Mangas_2012_Chem.Res.Toxicol_25_2393

Phenylmethylsulfonyl fluoride (PMSF) is a protease and esterase inhibitor that causes protection or potentiation/promotion of organophosphorus delayed neuropathy (OPIDN) depending on whether it is dosed before or after an inducer of delayed neuropathy. The molecular target of promotion has not yet been identified. Kinetic data of esterase inhibition were first obtained for PMSF with a soluble chicken brain fraction and then analyzed using a kinetic model with a multienzymatic system in which inhibition occurred with the simultaneous chemical hydrolysis of the inhibitor and ongoing inhibition (inhibition during the substrate reaction). The best fitting model was a model with resistant fraction, Ealpha (28%), and two sensitive enzymatic entities, Ebeta (61%) and Egamma (11%), with I(50) at 20 min of 70 and 447 muM, respectively. The estimated constant of the chemical hydrolysis of PMSF was kh = 0.23 min(-1). Ealpha, which is sensitive to mipafox and resistant to PMSF, became less sensitive to mipafox when the preparation was preincubated with PMSF. Its Ealpha I(50) (30 min) of mipafox increased with the PMSF concentration used to preincubate it. Egamma is sensitive to both PMSF and mipafox, and after preincubation with PMSF, Egamma became less sensitive to mipafox and was totally resistant after preincubation with 10 muM PMSF or more. The sensitivity of Ealpha to paraoxon (I(50) 30 min from 9 to 11 nM) diminished after PMSF preincubation (I(50) 30 min 185 nM) and showed no spontaneous reactivation capacity. The nature of these interactions is unknown but might be due to covalent binding at sites other than the substrate catalytic center. Such interactions should be considered to interpret the potentiation/promotion phenomenon of PMSF and to understand the effects of multiple exposures to chemicals.