Carbonic anhydrase metabolism is a key factor in the toxicity of CO 2 and COS but not CS2 toward the flour beetle Tribolium castaneum [Coleoptera: Tenebrionidae]

The analogues carbon dioxide (CO₂), carbonyl sulfide (COS) and carbon disulfide (CS₂) have been useful as substrate probes for enzyme activities. Here we explored the affinity of the enzyme carbonic anhydrase for its natural substrate CO₂, as well as COS and CS ₂ (1) by in vitro kinetic metabolism studies using pure enzyme and (2) through mortality bioassay of insects exposed to toxic levels of each of the gases during carbonic anhydrase inhibition. Hydrolysis of COS to form hydrogen sulfide was catalysed rapidly showing parameters K_m 1.86 mM and K_cat 41 s^-1 at 25°C; however, the specificity constant (K_cat/K_m) was 4000-fold lower than the reported value for carbonic anhydrase-catalysed hydration of CO₂. Carbonic anhydrase-mediated CS₂ metabolism was a further 65,000-fold lower than COS. Both results demonstrate the deactivating effect toward the enzyme of sulfur substitution for oxygen in the molecule. We also investigated the role of carbonic anhydrases in CO₂, COS and CS₂ toxicity using a specific inhibitor, acetazolamide, administered to Tribolium castaneum (Herbst) larvae via the diet. CO₂ toxicity was greatly enhanced by up to seven-fold in acetazolamide-treated larvae indicating that carbonic anhydrases are a key protective enzyme in elevated CO₂ concentrations. Conversely, mortality was reduced by up to 12-fold in acetazolamide-treated larvae exposed to COS due to reduced formation of toxic hydrogen sulfide. CS₂ toxicity was unaffected by acetazolamide. These results show that carbonic anhydrase has a key role in toxicity of the substrates CO₂ and COS but not CS₂, despite minor differences in chemical formulae.