EDTA-resistant and sensitive phosphotriesterase activities associated withalbumin and lipoproteins in rabbit serum

Phosphotriesterase (PTE) activities in mammalian serum are typically found in the lipoprotein fraction. This PTE requires Ca++ for activity and is con sequently inactivated by ethylenediaminetetraacetic acid (EDTA). There is a lso a little known PTE in mammal serum that is resistant to EDTA inactivati on. In this work, the PTE activities for the substrates O-hexyl O-2,5-dichl orophenyl phosphoramidate (HDCP) and O,O'-diethyl p-nitrophenyl phosphate w ere purified from rabbit serum by ultracentrifugation, molecular exclusion, and anion exchange chromotography. Rabbit serum produced two PTE activitie s. One was sensitive and the other was resistant to EDTA inhibition. The ED TA-resistant HDCP hydrolyzing activity and paraoxonase activities of rabbit serum were purified to homogeneity. These activities copurified and were a ssociated to albumin. This EDTA-resistant activity exhibited no stereoselec tivity in the hydrolysis of HDCP, The EDTA-sensitive activity was isolated in the lipoprotein fraction and stereoselectively hydrolyzed the S-HDCP ove r the R-HDCP. Other differences between the EDTA-sensitive paraoxonase and HDCP hydrolyzing activity were discovered in response to p-nitrophenylbutky rate, 5,5-dithio-bis(2-nitrobenzoic acid), caprylic acid, sodium ions, and ammonium ions. This work demonstrates the existence of two well differentia ted PTE activities in rabbit serum. One is sensitive to EDTA, stereoselecti ve, and found in the lipoprotein fraction, and the other is resistant to ED TA inhibition and nonstereospecific.