Theses and Dissertations

Issuing Body

Mississippi State University


Chambers, Janice

Committee Member

Howell, George Eli, III

Committee Member

Crow, John Allen

Committee Member

Carr, Russell

Committee Member

Pruett, Stephen

Date of Degree


Original embargo terms


Document Type

Dissertation - Open Access


Environmental Toxicology

Degree Name

Doctor of Philosophy


College of Veterinary Medicine


Environmental Toxicology Program


The mechanism of toxic action for organophosphates (OPs), originally developed as insecticides, is the persistent inhibition of acetylcholinesterase (AChE) resulting in accumulation of acetylcholine and subsequent hyperstimulation of the nervous system. Many OPs require bioactivation via cytochromes P450 to oxon metabolites which are anticholinesterases. Organophosphates display a wide range of acute toxicities. Differences in the OPs’ chemistries results in differences in the compounds' metabolism and toxicity. Acute toxicities of OPs appear to be principally dependent on compound specific efficiencies of detoxication, and less dependent upon efficiencies of bioactivation and sensitivity of AChE. Esterases, such as carboxylesterase (CaE) and butyrylcholinesterase (BChE), play a prominent role in OP detoxication. Organophosphates can stoichiometrically inhibit these enzymes, removing OPs from circulation thus providing protection for the target enzyme, AChE. This in vitro study investigated: 1) age-related sensitivity of AChE, BChE and CaE to structurally different OPs in rat tissues; 2) interspecies and intraspecies differences in bioactivation and detoxication of the OP insecticide malathion in rat and human hepatic microsomes; and 3) interspecies and intraspecies differences in sensitivity of AChE from erythrocyte ghost preparations to malaoxon. Sensitivities of esterases to 12 OPs was assessed by IC50s. The OPs displayed a wide range of AChE IC50s (low nM-µM) with no differences among ages; however, the CaE IC50s generally increased with age (up to 100old) reflecting greater protection in adults. Kinetic analysis of the bioactivation of malathion to the anticholinesterase metabolite, malaoxon, was measured in hepatic microsomes from rats (adult) and humans (various ages) of both sexes. No statistical interspecies (rat and human) or intraspecies (among humans) differences were found. The CaE degradation of malathion and malaoxon was determined in the microsomal samples using indirect measurements. No interspecies or intraspecies differences were found; however, CaE activity in rat microsomes was significantly higher than in humans. Inhibition of AChE by malaoxon was analyzed kinetically in erythrocyte ghost preparations from rats (adults) and humans (three age groups) of both sexes. No statistical interspecies or intraspecies differences were found. These results suggest the age-related differences in acute toxicities of OPs in mammals is primarily a result of their detoxication capacity.