Cholinesterase (ChE) is an enzyme that hydrolyzes the neurotransmitter acetylcholine into choline and acetic acid, and thus shuts off neural transmission (1, 2). There are two types of ChE: acetylcholinesterase (AChE, also known as erythrocyte cholinesterase or acetylcholine acetylhydrolase) and butyrylcholinesterase (BChE or BuChE, also known as plasma cholinesterase, pseudocholinesterase, or acylcholine acylhydrolase) (2). Both enzymes are present in cholinergic and noncholinergic tissues as well as in plasma and other body fluids. They differ in substrate specificity, behavior in excess substrate, and susceptibility to inhibitors. BChE is encoded by the gene, which is located in humans on chromosome 3q26.1-q26.2 (3). Mutations of the gene result in various genotypes and phenotypes (4), and some gene variants, such as atypical, K, J, and H variants, cause reduced activity of BChE. The silent variants lead to total loss of the enzyme activity (0–2% of normal activity). On the other hand, some variants result in increased activity, such as the C5+ variant (combination of BChE with an unidentified protein), the Cynthiana variant (increased amount of BChE than normal level), and the Johannesburg variant (increased BChE activity with normal enzyme level). In the absence of relaxants, there is no known disadvantage for individuals with these variants. BChE is synthesized in many tissues, including the liver, lungs, heart, and brain. Similar to AChE, a single gene gives rise to different protein products by alternative splicing in the coding region of the original transcript. This provides a series of diverse but related molecular forms of BChE (G1, G2, and G4). G4 is the predominant isoform in the mature brain. These forms have similar catalytic properties, but they exhibit different cellular and extracellular distributions and non-catalytic activities. BChE possesses three different enzymatic activities: esterase, aryl acylamidase, and peptidase. The esterase activity of BChE plays an important role in scavenging anti-AChE compounds such as cocaine, heroin, and organophosphate before they reach AChE at physiologically important sites. In the absence of AChE, BChE is believed to serve as a backup to AChE in supporting and regulating cholinergic transmission (5). BChE also inactivates some drugs, e.g., aspirin, amitriptyline, and bambuterol. The aryl acylamidase activity of BChE may be involved in the crosstalk between seratonergic and cholinergic neurotransmission systems, but it is still poorly understood. The peptidase activity of BChE is related to the development and progress of Alzheimer’s disease (AD) which is characterized by a loss of cholinergic neurons (6). In the brains of patients with AD, the level of the membrane-bound G4 form of AChE is selectively reduced by 90% or more in certain regions, while the level of the G1 form is largely unchanged. On the contrary, the G1 form of BChE shows a 30–60% increase, while the G4 form decreases or remains the same as in the normal brain (7). It has been indicated that BChE, which is found in the neuritic plaques and tangles, cleaves the amyloid precursor protein to the β-amyloid protein and helps β-amyloid diffusion to β-amyloid plaques. Abnormal expressions of BChE and AChE have also been observed in human tumors such as meningioma, glioma, acoustic neurinomas, and lung, colon and ovarian cancers (8). However, the relationship is not clear between altered BChE and AChE expressions and tumorigenesis. Because of the potential diagnostic and therapeutic values, investigators have synthesized various radiolabeled butyrylcholine analogs and tested their feasibilities as tracers for measurement of cerebral BChE activity (9-12). In an attempt to better understand the real-time distribution of BChE from injection site, Duysen and colleagues labeled the BChE directly with fluorescent dye and investigated the BChE pharmacokinetics in BChE knockout mice (13, 14). There is no detectable BChE activity in all tissues and plasma of the BChE−/− mice.
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