Antagonism of Acute Sulfide Poisoning in Mice by Nitrite Anion without Methemoglobinemia

Abstract

There are currently no FDA-approved antidotes for H2S/sulfide intoxication. Sodium nitrite, if given prophylactically to Swiss Webster mice, was shown to be highly protective against the acute toxic effects of sodium hydrosulfide (~LD40 dose) with both agents administered by intraperitoneal injections. However, sodium nitrite administered after the toxicant dose did not detectably ameliorate sulfide toxicity in this fast-delivery, single-shot experimental paradigm. Nitrite anion was shown to rapidly produce NO in the bloodstream, as judged by the appearance of EPR signals attributable to nitrosylhemoglobin and methemoglobin, together amounting to less than 5% of the total hemoglobin present. Sulfide-intoxicated mice were neither helped by the supplemental administration of 100% oxygen nor were there any detrimental effects. Compared to cyanide-intoxicated mice, animals surviving sulfide intoxication exhibited very short knockdown times (if any) and full recovery was extremely fast (~15 min) irrespective of whether sodium nitrite was administered. Behavioral experiments testing the ability of mice to maintain balance on a rotating cylinder showed no motor impairment up to 24 h post sulfide exposure. It is argued that antagonism of sulfide inhibition of cytochrome c oxidase by NO is the crucial antidotal activity of nitrite rather than formation of methemoglobin.

Environmental toxicology of hydrogen sulfide

“…Interestingly, it is not uncommon for individuals receiving a high dose of the toxicant for a very short duration to experience knockdown, but then spontaneously (and rapidly) recover, requiring no medical intervention and exhibiting no long-term sequelae [23]. Such observations have been well-replicated in laboratory animals [97]. …”

“…Significantly, at this time, these established characteristics of human poisonings have not been observed together in any of the reported animal models of which we are aware. For instance, mice given LD 40 doses of NaSH by injection either die in less than 4 min or fully recover within 15 min [97]. Furthermore, purified mouse hemoglobin can readily be manipulated to undergo the same conversion to sulfhemoglobin as the human protein.…”

“…While sulfide can clearly react with multiple biomolecules and there are tissue-specific variations in the toxic response, the crucial molecular target in acute cases is generally accepted to be cytochrome c oxidase (complex IV) of the mitochondrial electron-transport system (ETS)[23, 97, 127, 128, 149–151]. Sulfide is certainly a potent inhibitor of complex IV, but it is less well known that it also reacts with the enzyme resulting in catalytic turnover [89, 149, 152, 153].…”

“…As molecular H 2 S can freely diffuse through membranes, it readily crosses the blood-brain barrier to inhibit mitochondrial ETSs within the central nervous system. In unanesthetized laboratory animals, this results in clear behavioral signs of intoxication 2 min post-injection and can lead to death from respiratory paralysis within ~3 min [23, 97], or cardiac failure after ~7 min [147, 148]. At this time, it is not clear how to reconcile the observation that free H 2 S/HS − seemingly only persists for a matter of seconds in the bloodstream [145–148] yet onset of symptoms associated with complex IV inhibition by H 2 S/HS − begins at 2 min after the toxicant dose.…”

“…In this regard, quantitative understanding of the small molecule bioinorganic chemistry underpinning much of the field appears especially lacking. So, for example, while some authors argue that oxygen-dependent redox processes are involved in sulfide cytotoxicity observed in cultured cells [164, 165], other groups have pointed out that in the case of intact animals [97] and human patients [156] any effects of supplemental oxygen are indistinguishable from normal recovery. While less than helpfully informative, it is probably not disingenuous to describe the current status of the relevant redox biochemistry [133, 138] as complicated at best.…”