EDTA-resistant and sensitive phosphotriesterase activities associated with albumin 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 consequently inactivated by ethylenediaminetetraacetic acid (EDTA). There is also a little known PTE in mammal serum that is resistant to EDTA inactivation. In this work, the PTE activities for the substrates O-hexyl O-2, 5-dichlorophenyl phosphoramidate (HDCP) and O,O'-diethyl p-nitrophenyl phosphate were purified from rabbit serum by ultracentrifugation, molecular exclusion, and anion exchange chromotography. Rabbit serum produced two PTE activities. One was sensitive and the other was resistant to EDTA inhibition. The EDTA-resistant HDCP hydrolyzing activity and paraoxonase activities of rabbit serum were purified to homogeneity. These activities copurified and were associated to albumin. This EDTA-resistant activity exhibited no stereoselectivity in the hydrolysis of HDCP. The EDTA-sensitive activity was isolated in the lipoprotein fraction and stereoselectively hydrolyzed the S-HDCP over the R-HDCP. Other differences between the EDTA-sensitive paraoxonase and HDCP hydrolyzing activity were discovered in response to p-nitrophenylbutkyrate, 5,5-dithio-bis(2-nitrobenzoic acid), caprylic acid, sodium ions, and ammonium ions. This work demonstrates the existence of two well differentiated PTE activities in rabbit serum. One is sensitive to EDTA, stereoselective, and found in the lipoprotein fraction, and the other is resistant to EDTA inhibition and nonstereospecific.