BCHE

Human butyrylcholinesterase (BChE, EC 3.1.1.8) is a soluble sugar-coated globular molecule that is synthesized in the liver and secreted into the blood. Almost all tissues contain butyrylcholinesterase, but the enzyme in plasma is the most studied. Butyrylcholinesterase is a serine esterase, which is exquisitely sensitive to inhibition by organophosphorus pesticides and nerve agents. The ability of butyrylcholinesterase to scavenge organophosphorus poisons has made butyrylcholinesterase the new therapeutic for protection against nerve agent toxicity. The U.S. Department of Defense has invested millions of dollars to produce huge quantities of pure human butyrylcholinesterase to be used for protection against nerve agents.

Nomenclature

The name butyrylcholinesterase was assigned by the Human Gene Nomenclature committee in 1989. The abbreviation for the gene is BCHE and for the protein is BChE. The older literature uses the names pseudocholinesterase, plasma cholinesterase, serum cholinesterase, nonspecific cholinesterase, butyrylcholinesterase, and acylcholine acylhydrolase. The enzyme commission number is EC 3.1.1.8.

Structure

The human butyrylcholinesterase monomer contains 602 amino acids, which includes 28 residues in the signal peptide and 574 residues in the mature, secreted protein. BChE belongs to the family of serine hydrolases, defined as enzymes whose active site includes serine. Catalysis by BChE requires a functioning catalytic triad: Ser 198, Glu 325, and His 438. BChE activity is irreversibly inhibited by organophosphorus compounds. The active site serine is covalently modified by reaction with organophosphorus esters, in a reaction that simultaneously inactivates BChE and destroys the organophosphorus poison.

Figure 1. Tetrameric human BChE. (A) Top view of the BChE tetramer. (B) Side viewof the BChE tetramer.
(Lockridge O. 2015)

The BChE tetramer has four identical subunits (each 85 kDa) interacting with each other via a four-helix bundle at the C-termini. A polyproline-rich peptide lies in the center of the four-helix bundle. The four-helix bundle is the tetramerization domain. The polyproline-rich peptide in the center of the four-helix bundle is hydrogen bonded to Trp543, Trp 550, Trp557 and three other hydrophobic residues, all on the same side of the amphiphilic helix. The polyproline-rich peptide is not released from the tetramer when BChE is diluted.

Stability

Healthy human adults display a consistent level of BChE activity in plasma. However, plasma BChE activity is increased in obese subjects, as well as in the Cynthiana and C5+ variants of the BCHE gene. Low BChE activity is found in the plasma of pregnancy and in subjects who have liver disease, malnutrition, carcinoma, and other conditions as well as in subjects who have genetically inherited missense mutations of the BCHE gene.

BChE activity in the stored human plasma or serum was stable for at least 8 days when stored at 4°C, 22°C, or −20°C. Prolonged storage at room temperature (23°C) for 21 days saw BChE activity drop to 40% of its initial activity. However, plasma stored refrigerated (4°C) or frozen (−20°C) retained over 90% of BChE activity for 50 days.

Functions

Butyrylcholinesterase was one of the first examples in the new field of pharmacogenetics. The study of natural genetic variants of butyrylcholinesterase led to the realization that some people have no butyrylcholinesterase activity. Despite the complete absence of butyrylcholinesterase activity, people with silent butyrylcholinesterase are healthy. On this basis it was concluded that butyrylcholinesterase has no essential function that cannot be compensated by other enzymes. The only situation in which a function for butyrylcholinesterasewas apparent was in response to drugs administered intravenously. The esterase activity of butyrylcholinesterase was responsible for destroying a large percentage of the drug before it reached its site of activity. People with silent or atypical butyrylcholinesterase were unable to hydrolyze the drug and therefore got an overdose.

The butyrylcholinesterase knockout mouse and the acetylcholinesterase knockout mouse were made to try to uncover a physiological function for butyrylcholinesterase. It is proved that butyrylcholinesterase has a supporting role in neurotransmission, in that butyrylcholinesterase hydrolyzes acetylcholine that has diffused out of the nerve synapse. This role becomes evident only when acetylcholinesterase is inhibited. Drugs currently approved for slowing the progression of Alzheimer’s disease are inhibitors of acetylcholinesterase.

Applications

BChE is an excellent biomarker of exposure to nerve agents and organophosphorus pesticides. The United States Department of Defense plans to stockpile lyophilized human BChE to use for protection against the toxicity of nerve agents. Human BChE was selected for this indication because studies in animals have demonstrated that BChE protects from OP toxicity. The mechanism that explains the protective effect of BChE is that BChE is a highly reactive scavenger that intercepts and destroys the poison before it reaches its AChE target. The disadvantage of using plasma-derived human BChE for protection from nerve agents is that the reaction with OP is stoichiometric. One molecule of BChE destroys one molecule of nerve agent and in the process inactivates BChE from further bio-scavenging activity. It would be advantageous to have a catalytic scavenger that destroys many molecules of nerve agent because a lower dose of the catalytic scavenger would be required for protection.

References

1. Lockridge O. Review of human butyrylcholinesterase structure, function, genetic variants, history of use in the clinic, and potential therapeutic uses. [J]. Pharmacology & Therapeutics, 2015, 148:34-46.
2. Satoh T, Gupta R C. 3. Butyrylcholinesterase: Overview, Structure, and Function [M] // Anticholinesterase Pesticides: Metabolism, Neurotoxicity, and Epidemiology. John Wiley & Sons, Inc. 2011:25-41.