M9 AM Ecotoxicology of Reptiles
Monday, 14 November 2005: 8:00 AM - 11:40 AM in 339-340

(SCH-1117-832186) Reptile cholinesterase characterization, sensitivity to organophosphates and use in monitoring.

Schmidt, S¹, Hooper, M¹, McMurry, S¹, Rainwater, T¹, Finger, A¹, Keller, J^{2, 3}, ¹ Texas Tech University, Lubbock, Texas, USA² Hollings Marine Laboratory, Charleston, South Carolina, USA³ Duke University, Beaufort, North Carolina, USA

ABSTRACT- Cholinesterase (ChE) activity measurement in blood, brain, or liver of an animal is a useful tool for obtaining evidence of exposure to ChE-inhibiting compounds such as organophosphate or carbamate insecticides. Tissues from 11 species of Reptilia were characterized with regard to substrate affinity and inhibitor concentrations in the Ellman ChE assay. Optimized substrate (acetylthiocholine) and iso-OMPA (a differential BChE inhibitor) concentrations ranged from 1.0 mM to 3.16 mM and 100 uM to 5,620 uM, respectively. Total plasma ChE (umoles AThCh hydrolyzed/min/ml) and percent acetylcholinesterase (AChE) ranged from 0.09 to 1.53 and 4.9% to 7.3%, respectively. In vitro inhibitory concentrations for chlorpyrifos-oxon (CPF-O), paraoxon (PAR-O), and diazoxon (DIA-O) were tested in brain and plasma tissue samples of each reptile species to determine species-specific sensitivity patterns. Overall, brain AChE IC50 values ranged from 12.9 nM to 98.6 nM, 70.3 nM to 617 nM, and 206 nM to 3,150 nM for CPF-O, PAR-O, and DIA-O, respectively. Generally, brain AChEs were more sensitive than plasma AChEs, with sensitivity to CPF-O > PAR-O > DIA-O. Within their respective families, turtles, horned lizards, and snakes had similar AChE IC50s regardless of tissue or compound. Between families, terrestrial turtle brain AChE was the least sensitive, followed by lizards and then snakes. Two field-oriented studies applied the methods developed in the ChE characterization to investigate the relationship between age and ChE activity in Morelet's Crocodile (Crocodylus moreletii) and seasonal ChE variability in sub-adult Loggerhead Sea Turtles. Decreasing ChE with age was found in the crocodiles with the juvenile ChE significantly different from sub-adult and adult ChE. Loggerhead ChEs did not show a relationship with month or water temperature nor did they indicate exposure to anticholinesterase compounds. These studies show that reptilian ChEs share characteristics similar to birds and mammals, are sensitive to OPs and are suitable as biomarkers for monitoring exposure to cholinesterase-inhibiting compounds.

Key words: biomarker, cholinesterase, reptile, organophosphate