1 (BACE1) inhibitory activity was also evaluated. The total alkaloidal extract was administered orally in three doses (10, 20 and 30 mg/kg) for 15 days to different groups of young and aged mice. Elevated plus maze and passive avoidance apparatus served as the exteroceptive behavioral models for testing memory. Diazepam-, scopolamine-, and ageing-induced amnesia served as the interoceptive behavioral models. MKA (20 and 30 mg/kg, p.o.) showed significant improvement in memory scores of young and aged mice. Furthermore, the same doses of MKA reversed the amnesia induced by scopolamine (0.4 mg/kg, i.p.) and diazepam (1 mg/kg, i.p.). Interestingly, the brain cholinesterase activity was also reduced significantly by total alkaloidal extract of M. koenigii leaves. The IC50 value of MKA against BACE1 was 1.7 µg/mL. In conclusion, this study indicates MKA to be a useful remedy in the management of Alzheimer's disease and dementia.

Antidiabetic complications and anti-Alzheimer activities of sophoflavescenol, a prenylated flavonol from Sophora flavescens, and its structure-activity relationship.

It was previously reported that prenylated flavonols from Sophora flavescens are inhibitors of rat lens aldose reductase (RLAR), human recombinant aldose reductase (HRAR), advanced glycation endproducts (AGE), ß-secretase (BACE1) and cholinesterases (ChE). Based upon structure-activity relationships, 3,4'-dihydroxy flavonols with a prenyl or lavandulyl group substitution at the C-8 position, and a hydroxy group at the C-5, are important for such inhibition. In our ongoing study to isolate active principles from S. flavescens by an activity-guided isolation procedure, further detailed phytochemical investigations of the CH(2)Cl(2) fraction were conducted via repeated chromatography over silica gel and Sephadex LH-20 columns. This ultimately resulted in the isolation of a promising active sophoflavescenol with higher inhibitory activities among the current prenylated flavonols isolated from S. flavescens against RLAR, HRAR, AGE, BACE1 and ChEs. The results further support that 3,4'-dihydroxy flavonols with a prenyl or lavandulyl substitution at the C-8 position and a methoxy group at C-5 represent a new class of RLAR, HRAR and AGE inhibitors. Nevertheless, the C-5 hydroxyl group of prenylated flavonoids is important for inhibition of BACE1 and ChEs, indicating that the hydroxyl group at C-5 might be the main contributor to the augmentation and/or modification of prenylated flavonol activity.

Selective inhibition of prenylated flavonoids from Sophora flavescens against BACE1 and cholinesterases.

It was previously reported that certain lavandulylated flavanones from Sophora flavescens are beta-site APP cleaving enzyme 1 (BACE1) inhibitors; however, based upon their levels within the extract, their inhibitory effects should be higher than expected. Moreover, chalcones and flavonols were reported to exert higher bioactivities than flavanones. These findings have led to a further search for other possible constituents potentially contributing to the strong inhibitory activity of the S. flavescens extract. In this study, BACE1 activities were significantly inhibited by 8-lavandulylkaempferol (IC(50) 7.29 microM), kuraridinol (IC(50) 7.10 microM), kuraridin (IC(50) 6.03 microM), and kushenol C (IC(50) 5.45 microM) from the ethyl acetate fraction, along with desmethylanhydroicaritin (IC(50) 1.86 microM), xanthohumol (IC(50) 7.19 microM), and leachianone G (IC(50) 8.56 microM) from the dichloromethane fraction of the extract. The results indicate that the prenyl group, rather than the lavandulyl group, and the flavonols and chalcones, rather than flavanones, might make predominant contributions to BACE1 inhibition. In particular, 8-lavandulylkaempferol exhibited significant inhibitory effects with IC(50) values of 7.10 and 8.11 microM for butyrylcholinesterase and acetylcholinesterase, respectively, when compared to its counterpart, desmethylanhydroicaritin. This indicates that the lavandulyl group might play a predominant role in both cholinesterase inhibitions. This is the first study indicating that prenylated flavonoids exert varying degrees of inhibition primarily through their skeleton (flavonols, chalcones, flavanones), as well as their lipophilic chain length (prenyl and lavandulyl groups). Therefore, S. flavescens and its prenylated flavonoids, possessing low molecular weights and lipophilic moieties may be potent preventive and therapeutic candidates for Alzheimer's disease.

