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A mutant enzyme that breaks down cocaine in the bloodstream 2000 times faster than the body's natural enzymes could lead to a rapid-response treatment for acute overdose or lead to a new therapeutic approach to treating drug addiction. Writing in the latest issue of the Journal of the American Chemical Society, Chang-Guo Zhan of the University of Kentucky in Lexington and colleagues, describe the discovery of what they refer to as the most powerful substance ever found to eliminate cocaine from the body. The advance could ultimately lead to an effective medicine for fighting overdose and addiction of this illicit drug. "Our enzyme mutant is currently the most promising candidate for an effective medicine," Zhan told Spectroscopynow. Cocaine is an alkaloid extracted from the leaves of the coca plant. It acts as a central nervous system stimulant and an appetite suppressant by blocking reuptake of dopamine, serotonin, and norepinephrine in the brain. However, while it is used as a topical anaesthetic, particularly in eye, nose and throat surgery, it also interferes with one of the brain's reward biochemical pathways, the mesolimbic reward pathway. This means it can be highly addictive and so its uses therapeutically are limited. The drug's effects on the brain's reward centres make it a serious drug of abuse across the globe and in almost every niche of society to some extent. Cocaine's unlicensed production, distribution, and use are today illegal in most countries. "The disastrous medical and social consequences of cocaine addiction have made the development of an anticocaine medication a high priority," the researchers say. In the new study, Zhan and colleagues, Fang Zheng, Wenchao Yang, Mei-Chuan Ko, Junjun Liu, Hoon Cho, Daquan Gao, Min Tong, Hsin-Hsiung Tai, and James Woods, point out that there is no effective anti-cocaine medication currently available. Researchers have followed several avenues of research in the quest for an anti-addiction compound. Focus on the plasma enzyme butyrylcholinesterase (BChE) is one of the most promising. "The primary pathway for cocaine metabolism in primates is hydrolysis at the benzoyl ester or methyl ester group," the researchers explain. The BChE enzyme catalyses this process and was discovered to break down and so deactivate cocaine. However, natural BChE is too weak and ineffective for medical therapeutic use, the researchers note. "Wild-type BChE has a low catalytic efficiency against abused cocaine," they explain, "Design of a high-activity enzyme mutant is extremely challenging, particularly when the chemical reaction process is rate-determining for the enzymatic reaction." As such, the researchers have used a novel, systematic computational design approach based on simulating on the computer the transition states of cocaine within the enzyme active site and calculating the activation energies. This has allowed them to identify a more potent and stable mutant version of the BChE structure. In their preliminary tests, their novel variant on BChE could metabolize cocaine at a rate 2000 times faster than natural human BChE. The researchers point out that reducing circulating levels of the drug in the blood is a key to reducing the chance of overdose in someone that has been hospitalised because of cocaine abuse. Their initial studies with laboratory mice overdosed on cocaine demonstrate that mice injected with the mutant BChE do not suffer convulsions and death, whereas control overdosed mice do. "We hope to see it in clinical trials after a few years from now," Zhan told SpectroscopyNOW, "but it will be dependent on how soon we can make a sufficiently large amount of the enzyme mutant." An additional important aspect of the research is not directly related to cocaine addiction but concerns the method the team developed to design the potent mutants of the natural enzyme. "We have developed a novel, generally applicable computational design approach based on a systematic virtual screening of transition states of enzymatic reaction and activation energy calculations," they explain. The same approach could be applied to the rational enzyme engineering of other enzymes for drug discovery and other applications. "We plan to use the same approach in another project for design and discovery of high-activity mutants of human hydrolases against chemical warfare nerve agents (for the purpose of chemical defense)," Zhan adds. As to the role of NMR spectroscopy in this discovery, with every enzyme there is a natural substrate. In this case, the substrate is the cocaine molecule. However, in the creation of designer enzymes for the rapid metabolism of this substance, NMR could provide unique solution structure insights. Its increasing prowess in investigating proteins means it could be used to look at the activity of the experimental enzymes and how they dock with cocaine. Reference:
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 Cracking the problem of cocaine
 Cocaine chemical structure, substrate for mutant enzyme |
