Effects of Chlorpyrifos-oxon on Prohormone Convertase Enzyme Activity

by Harshman, Sean William

Organophosphate (OP) compounds have been synthesized for various applications including medicine, pest control and weapons of terror. OPs irreversibly phosphorylate the active site of acetylcholinesterase, the primary target of toxicity, rendering it inactive causing a hypercholinergic state in the post synaptic cleft. However, very little is known about the effects of OPs on secondary targets of toxicity i.e. other serine hydrolases. The prohormone convertase (PC) enzymes, specifically PC1/3 and PC2, are serine hydrolases which are responsible for the processing of pro-opiomelanocortin in the anterior and intermediate lobes of the pituitary. To test the direct interaction between OPs and the PC enzymes, purified PC1/3 and PC2 were treated in vitro with chlorpyrifos-oxon (CHP-O) or vehicle, acetonitrile (ACN), endoproteolyically digested, and analyzed for any evidence of phosphorylation on a MALDI-TOF/TOF mass spectrometer (MS). To further describe the interaction, enzyme assays was performed by pretreating the purified PC1/3 and PC2 with micromolar concentrations of CHP-O over 0.5 h – 4 h. Substrate was added, adrenocorticotropic hormone 1-39 (ACTH 1-39), and analyzed on the MALDI-TOF/TOF MS for processing products and any subsequent reduction in peptide production caused by CHP-O. As a result of these experiments, a statistically significant (p <0.05) reduction in peptide processing was observed for both PC1/3 and PC2 in the presence of CHP-O in vitro. However, evidence of direct interaction of CHP-O on the PC enzymes was inconclusive.