Pharmacokinetic and pharmacodynamic interaction for a binary mixture of chlorpyrifos and diazinon in the rat.

Chlorpyrifos (CPF) and diazinon (DZN) are two commonly used organophosphorus (OP) insecticides and a potential exists for concurrent exposures. The primary neurotoxic effects from OP pesticide exposures result from the inhibition of acetylcholinesterase (AChE). The pharmacokinetic and pharmacodynamic impact of acute binary exposures of rats to CPF and DZN was evaluated in this study. Rats were orally administered CPF, DZN, or a CPF/DZN mixture (0, 15, 30, or 60 mg/kg) and blood (plasma and RBC), and brain were collected at 0, 3, 6, 12, and 24 h postdosing, urine was also collected at 24 h. Chlorpyrifos, DZN, and their respective metabolites, 3,5,6-trichloro-2-pyridinol (TCP) and 2-isopropyl-4-methyl-6-hydroxypyrimidine (IMHP), were quantified in blood and/or urine and cholinesterase (ChE) inhibition was measured in brain, RBC, and plasma. Coexposure to CPF/DZN at the low dose of 15/15 mg/kg did not alter the pharmacokinetics of CPF, DZN, or their metabolites in blood. A high binary dose of 60/60 mg/kg increased the C(max) and AUC and decreased the clearance for both parent compounds, likely due to competition between CPF and DZN for CYP450 metabolism. At lower doses, most likely to be encountered in occupational or environmental exposures, the pharmacokinetics were linear. A dose-dependent inhibition of ChE was noted in tissues for both the single and coexposures, and the extent of inhibition was plasma > RBC > or = brain. The overall relative potency for ChE inhibition was CPF/DZN > CPF > DZN. A comparison of the ChE response at the low binary dose (15/15 mg/kg), where there were no apparent pharmacokinetic interactions, suggested that the overall ChE response was additive. These experiments represent important data concerning the potential pharmacokinetic and pharmacodynamic interactions for pesticide mixtures and will provide needed insight for assessing the potential cumulative risk associated with occupational or environmental exposures to these insecticides.

[A comparative study on the sensitivity and specificity of cholinesterase and glutathione s-transferase in Gammarus pulex L].

Studies on the influences of lindane, pirimiphos methyl, permethrin, zinc and dodecyl linear alkybenzene sulfonate (LAS) on the activity and toxicity of cholinesterase (ChE) and glutathione s-transferase (GST) in Gammarus pulex L. showed that only pirimiphos methyl caused a change in ChE activity in Gammarus, with a significant reduction in enzyme activity after 24 h and 48 h exposure. Both lindane and permethrin caused a change in GST activity in Gammarus, with a significant increase in enzyme activity after 48 h exposure. Lindane alos caused a significant increase in GST activity after 24 h exposure. Biomarkers ChE and GST were demonstrated a high degree of specificity and sensitivity in comparison to the lethality assay, but GST activity was less specific than ChE activity.

Effect of Rhazya stricta Decne on monoamine oxidase and cholinesterase activity and brain biogenic amine levels in rats.

The effect of treatment with the medicinal plant Rhayva stricta Decne, on monoamine oxidase (MAO) and cholinesterase activity, and on the concentration of brain biogenic amines was studied in rats. R. stricta extract, at doses of 0.2 and 0.5 g kg(-1), significantly (P < 0.05-0.01) increased the hepatic and cerebral activity of MAO by 36-127%. The higher doses used (2.0 and 8.0 g kg(-1)) produced smaller (10-26%) and statistically insignificant increases in MAO activity in liver and brain. Cholinesterase activity in blood, liver and brain was not significantly influenced by treatment with R. stricta. The concentrations of the measured biogenic amines (noradrenaline, adrenaline, 5-hydroxytryptamine and dopamine) were significantly lowered in rats treated with R. stricta. The observed increase in MAO activity may be responsible for the lowered biogenic amines levels and may, in part, be responsible for the pharmacological effects of R. stricta extract in rats.

Huperzine A, a potential therapeutic agent for treatment of Alzheimer's disease.

