Chemistry labs just don’t smell like they used to.  When I were a lad, chemistry lessons involved a procedure called Qualitative Inorganic Analysis.  This involved bubbling hydrogen sulfide H2S, produced in a Kipp’s apparatus, through solutions to precipitate various metals group by group.

Now hydrogen sulfide (as in rotten eggs) is known to be as toxic as hydrogen cyanide, having roughly the same lethal concentration.  But these two gases are definitely not two of a kind.  Even at low concentrations, hydrogen cyanide does no-one any good [1], and if Miss Marple is on the guest list, watch out! 

However, it now turns out that dear old H2S, while one definitely doesn’t want an overdose of it [2], is not only a necessary part of your metabolism, it may one day save your life.

Browsing in our university library, my eye fell upon Toxic Gas, Lifesaver featured on the cover of the March 2010 Scientific American.  It begins:

Imagine walking into a hospital emergency room, with its hand-sanitizer-adorned walls and every surface meticulously scrubbed free of contaminants, only to encounter the stench of rotten eggs.  Distasteful though this juxtaposition might sound, the toxic gas synonymous with that smell – hydrogen sulfide (H2S) – may well become a fixture in such settings in the future.  Over the past decade scientists have discovered that H2S is actually essential to a number of processes in the body, including controlling blood pressure and regulating metabolism. Our findings indicate that if harnessed properly, the gas could, among other benefits, help treat heart attack patients and keep trauma victims alive until they can undergo surgery or receive a blood transfusion. 

Scholars have known about H2S’s toxic effects on humans for centuries …  Today it constitutes the number-one occupational safety hazard at oil and gas field wellheads, along pipelines, in ... 
So writes Rui Wang, Vice-President (Research) at Lakehead University, Thunder Bay (on the Canadian shore of Lake Superior.)  

There was, to say the least, considerable excitement surrounding the discovery of how nitric oxide (NO) functions in the body.  I remember watching a programme on how the life of a patient suffering from anaphylactic shock was saved by the insight of the surgeon. As I remember, the dilemma was this: more
adrenaline would stimulate the heart, but would also further relax the blood vessels leading to a catastrophic drop in pressure.  Simply based on that equation, things had got beyond the point of no return.  Something occurred to the surgeon’s mind, namely a recent discovery that NO would relax the blood vessels, so perhaps a drug suppressing NO production might save the patient’s life.  It worked.  Search through the literature suggests that this is now standard treatment.

Back to Rui Wang.  Coming home one day, he smelt H2S in his house.  Tracking down the source, it turned out to be a cracked specimen among the eggs his daughter had painted for Easter.  This set his mind on to the possibility that H2S might be another small signalling molecule active in the body.

His studies showed that the gas was present in the blood vessels in rodents, and that they contained appropriate enzymes to produce it.  A recent paper of his in Science [3] begins:
Studies of nitric oxide over the past two decades have highlighted the fundamental importance of gaseous signaling molecules in biology and medicine. The physiological role of other gases such as carbon monoxide and hydrogen sulfide (H2S) is now receiving increasing attention.
What was significant about this work was showing how mice genetically engineered to knock out the H2S producing enzyme suffered from hypertension which could be alleviated by administration of the gas.

H2S and NO can not, however, be simply substituted one for another.  There mode of action is quite different, and while NO is effective in the big blood vessels close to the heart, H2S operates on the tiny peripheral blood vessels.

The Scientific Amercian article ends on a quite bullish note regarding the clinical prospects for H2S.  But two speculations of his stick out.

Garlic was already widely hailed as being good for hypertension, and a 1998 study Garlic prevents hypertension induced by chronic inhibition of nitric oxide synthesis showed that it could come to the rescue of rats which had been doped to prevent NO synthesis in their system.  But Wang highlights some studies showing that chemicals in garlic (a very sulphurous vegetable anyway) can be broken down by enzymes to release H2S.

At the end of the Permian, there was a mass extinction, and one theory is that the deep oceans became quite anoxic, allowing H2S-producing bacteria to thrive.  This knocked out about 95% of marine species and 70% of land species.  The survivors were those that could cope with H2S, and may even have incorporated it into their metabolism.  Ratty and ourselves would be beneficiaries of this unlikely inheritance.

[1] (a) Hydrogen cyanide as a toxin and a weapon (b) many species of plant incorporate it as part of their defences in cyanogenic glucosides

[2] Hydrogen sulfide toxicity.

[3] H2S as a Physiologic Vasorelaxant: Hypertension in Mice with Deletion of Cystathionine g-Lyase

Guangdong Yang, Lingyun Wu, Bo Jiang, Wei Yang, Jiansong Qi, Kun Cao, Qinghe Meng, Asif K. Mustafa, Weitong Mu, Shengming Zhang, Solomon H. Snyder, Rui Wang

Science Vol 322 24 October 2008 pp 587-588