Serine hydrolase targets of organophosphorus toxicants

Acetylcholinesterase (AChE) is one of several hundred serine hydrolases in people potentially exposed to about 80 organophosphorus (OP) compounds important as insecticides or chemical warfare agents. The toxicology of OPs was interpreted until recently almost solely on the basis of AChE inhibition. It is assumed that each serine hydrolase has a specific function and proposed that every OP compound has a unique inhibitory profile. This review considers the progress in sifting the expanding list of potential serine hydrolase toxicological targets. About 50 serine hydrolase targets have been recognized but only a few studied thoroughly. The toxicological relevance of known secondary OP targets is established mainly from observations with humans (butyrylcholinesterase and neuropathy target esterase-lysophospholipase) and studies with mice (cannabinoid CB1 receptor, carboxylesterase, lysophospholipase and platelet activating factor acetylhydrolase) and hen eggs (arylformamidase or kynurenine formamidase). Pesticides most commonly shown to inhibit these targets in experimental vertebrates are chlorpyrifos and tribufos. Generally the levels of environmental and occupational OP pesticide exposure are well below those causing in vivo inhibition of secondary serine hydrolase targets. Although exposure to OP insecticides is decreasing from stricter regulations and the development of resistant pest strains, it will continue to some degree for decades in the future. Only two OPs are used as pharmaceuticals, i.e. echothiophate as an ophthalmic for treatment of glaucoma and metrifonate as an anthelmintic for Schistosoma (and formerly as a candidate drug for improved cognitive function in Alzheimer's disease). In safety evaluations, knowledge on known OP targets must be balanced against major gaps in current understanding since more than 75% of the serine hydrolases are essentially unknown as to OP targeting and relevance, i.e. it is not clear if they play a role in OP toxicology.