Earl Martin

Inhibition of human cholinesterases by drugs used to treat Alzheimer disease.

&NA; Current approaches to the treatment of cognitive and behavioral symptoms of Alzheimer disease emphasize the use of cholinesterase inhibitors. The kinetic effects of the cholinesterase inhibitors donepezil, galantamine, metrifonate, physostigmine, rivastigmine, and tetrahydroaminoacridine were examined with respect to their action on the esterase and aryl acylamidase activities of human acetylcholinesterase (AChE) and human butyrylcholinesterase (BuChE). Each of these drugs inhibited both AChE and BuChE, but to different degrees. Inhibition of BuChE by these compounds was approximately the same, or better, when acetylthiocholine, the analog of the neurotransmitter acetylcholine, was used as the substrate, instead of butyrylthiocholine. In addition, for these drugs, the inhibition of aryl acylamidase activity paralleled that observed for inhibition of esterase activity of AChE and BuChE. Given that drugs that are currently in use for the treatment of Alzheimer disease inhibit both AChE and BuChE, the development of drugs targeted toward the exclusive inhibition of one or the other cholinesterase may be important for understanding the relative importance of inhibition of BuChE and AChE in the treatment of this disease.

Carbamates with Differential Mechanism of Inhibition Toward Acetylcholinesterase and Butyrylcholinesterase

Most carbamates are pseudoirreversible inhibitors of cholinesterases. Phenothiazine carbamates exhibit this inhibition of acetylcholinesterase but produce reversible inhibition of butyrylcholinesterase, suggesting that they do not form a covalent bond with the catalytic serine. This atypical inhibition is attributable to pi-pi interaction of the phenothiazine moiety with F329 and Y332 in butyrylcholinesterase. These residues are in a helical segment, referred to here as the E-helix because it contains E325 of the catalytic triad. The involvement of the E-helix in phenothiazine carbamate reversible inhibition of butyrylcholinesterase is confirmed using mutants of this enzyme at A328, F329, or Y332 that show typical pseudoirreversible inhibition. Thus, in addition to various domains of the butyrylcholinesterase active site gorge, such as the peripheral anionic site and the pi-cationic site of the Omega-loop, the E-helix represents a domain that could be exploited for development of specific inhibitors to treat dementias.

Butyrylcholinesterase Is Associated With β-Amyloid Plaques in the Transgenic APPSWE/PSEN1dE9 Mouse Model of Alzheimer Disease

Abstract Histochemical analysis of Alzheimer disease (AD) brain tissues indicates that butyrylcholinesterase (BuChE) is present in &bgr;-amyloid (A&bgr;) plaques. The role of BuChE in AD pathology is unknown, but an animal model developing similar BuChE-associated A&bgr; plaques could provide insights. The APPSWE/PSEN1dE9 transgenic mouse (ADTg), which develops A&bgr; plaques, was examined to determine if BuChE associates with these plaques, as in AD. We found that in mature ADTg mice, BuChE activity associated with A&bgr; plaques. The A&bgr;-, thioflavin-S– and BuChE-positive plaques mainly accumulated in the olfactory structures, cerebral cortex, hippocampal formation, amygdala, and cerebellum. No plaques were stained for acetylcholinesterase activity. The distribution and abundance of plaque staining in ADTg closely resembled many aspects of plaque staining in AD. Butyrylcholinesterase staining consistently showed fewer plaques than were detected with A&bgr; immunostaining but a greater number of plaques than were visualized with thioflavin-S. Double-labeling experiments demonstrated that all BuChE-positive plaques were A&bgr; positive, whereas only some BuChE-positive plaques were thioflavin-S positive. These observations suggest that BuChE is associated with a subpopulation of A&bgr; plaques and may play a role in AD plaque maturation. A further study of this animal model could clarify the role of BuChE in AD pathology.