HupA is a potent, reversible and selective inhibitor of AChE with a rapid absorption and penetration into the brain in animal tests. It exhibits memory-enhancing activities in animal and clinical trials. Compared to tacrine and donepezil, HupA possesses a longer duration of action and higher therapeutic index, and the peripheral cholinergic side effects are minimal at therapeutic doses. This review article deals with a comprehensive survey of the progress in chemical and pharmacological studies of HupA including the isolation and structure elucidation, pharmacological actions, total synthesis, SAR studies and the future development of HupA. Recently, it has been reported that HupA could reduce neuronal cell death caused by glutamate. The dual bio-activities of HupA would further enhance its value and potentiality as the therapeutic agent for Alzheimer s disease.

Dimethylphosphoryl-inhibited human cholinesterases: inhibition, reactivation, and aging kinetics.

Human poisoning by organophosphates bearing two methoxy groups, e.g. by malathion, paraoxon-methyl, dimethoate and oxydemeton-methyl, is generally considered to be rather resistant to oxime therapy. Since the oxime effectiveness is influenced not only by its reactivating potential but also by inhibition, aging and spontaneous reactivation kinetics, experiments were performed with human acetyl- (AChE) and butyrylcholinesterase (BChE) to determine the respective kinetic constants. The efficacy of obidoxime in reactivating dimethylphosphoryl-AChE was 40, 9 and 3 times higher than of HI 6, pralidoxime and HLö 7, respectively. Aging (t1/2 3.7 h) and spontaneous reactivation (t1/2 0.7 h) occurred concomitantly, with the portion of the aged enzyme being dependent on the presence of excess inhibitor. Calculation of steady-state AChE activity in the presence of inhibitor and oxime revealed that obidoxime was superior to pralidoxime. In addition, organophosphate concentrations up to 10(-6) M (paraoxon-methyl) and 10(-4) M (oxydemeton-methyl) could be counteracted at clinically relevant oxime concentrations (10 microM). These data indicate that oximes may effectively reactivate human dimethylphosphoryl-AChE. Failure of oximes may be attributed to megadose intoxications and to prolonged time intervals between poison uptake and oxime administration. The potency of the oximes to reactivate dimethylphosphoryl-BChE was much lower and the spontaneous reactivation slower (t1/2 9 h), while aging proceeded at a comparable rate. Thus, BChE activity determination for diagnosis and therapeutic monitoring may give no reliable information on AChE status.

[The effect of the parenteral administration of alpha-ketoglutarate on the resistance of rats to ionizing radiation and on their cholinergic systems]. / Vplyv parenteral'noho vvedennia al'fa-ketohlutaratu na rezystentnist' shchuriv do ionizuiuchoho oprominennia ta kholinerhichnu systemu ïkh orhanizmu.

We have investigate the effect of sodium ketoglutarate intraperitoneal injection (20 mg/100 g body weight) made 0.5 hour before and 1, 2 and 3 hours after total X-ray treatment (259 mKl/kg) on the survival of rats. Simultaneously we have define changes in cholinesterase-acetylcholine system and content of adrenaline and noradrenaline in liver and pancreas tissues, small intestines mucous and in blood. All the data were taking at 1, 2, 5, 4, 24 and 72 hours after treatment and sodium ketoglutarate injection. We have found that sodium ketoglutarate injection made 0.5 hour before the treatment results in increasing of percentage death rate and injection made 1 and 2 hours after treatment results in increase in survival of rats. The effect of alpha-ketoglutarate injection made 1 hour after treatment in more pronounced. It is accompanied with increase of cholinergic status of organism on the catecholamine deficit background, decrease in the cholinesterase activity and increase of acetylcholine content in tissues of treated organism.

[Aminostigmine as a cholinesterase inhibitor and as an agent for treating poisonings by cholinergic blockers]. / Aminostigmin kak ingibitor kholinésterazy i kak sredstvo dlia lecheniia otravlenii kholinoblokatorami.

Experiments on non-inbred albino mice have demonstrated that aminostigmine is an active reversible centrally active cholinesterase inhibitor close to the properties of physostigmine, but greatly superior to it in its action duration. Clinical examinations of healthy volunteers and patients have shown that aminostigmine-induced inhibition of cholinesterase activity persists 6 hours. The agent have been found to be more highly effective in treating cholin blocker-induced intoxications than galanthamine, which manifests itself in its greater stability of the therapeutical effect achieved and in its higher ability to prevent cardiovascular events occurring in intoxication.

