REHOVOT, Israel -- January 1, 1997 -- A new Weizmann Institute study shows exactly how a substance derived from a moss long used in Chinese folk medicine blocks a brain enzyme that may be involved in Alzheimer disease.
In the study, reported in the January issue of Nature Structural Biology, the scientists solved the 3-D structure of a complex made up of the natural substance Huperzine A (HupA) -- extracted from the Huperzia serrata moss used in China for centuries as a medicine called Qian Ceng Ta -- and the brain enzyme acetylcholinesterase (AChE). The determination of the structure revealed a strikingly good fit between HupA and the enzyme and may provide a possible starting point for designing a new generation of Alzheimer drugs with improved properties.
"It is as if this natural substance were ingeniously designed to fit into the exact spot in AChE where it will do the most good," says crystallographer Prof. Joel Sussman, one of the authors of the study.
The research was performed by graduate student Mia Raves together with crystallographer Dr. Michal Harel and Profs. Sussman and Israel Silman, all of the Weizmann Institute. It involved close collaboration with Prof. Alan Kozikowski, a medicinal chemist at Georgetown University in Washington, D.C., who was the first to synthesize HupA in a test tube, and Dr. Yuan-Ping Pang, a chemist at the Mayo Clinic in Jacksonville, Florida, who had made theoretical predictions of the HupA-AChE interaction.According to one theory, memory loss and other cognitive deficits in Alzheimer patients result from degeneration of nerve cells which release the message- carrying chemical acetylcholine. The acetylcholine shortage that ensues is compounded by the action of AChE, the enzyme that breaks down acetylcholine in the body. Two Alzheimer drugs approved by the US Food and Drug Administration, tacrine (COGNEX) and E2020 (ARICEPT), work by inhibiting AChE. HupA, which differs from these drugs in chemical structure but also inhibits AChE, is currently under investigation in China and elsewhere as a possible Alzheimer's drug.
The new study -- performed by X-ray crystallography -- revealed exactly how the blocking of the enzyme by HupA takes place: HupA slides smoothly into the active site of AChE where acetylcholine is broken down, and latches onto this site via a very large number of subtle chemical links. This binding closes off the enzyme's "cutting" machinery and keeps acetylcholine out of danger.
According to neurochemist Prof. Silman, "such specific binding means that HupA could be a potent drug even when used in small quantities, so that the risk of side effects would be minimal." These risks are relatively small to begin with because HupA is believed to possess very low toxicity.
The scientists worked with high-quality crystals of AChE derived from electric organ tissue of the Torpedo, a fish which is one of the richest sources of this enzyme. Due to the very high degree of similarity in the amino-acid sequence of Torpedo and human AChE, it is likely that the Torpedo 3-D structure is a very good model of the human enzyme. The Torpedo AChE crystals were soaked with HupA, and then exposed to a narrow X-ray beam, producing a diffraction pattern from which a 3-D computer image of AChE-HupA binding could be obtained.
In the past few years, Harel, Silman and Sussman have conducted a number of studies that shed new light on medications used to treat Alzheimer disease. Several years ago, they determined the structure of AChE and showed that it has a very deep chasm -- known as the "aromatic gorge" -- inside of which axcetyline is broken down. They then solved the structure formed by AChE and tacrine, and found that tacrine works by binding to the active site of the enzyme in place of acetylcholine.
The study of HupA-AChE binding was supported by the US Army Medical, Research and Development Command, the Weizmann Institute's Helen and Milton A. Kimmelman Center for Biomolecular Structure and Assembly, and the Scientific Cooperation of the European Union with Third Mediterranean Countries through the israel Ministry of Science. Prof. Silman, a member of the Weizmann Institute's Neurobiology Department, holds the Bernstein-Mason Chair of Neurochemistry. Prof. Sussman and Dr. Harel are members of the Weizmann Institute's Structural Biology Department. Sussman also heads the Protein Data Bank of the Brookhaven National Laboratory in Upton, New York.
TV JOURNALISTS: A 5-minute Weizmann Institute film that can provide background footage for this study is available (broadcast quality, Betacam PAL). The film shows how the 3-D structure of the AChE enzyme was determined and contains visuals of an X-ray crystallography lab, Prof. Silman and Sussman at work, animation of a brain cell activated by the neurotransmitter acetylcholine, and the 3-D computer model of AChE. Please let us know if you'd like a copy.
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