A method to describe enzyme-catalyzed reactions by combining steady state and time course enzyme kinetic parameters

BACKGROUND Complete analysis of single substrate enzyme-catalyzed reactions has required a separate use of two distinct approaches. Steady state approximations are employed to obtain substrate affinity and initial velocity information. Alternatively, first order exponential decay models permit simulation of the time course data for the reactions. Attempts to use integrals of steady state equations to describe reaction time courses have so far met with little success. METHODS Here we use equations based on steady state approximations to directly model time course plots. RESULTS Testing these expressions with the enzyme beta-galactosidase, which adheres to classical Michaelis-Menten kinetics, produced a good fit between observed and calculated values. GENERAL SIGNIFICANCE This study indicates that, in addition to providing information on initial kinetic parameters, steady state approximations can be employed to directly model time course kinetics. Integrated forms of the Michaelis-Menten equation have previously been reported in the literature. Here we describe a method to directly apply steady state approximations to time course analysis for predicting product formation and simultaneously obtain multiple kinetic parameters.

Butyrylcholinesterase-Mediated Enhancement of the Enzymatic Activity of Trypsin

Abstract1. Acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BuChE, EC 3.1.1.8) are enzymes that catalyze the hydrolysis of esters of choline.2. Both AChE and BuChE have been shown to copurify with peptidases.3. BuChE has also been shown to copurify with other proteins such as transferrin, with which it forms a stable complex. In addition, BuChE is found in association with β-amyloid protein in Alzheimer brain tissues.4. Since BuChE copurifies with peptidases, we hypothesized that BuChE interacts with these enzymes and that this association had an influence on their catalytic activities. One of the peptidases that copurifies with cholinesterases has specificity similar to trypsin, hence, this enzyme was used as a model to test this hypothesis.5. Purified BuChE causes a concentration-dependent enhancement of the catalytic activity of trypsin while trypsin does not influence the catalytic activity of BuChE.6. We suggest that, in addition to its esterase activity, BuChE may assume a regulatory role by interacting with other proteins.

Cholinesterase inhibition in Alzheimer's disease: is specificity the answer?

Cholinesterase inhibitors are the standard of care for Alzheimers disease (AD). Acetylcholinesterase (AChE) catalyzes the hydrolysis of the cholinergic neurotransmitter acetylcholine. However, the related enzyme butyrylcholinesterase (BuChE) also breaks down acetylcholine and is likewise targeted by the same clinical cholinesterase inhibitors. The lack of clinical efficacy for the highly specific and potent AChE inhibitor, (-) huperzine A, is intriguing, given the known cholinergic deficit in AD. Based on the proven efficacy of inhibitors affecting both cholinesterases and the apparent failure of specific AChE inhibition, focused BuChE inhibition seems important for more effective treatment of AD. Therefore, BuChE-selective inhibitors provide promise for improved benefit.

Biochemical and histochemical comparison of cholinesterases in normal and Alzheimer brain tissues.

Cholinesterase activity associated with neuritic plaques (NPs) and neurofibrillary tangles (NFTs) in Alzheimers disease (AD) brains exhibit altered histochemical properties, such as requiring lower pH (6.8) for optimal cholinesterase staining compared to the pH (8.0) for best visualization of cholinesterases in neurons. Furthermore, visualization of NPs and NFTs can be prevented by agents like the peptidase inhibitor/metalloantibiotic bacitracin. The anomalous behavior of cholinesterases associated with pathological lesions needs to be elucidated because of the putative links between these enzymes and the disease process in AD. In this study, cholinesterases were extracted from AD and normal brain tissue to determine whether the differences observed in histochemical analyses in the two sources were reflected in kinetic properties measured in solubilized enzymes. Isolated brain enzymes from both these sources exhibited comparable kinetic parameters with respect to pH dependence, substrate affinity and inhibitor sensitivity and were not significantly affected by other agents that blocked cholinesterase histochemical visualization, such as the structurally diverse metal-chelating antibiotics bacitracin, doxycycline, minocycline and rifampicin. Although the cholinesterases from AD brain tissue examined here represented a total pool of these enzymes from AD brain, rather than enzymes specifically from NPs and NFTs, their kinetic behavior being comparable to cholinesterases isolated from normal brain tissues implies that these enzymes do not undergo disease-related modification in their primary structures. This suggests that the atypical histochemical behavior of cholinesterases in NPs and NFTs may result from interaction of cholinesterases with other molecules within these lesions, mediated by transition metal ions known to be present in AD pathology lesions.