Bioavailability to rats and toxicological potential in mice of bound residues of malathion in beans.

Under conditions of local practice, the application of 2,3-succinate-14C-malathion on beans led to the formation of 17-18% of bound 14C-residues after 30 weeks. When fed to rats, 75% of these residues became bioavailable after 2 days with the major part, excreted via expired air (8%) and urine (60%). The main radioactive metabolites detected in urine were malathion monocarboxylic acid and malathion dicarboxylic acid. Feeding of bound residues to mice (1.8 ppm in feed) for 90 days resulted in a reduction in body weight gain after 60 days and inhibition of erythrocyte cholinesterase activity after 90 days. Increased levels of serum glutamic oxaloacetic transaminase and alkaline phosphatase were also observed. The results strongly suggest that bean-bound malathion residues can cause adverse biological effects in mice.

Bioavailability and toxicological potential of lentil-bound residues of malathion in rats.

Lentil grains treated with malathion and stored under laboratory conditions for 12 months formed bound residues. Bioavailability and the effects of lentil-bound residues of malathion in rats were studied. The amount of bound residues in lentils treated with 14C-malathion at 10 ppm and 50 ppm gradually increased to 9.52% and 13.01% of the initially applied doses after 12 months. When rats were fed these 14C-bound residues, radioactivity excreted in urine accounted for 34.49% of the administered dose. In feces, 26.15% of given dose was methanol-extractable while 18.67% was determined as nonextractable. Various tissues including liver, kidney, fat and lungs contained 8.93% while radioactivity in expired air (14CO2) was low (1.51%). The results indicate that lentil-bound malathion residues are highly bioavailable to rats. Analysis of the lentil material containing bound residues indicated that the main compound was malathion. Lentil-bound malathion residues were administered to albino rats at 0.95 and 6.51 ppm in the feed for 3 months. Body weights were determined during and at the end of the experiment. Terminal organ weights were also determined and a number of blood chemistry parameters were examined. A significant reduction in serum cholinesterase activity and an increase in blood urea nitrogen and in white cell count suggest a toxocological potential of the bound residues.

Bioavailability to rats and toxicity in mice of carbofuran residues bound to faba beans.

14C-carbofuran penetrated readily into seeds of Vicia faba and the rate of penetration was found to be dose dependent. The percentage of bound residues was generally low and did not exceed 3% of the applied dose. When the bound residues were fed to rats 46% of the radioactivity was eliminated via CO2 and urine, while tissues contained 25%. Carbofuran phenol and 3-hydroxy carbofuran represented the main metabolites in the urine. These data indicate that bean-bound carbofuran residues are highly bioavailable to rats. Feeding mice with bound carbofuran residues for 90 days led to inhibition of erythrocyte cholinesterase activity after 30 days (35-40%) while the plasma enzyme remained unaffected. Serum transaminases and blood urea nitrogen were significantly elevated, indicating injury to hepatic and renal structures. The results strongly suggest that the bound residues can induce adverse biological effects in mice.

Phosphonylation of purified human, canine and porcine cholinesterase by soman. Stereoselective aspects.

Cholinesterases (EC 3.1.1.8, acylcholine acylhydrolase) from the sera of man, dog and pig were purified 400-600-fold using a combination of ion-exchange and affinity chromatography. In a first approach, phosphonylation by soman was studied by using the half-resolved epimers C(+)P(+/-)-soman and C(-)P(+/-)-soman. The degradation of soman at the nanomolar level was followed in time by determining the remaining soman by capillary gas chromatography with NP detection. In the three sera investigated the P-(-)-epimer phosphonylates at a higher rate than its corresponding P(+)-counterpart and the stereoselectivity is greater for the C(+)-epimers than for the C(-)-epimers. Individual soman isomers were isolated from C(+)- and C(-)-epimers and quantified by gas chromatography. Second-order rate constants were determined for the phosphonylation of purified cholinesterase by isolated soman isomers. The C(+)P(-)-isomer has the highest phosphonylation rate for the three species; the other toxic isomer, C(-)P(-), has a five to ten-fold lower rate. The overall stereoselectivity is more marked in human cholinesterase than in canine. Porcine serum cholinesterase is phosphonylated by the P(-)-isomers at a slightly higher rate than the human enzyme.