Interactions of Nitric Oxide and Redox-related species with biological targets

Abstract

The toxicity of NO and the redox-related species NO⁺( the nitrosonium cation, supplied as the nitroprusside anion) and NO^–( the nitroxyl anion, supplied by decomposition of the monoprotonated trioxodinitrate anion or Piloty's acid, N-hydroxybenzenesulfonamide) to the food spoilage organism Clostridium sporogenes, together with that of nitrite, Roussin's black salt and SIN-1, was assessed. Distinct toxic effects were shown by each of the three species. The nitroprusside anion inhibits growth, with K_i(50)= 20 µmol l^–1, and hydrogen production to similar extents. At higher concentration of nitroprusside, the cell suspension is clarified, due either to clumping or lysis. Electron microscopy shows that sodium nitroprusside at 10 µmol l^–1 causes blistering of cells and, at 20 µmol l^–1, lysis of cells. This results from nitrosation of thiol groups in the cell wall. Analysis for thiols shows a decrease of about 90% of the thiol groups in the cell wall for nitroprusside-treated cells, while electron paramagnetic resonance (EPR) spectrometry confirms the production of reduced nitroprusside species resulting from reaction with the thiol groups. EPR also shows the formation of mononuclear dinitrosyliron–sulfur complexes with g= 2.03, which is probably due to the NO released during the reactions of the reduced nitroprusside. Nitric oxide and the nitroxyl ion do not cause lysis of cells but both cause clumping at sufficiently high concentrations. The effects of NO were shown equally on both growth and dihydrogen production, but the nitroxyl anion differs from the other two species in showing greater toxicity towards dihydrogen production than to growth, reflecting the high affinity of this species towards iron–sulfur clusters.