Differential binding of phenothiazine urea derivatives to wild-type human cholinesterases and butyrylcholinesterase mutants.

A series of N-10 urea derivatives of phenothiazine was synthesized and each compound was evaluated for its ability to inhibit human cholinesterases. Most were specific inhibitors of BuChE. However, the potent inhibitory effects on both cholinesterases of one sub-class, the cationic aminoureas, provide an additional binding mechanism to cholinesterases for these compounds. The comparative effects of aminoureas on wild-type BuChE and several BuChE mutants indicate a binding process involving salt linkage with the aspartate of the cholinesterase peripheral anionic site. The effect of such compounds on cholinesterase activity at high substrate concentration supports ionic interaction of aminoureas at the peripheral anionic site.

Early Detection of Cerebral Glucose Uptake Changes in the 5XFAD Mouse

Brain glucose hypometabolism has been observed in Alzheimer’s disease (AD) patients, and is detected with 18F radiolabelled glucose, using positron emission tomography. A pathological hallmark of AD is deposition of brain β-amyloid plaques that may influence cerebral glucose metabolism. The five times familial AD (5XFAD) mouse is a model of brain amyloidosis exhibiting AD-like phenotypes. This study examines brain β-amyloid plaque deposition and 18FDG uptake, to search for an early biomarker distinguishing 5XFAD from wild-type mice. Thus, brain 18FDG uptake and plaque deposition was studied in these mice at age 2, 5 and 13 months. The 5XFAD mice demonstrated significantly reduced brain 18FDG uptake at 13 months relative to wild-type controls but not in younger mice, despite substantial β-amyloid plaque deposition. However, by comparing the ratio of uptake values for glucose in different regions in the same brain, 5XFAD mice could be distinguished from controls at age 2 months. This method of measuring altered glucose metabolism may represent an early biomarker for the progression of amyloid deposition in the brain. We conclude that brain 18FDG uptake can be a sensitive biomarker for early detection of abnormal metabolism in the 5XFAD mouse when alternative relative uptake values are utilized.

Homocysteine thiolactone and human cholinesterases.

1. The cholinergic system is important in cognition and behavior as well as in the function of the cerebral vasculature.2. Hyperhomocysteinemia is a risk factor for development of both dementia and cerebrovascular disease.3. Acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) are serine hydrolase enzymes that catalyze the hydrolysis of the neurotransmitter acetylcholine, a key process in the regulation of the cholinergic system.4. It has been hypothesized that the deleterious effects of elevated homocysteine may, in part, be due to its actions on cholinesterases.5. To further test this hypothesis, homocysteine and a number of its metabolites and analogues were examined for effects on the activity of human cholinesterases.6. Homocysteine itself did not have any measurable effect on the activity of these enzymes.7. Homocysteine thiolactone, the cyclic metabolite of homocysteine, slowly and irreversibly inhibited the activity of human AChE.8. Conversely, this metabolite and some of its analogues significantly enhanced the activity of human BuChE.9. Structure–activity studies indicated that the unprotonated amino group of homocysteine thiolactone and related compounds represents the essential feature for activation of BuChE, whereas the thioester linkage appears to be responsible for the slow AChE inactivation.10. It is concluded that hyperhomocysteinemia may exert its adverse effects, in part, through the metabolite of homocysteine, homocysteine thiolactone, which is capable of altering the activity of human cholinesterases, the most pronounced effect being BuChE activation.