Cold Comfort

There was an amazing BBC Horizon on TV recently (well, actually on iPlayer, where I do the vast majority of my viewing): Horizon – 2010-2011 – Back from the Dead.

The synopsis went:

Dr Kevin Fong investigates a pioneering technique of extreme cooling that is being used to bring people back from the dead.

In the operating theatre, a patient’s heart is stopped and their brain shows no activity. They are indistinguishable from someone who is dead. Yet patients can then be warmed up and brought back to life.

Kevin Fong meets the doctors who have developed this procedure, finds out how it could revolutionise intensive care and trauma medicine, and meets some of the remarkable people who have been brought back from the dead.

The stories about a woman trapped beneath ice, saved only by her core temperature plummeting to 13 celsius, and the man whose brain had to be stopped cryogenically so that its blood supply could be severed, were amazing. There was also a touching story about a baby’s treatment with Xenon, as a result of the research described in the following clip from the programme:

A thought occurred while watching this; see if you agree …

Disclaimer: it’s a long time since I did any biochemistry since finishing my first degree in 1989!

Mitochondria were very likely endosymbiotic prokaryotes, colonising a new environment while offering survival benefits, when they became incorporated into the cytoplasm of our eukaryote ancestors. This is slightly contentious and the theory is being re-examined, however, what did take place did so at a time of increasing oxygen levels in a predominantly anoxic environment. Initially, oxygen would have been toxic, so the symbiotic contribution coming from mitochondria was likely to confer an ability to deal with that oxygen in cells that otherwise did not have that capacity, and would have died as a result: mitochondria were oxygen dustbins.

The electronic byproduct of dealing with that oxygen, hence “powerhouses of our cells”, became useful in it’s own right as the metabolic pathways of the citric acid cycle within mitochondria evolved, which incredibly may have been piecemeal, different stages having evolved within different hosts. This is possible as, famously, mitochondria have genomes independent of their hosts.

Nonetheless, handling oxygen is still a compromise position for mitochondria (it’s part of the deal, the rent that they pay for their cellular home), so, when those cells are starved of oxygen, the mitochondria remain quite happy. The corollary is of course that the cell cannot rely on mitochondria to help them out in such difficulty: anoxia is their preferred place.

Quite how the cell switches into “suicide” mode as described in the clip is unknown but, as with all that we discover about them, that the effect of oxygen starvation on mitochondria is in keeping with their evolutionary origins is fascinating.

Note switching is also consistent with other research from a few years ago which found that, “reducing oxygen levels to near zero, called anoxia, triggers an entirely different protective response that is independent of the mitochondria. This response involves the enzyme PHD as the oxygen sensor” (Mitochondria monitor oxygen concentration in the cell).