Cocaine hydrolysis: correlating activity for addiction therapy

Enzymes against cocaine

The interaction of novel substrates for the enzyme butyrylcholinesterase (BChE) and mutants have been investigated using computational and correlation studies. The insights revealed could improve our understanding of how this enzyme, which metabolizes cocaine, might be modulated in drug therapy and the development of anti-addiction drugs.

Fang Zheng, Wenchao Yang, Liu Xue, Shurong Hou, Junjun Liu and Chang-Guo Zhan of the Department of Pharmaceutical Sciences, at the University of Kentucky, in Lexington, have investigated the link between protein structure and catalytic activity in butyrylcholinesterase (BChE) mutants against the well-known drug of abuse, (−)-cocaine. They did so by modelling the rate-determining transition state, which they explain is the transition state for the first step in cocaine hydrolysis and compared efficiencies with the naturally occurring "wild type" enzyme. Ultimately, the work should provide insights into the mode of action of the enzyme and how it might be controlled by pharmaceutical intervention in treating cocaine abuse.

Cocaine benzoylmethylecgonine is a crystalline tropane alkaloid produced by the leaves of the coca plant as a natural pesticide. South American indigenous peoples discovered the effects of chewing the leaves of the coca plant at least a thousand years ago. It acts as a potent stimulant of the central nervous system, an appetite suppressant, and a topical anaesthetic. Technically, it is a serotonin-norepinephrine-dopamine reuptake inhibitor that mediates these neurotransmitters with various physiological and psychological. Its interaction with the brain's mesolimbic reward pathway makes cocaine highly addictive as a drug and it is illegal for non-medicinal and licensed use across most of the world.

Cocaine disaster

The team explains that, "The disastrous medical and social consequences of cocaine abuse have made a high priority the development of a medication for treatment of cocaine overdose and addiction." They point out that developing an anti-cocaine medication that would accelerate cocaine metabolism to its inactive metabolites could be used to wean addicts off the drug. Butyrylcholinesterase might itself be useful as an anti-addiction compound but for the fact that it has a rather low catalytic turnover against naturally occurring (-)cocaine, it takes the body at least an hour and a half to metabolise even a small dose of cocaine. Developing mutant enzymes based on BChE might lead to a better cocaine hydrolase (CocH) for further research or perhaps even as a treatment itself that could be used to "flush" cocaine out of the system and in effect act as an antidote in overdose cases.

The researchers have found through molecular modelling of the TS1 structures that specific mutations on non-active sites in the enzyme can indirectly boost the mutant's catalytic efficiency relative to the wild type. Changing amino acid residues at those non-active sites apparently enhances or weakens the overall hydrogen bonding between the carbonyl oxygen of (-)-cocaine benzoyl ester and the oxyanion hole of the enzyme, with either a positive or negative effect on catalysis depending on the actual amino acid substitutions in the mutant enzyme. Further, they add that computational insights and predictions are supported by experimental data on mutants and they have reported five new mutants for the first time that are some thousand times more effective in hydrolysing (-)-cocaine than the natural enzyme.

"The combined computational and experimental data demonstrate a reasonable correlation relationship between the hydrogen-bonding distances in the TS1 structure and the catalytic efficiency of the enzyme against (-)-cocaine," the team says. Zhan and his colleagues have been working on building a better enzyme for many years and there are many more steps to take before an anti-cocaine medication is available, not least preclinical tests and animal modelling once they find the optimal enzyme to break down cocaine. After that, one could imagine small-scale human trials and eventually a mainstream anti-addiction product but that is likely to be several years in the future.