Organophosphates essentially irreversibly inhibit acetylcholinesterase by leaving a phosphate group bound to the enzyme. Oximes, such as pralidoxime, reversibly bind to acetylcholinesterase and have high affinity for binding to phosphate groups. They can, therefore, bind to acetylcholinesterase, pick up the phosphate group inhibiting the acetylcholinesterase, and take the phosphate group with them when they leave the acetylcholinesterase. Thus pralidoxime can be used to regenerate acetylcholinesterase after organophosphate poisoning.
A limitation of pralidoxime is that it is only effective in a limited time window before ageing of the organophosphate inhibition of acetylcholinesterase occurs. Pralidoxime itself binds to and competitively inhibits acetylcholinesterase. Therefore, if pralidoxime is administered after all the organophosphate-inhibited acetylcholinesterase has already aged, pralidoxime will just make the anticholinesterase poisoning worse. It is therefore important to administer pralidoxime in the appropriate time window.