Human pharmacology of hydrogen sulfide, putative gaseous mediator

Abstract

Nitric oxide (NO), the first low molecular weight inorganic gas to be established as a biological mediator, had previously been regarded merely as a toxic pollutant. It was joined, similarly implausibly to the minds of a generation brought up with the hazards of coal gas as domestic heating, by carbon monoxide (CO) – potentially lethal as an exhaust gas, but also formed in mammalian tissues together with biliverdin by inducible and/ or constitutive forms of haem oxygenase, and implicated subsequently as a signalling molecule, not only in the central nervous system (especially olfactory pathways) and cardiovascular system but also in respiratory, gastrointestinal, endocrine and reproductive functions [1]. Hydrogen sulfide (H2S) was known to generations of schoolboys in one biological context – as the source of the odour of rotten eggs – but even so, the proposal in the early 2000s [2] that it too is a gaseous mediator was met with some scepticism.The toxicology of H2S had previously been studied [3], and actions on enzymes including monoamine oxidase and carbonic anhydrase identified, but more recent work has also shown a rich and diverse pharmacology consistent with functions as a signalling molecule under physiological conditions. It has also been implicated both as a harmful and also as a protective factor in the pathophysiology of a range of experimental models of disease [4], and its therapeutic potential has attracted comment [5]. There are striking similarities between these three gaseous mediators, as well as contrasts, that underlie complex and imperfectly understood functional interactions between them. All three are highly diffusible labile molecules that are rapidly eliminated from the body: NO as nitrite and nitrate in urine as well as NO in exhaled air; CO in exhaled air; H2S as thiosulfate, sulfite and sulfate in urine (Figure 1) as well as (it emerges, see below) in exhaled breath. All three react with haemoglobin, yielding methaemoglobin or nitrosylhaemoglobin (respectively inactive and active metabolites) from distinct reactions of haem and of globin with NO; carboxyhaemoglobin or sulfhaemoglobin from CO or H2S. All three affect cellular energetics via actions on cytochrome c oxidase. All three have vasodilator effects, and all have anti-inflammatory and cytoprotective effects at low concentrations in contrast to causing cellular injury at higher concentrations – consistent with Paracelsus’s aphorism that the distinction between drugs and poisons is determined exclusively by the dose.