Dichlorophenyl phosphoramidates as substrates for avian and mammalian liver phosphotriesterases: activity levels, calcium dependence and stereospecificity

The present study shows the existence of both Ca2+-dependent and EDTA-resis tant hydrolysing activities against HDCP and paraoxon in the particulate an d soluble fractions of hen, rat and rabbit liver. HDCP was more extensively hydrolysed than paraoxon in both subcellular fractions and each of three i ndividuals of the three animal species under study in spite of wide interin dividual variations. However the ratio of HDCP versus paraoxon hydrolysing activity (HDCPase/paraoxonase), although within the same order of magnitude , cannot be considered as constant as it ranges one- to seven-fold between individuals of the same species. Also there is no constant ratio of Ca2+-de pendent/EDTA-resistant activities. Rabbit liver showed the highest rates of Ca2+-dependent hydrolysis for both organophosphorus compounds whereas the hen paraoxonase activity was not inhibited by EDTA. The stereospecific hydr olysis of HDCP was mostly a Ca2+-dependant one, the S-HDCP isomer being hyd rolysed faster than the R-HDCP one. The suggestion is made that HDCP could be conveniently used to measure PTE activity in the liver. (C) 1999 Elsevie r Science Ireland Ltd. All rights reserved.