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Submitted to: International Journal of Toxicology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/13/2002
Publication Date: 8/18/2003
Citation: Yi, G.B., Mcclendon, W.D., Desaiah, D., Goddard, J., Lister, A., Moffitt, J., Vander Meer, R.K., Deshazo, R., Lee, K.S., Rockhold, R.W. 2003. The Fire Ant Venom Alkaloid, Isosolenopsin A, a Potent and Selective Inhibitor of Neuronal Nitric Oxide Synthase. International Journal of Toxicology. 22:81-86. Interpretive Summary: The red imported fire ant infests over 150 million hectares throughout the southern tier of the United States and presents medical, agricultural, and ecological problems because of its high population densities and potent sting. The unique fire ant venom contains mainly piperidine alkaloids that have a wide range of physiological activities. However, the often life-threatening allergic reactions to stings are attributed to small amount of proteins found in the venom. While hypersensitivity to fire ant venom accounts for many of the reported medical problems associated with fire ants, there are reports of deaths and neurological problems after individuals receive large numbers of fire ant stings that are more readily explained by direct toxicity. Nitric oxide, a neurotransmitter, plays important roles in various physiological and inflammatory processes throughout the body. Scientists at the Center for Medical, Agricultural and Veterinary Entomology, Gainesville, FL, The University of Mississippi Medical Center, the Mississippi Department of Health, and Jackson State University, used a rat mammalian model to determine that fire ant venom alkaloids inhibit the production of nitric oxide by several forms of nitric oxide synthase. The inhibition of nitric oxide production could cause the reported systemic neurotoxicity, hypercoagulability, or other systematic toxic effects in man after receiving massive stings, independent of the recognized protein-based hypersensitivity reactions. This study indicates that a more aggressive rather than passive approach to management of massive fire ant stings may be necessary.
Technical Abstract: Massive, multiple fire ant, Solenopsis invicta, stings may be treated aggressively, particularly in the elderly, despite limited evidence of systemic toxicity. Over 95% of the S. invicta venom is composed of piperidine alkaloids of unknown toxicity. To assess a basis for systemic toxicity, the inhibitory activity of an alkaloid-rich, protein-free methanol extract of whole ant venom was tested against three nitric oxide synthase (NOS) isoforms, rat cerebellar neuronal (nNOS), bovine recombinant endothelial (eNOS) and murine recombinant inducible (iNOS), by determining the conversion of [3H]arginine to [3H]citrulline in vitro. Concentration-dependent inhibition of nNOS activity was noted, with inhibition of approximately 50% with 0.33±0.06 (g of the venom extract, and over 95% inhibition of nNOS, eNOS and iNOS, with 60 ug in 60 ?L reaction mixture. S. invicta venom alkaloids can potently inhibit all major NOS isoforms. Isosolenopsin A (cis-2-methyl-6-undecylpiperidine), a naturally occurring fire ant piperidine alkaloid, was synthesized and tested against NOS isoforms. Enzyme activities for nNOS and eNOS were over 95% inhibited with 1000 uM of isosolenopsin A, while the activity of iNOS was inhibited by only about 20% at the same concentration. The IC50s were 18+3.9 uM for nNOS, 156+10 uM for eNOS, and >1000 uM for iNOS. Isosolenopsin A inhibition was noncompetitive with L-arginine (Ki = 19±2 uM) and its nNOS inhibitory potency compares favorably with that of widely used nNOS inhibitors. Inhibition of NOS isoforms by isosolenopsin A and structurally similar compounds may have toxicological significance with respect to adverse reactions to fire ant stings.