On the Role of Unspecific Carboxylesterases (EC 3.1.1.1) in the Metabolism of Acetylsalicylic Acid and Procaine

Abstract

Unspecific carboxylesterases (CE) are involved in drug metabolism by hydrolyzing ester and amide bonds. Their significance in vivo, however, compared to the contributions of other esterases is unknown so far in most cases. Since previous studies (Block and Wassermann, 1976 and 1977) provided evidence of the high specificity of bis-p-nitrophenylphosphate (BNPP) as an irreversible inhibitor of CE in the rat, the influence of BNPP-pretreatment on the fate of (carboxyl-¹⁴C)procaine (P) and of acetyl(carboxyl-¹⁴C) salicylic acid (ASA) was investigated. 12 hrs after i.p. inj. of 100 mg BNPP/kg b.w. (or of 0.9% NaCl = controls) 10 mg of ¹⁴C-P or ¹⁴C-ASA were i.p. applied to rats (Wistar, female, 150 g). During 24 hrs 20.1% (12.3%) of the dose were found in the urine as unchanged P and 2.8% (0.7%) as unchanged ASA (values in brackets = controls). In mice the toxicity of P is augmented by BNPP-pretreatment, the LD₅₀ decreased by a factor of 1.8. After preincubation of mouse liver homogenate with 10⁻⁴M BNPP (30 min, 37°C) the “procaine esterase” was almost totally inhibited. Using a continuous spectrophotometric test, 40% of the procaine hydrolyzing activity of mouse plasma and 60% of the ASA hydrolyzing activity of rat serum were found sensitive to BNPP. Values for v_max and k_M of controls and of the non-BNPP-sensitive fractions are provided by Lineweaver-Burk plots. In human plasma or serum neither procaine esterase nor ASA esterase was inhibited after preincubation with BNPP; this is in good agreement with the known fact, that human plasma contains no CE, and provides an additional argument for the high specificity of BNPP. — The application of BNPP in toxicologic and teratologic studies on the molecular mechanisms of the known side-effects of ASA appears promising. Supported by the DFG.