Haux_2000_Chem.Res.Toxicol_13_646

Ethephon [(2-chloroethyl)phosphonic acid] has two seemingly unrelated types of biological activity. It is a major agrochemical absorbed by crops, slowly releasing ethylene as a plant growth regulator. Ethephon also inhibits the activity of plasma butyrylcholinesterase (BuChE) in humans, dogs, rats, and mice. This is totally unexpected for an ionized phosphonic acid (mostly the dianion at physiological pH), in contrast to the classical inhibitors (nonionized triester phosphates) which phosphorylate serine at the active site. This study tests the hypothesis that ethephon (as the dianion) also acts as a phosphorylating agent in inhibiting BuChE activity. The sensitivity of plasma BuChE to ethephon (90 min preincubation at 25 degrees C) is greatest for humans, dogs, and mice (IC(50) = 6-23 microM), intermediate for chickens, rabbits, rats, and guinea pigs (IC(50) = 26-53 microM), and lowest for pigs and horses (IC(50) = 92-172 microM). The IC(50) decreases linearly with time on a log-log scale to values of 0.15-0. 3 microM for human, dog, and horse BuChE at 24 h. The inhibition rate is generally related to ethephon concentration, consistent with a bimolecular reaction, e.g., phosphorylation. The extent of inhibition of the esteratic activity of BuChE by ethephon is directly proportional to the extent of inhibition of [(3)H]diisopropyl phosphorofluoridate ([(3)H]DFP) postlabeling which is not reversible on removing the ethephon, either directly or after further incubation for 24 h at 25 degrees C. These observations strongly suggest that ethephon, as DFP, phosphorylates human plasma BuChE at Ser-198 of the esteratic site, or more generally, the formation of a phosphobutyrylcholinesterase. With human plasma BuChE, (2-bromoethyl)- and (2-iodoethyl)phosphonic acids have lower affinities for the site than ethephon but higher phosphorylation rate constants, consistent with their relative hydrolysis rates at pH 7.4 (phosphorylation of water). (2-Chlorohexyl)phosphonic acid is a poor inhibitor, perhaps being too reactive with water. Thus, potency differences for ethephon and its analogues with BuChE of various species depend on both the affinities and phosphorylation rates, i.e., the binding and reactivity of the (2-haloalkyl)phosphonic acid dianion in the esteratic site.