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1-11C-Methyl-4-piperidinyl n-butyrate.

Authors

Shan L1.

Source

Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2009 Oct 20 [updated 2009 Dec 14].

Author information

1
National Center for Biotechnology Information, NLM, NIH

Excerpt

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). 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 (1, 2). BChE is encoded by the BCHE gene, which is located in humans on chromosome 3q26.1-q26.2 (3). Mutations of the BCHE gene result in various genotypes and phenotypes (4), and some BCHE 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 muscle 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 BCHE 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 (1). 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 (1, 6). The aryl acylamidase activity of BChE may be involved in the crosstalk between serotonergic 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) (7, 8), which is characterized by a loss of cholinergic neurons. 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. 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 (6). 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 (9, 10). However, the relationship between altered BChE and AChE expressions and tumorigenesis is not clear, nor is the efficacy of specific inhibitors as chemotherapeutic agents. Because of the potential diagnostic and therapeutic values, investigators have synthesized various radiolabeled acetylcholine and butyrylcholine analogs as positron emission tomography (PET) tracers (11-16). These tracers have been used to measure ChE activity, detect diseases with cholinergic deficits, and study the efficacy of ChE inhibitors. N-methylpiperidinyl esters are a group of synthetic AChE substrates; of them, 1-11C-methyl-4-piperidinyl acetate (11C-MP4A) and 1-11C-methyl-4-piperidinyl propionate (11C-MP4P) have already been used in the clinic as PET tracers for in vivo assessment of AChE activity associated with AD. 1-11C-Methyl-4-piperidinyl n-butyrate (11C-MP4B or [11C]BMP), a specific radiolabeled substrate of BChE, was developed for in vivo assessment of BChE activity with PET (15-18).

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