METABOLISM OF CARBONYL SULFIDE TO HYDROGEN SULFIDE IN INSECTS IS CATALYSED BY CARBONIC ANHYDRASE

Carbonyl sulfide (COS) is a new fumigant under development as a methyl bromide replacement for fumigation of durable commodities. COS has been shown to be relatively fast acting and toxic to a broad range of stored-product pests but little is known of the mechanism of toxicity of COS to insects. In rats, COS is metabolised to hydrogen sulfide by the enzyme carbonic anhydrase, a widely distributed family of enzymes that usually catalyse the reversible hydration of carbon dioxide. The present study investigated whether hydrogen sulfide was the toxic agent of COS in insects. Firstly, adult stored-product insects Sitophilus oryzae (L.), Tribolium castaneum (Herbst) and Rhyzopertha dominica (F.) were fumigated with hydrogen sulfide for 6 h at a range of concentrations from 1-50 mg L -1 . The percentage mortality of insects was !84% at concentrations of 5 mg L -1 and above suggesting hydrogen sulfide is potently toxic to insects. To determine the role of metabolism in COS toxicity, 17-21 day old larvae of T. castaneum were raised on culture medium containing carbonic anhydrase inhibitors, acetazolamide or methazolamide, at concentrations up to 20 mg g -1 media. Larvae raised on inhibitors and untreated larvae were then fumigated with 60 mg L -1 COS for 5 h, a concentration - time product that had been shown to produce approximately 90% mortality in untreated larvae. The mortality of larvae raised on acetazolamide-containing medium (20 mg g -1 ) was much lower than untreated larvae (40% versus 95%) after fumigation. Similarly, mortality was reduced in insects administered methazolamide (20 mg g -1 ) resulting in 53% mortality. Administration of carbonic anhydrase inhibitors to insects protected them from the acute toxicity of COS. This work demonstrates that the acute toxicity of COS to insects is dependent on carbonic anhydrases metabolism to hydrogen sulfide, the toxic agent of COS.