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Factbox: Cholinesterase inhibitors - What are they and what do they do?

LONDON (Reuters) - German doctors said on Monday that tests on Russian opposition figure Alexei Navalny indicate poisoning with some kind of cholinesterase inhibitor, although the specific substance is not yet known.

Here are some facts about cholinesterase inhibitors:

- Cholinesterase inhibitors are a group of chemical compounds used in everything from chemical weapons to pesticides designed to kill bugs, and human medicines designed to alleviate symptoms of Alzheimer’s and other types of dementia.

- Nerve gases and the so-called “Novichok” group of chemicals are also cholinesterase inhibitors.

- Cholinesterase is an enzyme that is critical for the normal function of the nervous system in humans, other vertebrates, and in insects.

- Chemicals that block, or inhibit, cholinesterase can cause multi-system acute effects that can be lethal. The chemicals interfere with nerve to muscle messaging and muscles are no longer able to contract and relax.

“They go into a sort of spasm,” said Alastair Hay, a professor environmental toxicology at Britain’s University of Leeds. “All muscles are affected, with the most crucial being those which affect breathing.”

- Certain pesticides, such as organophosphates and carbamates, work by inhibiting cholinesterase. These chemicals can also be poisonous to humans in some situations.

- Human exposure to cholinesterase inhibiting chemical pesticides can result from inhalation, ingestion, or eye or skin contact during their manufacture, mixing, or application.

Reporting and writing by Kate Kelland, editing by Giles Elgood, William Maclean