We breathe oxygen; it can’t be dangerous, can it?
Millions and millions of chemical reactions are going on every second in the cells of our body. The oxygen in the air we breathe is involved in a large part of them. When we say that alcohol “burns” in our organism at a constant speed, it does not burn in blue flames like in a lamp, of course. The enzymes of our organism, the proteins regulating chemical reactions in our body, take care of burning or combustion in very accurately controlled reactions. The energy produced by the reaction does not dissipate into the environment as heat, but it is bound to new chemical compounds that can be utilized later in the same cell or even in another cell. A very good example of an energy-carrying molecule is adenosine triphosphate or ATP. The development of this kind of controlled “burning” is one of the crucial factors that made possible the life of all cells including our own cells.
Nothing is perfect
There is nothing in the world without fault, even if it has had time to gradually perfect itself over three thousand million years. When oxygen is consumed in the burning process (oxidation), it has to change from an oxygen molecule consisting of two oxygen atoms into oxygen radicals that are much more aggressive substances than molecular oxygen. Since they are so very dangerous and reactive, they have to be kept under much stricter control than a fire in a closed stove. It is possible that some sparks can jump out of a stove or even a burning firebrand may drop onto the floor causing a lot of damage to the environment. This is what happens sometimes to oxygen radicals.
Oxygen as a weapon
Oxygen is also a weapon. The body has certain cell types (such as macrophages) that are intended to attack and destroy bacteria and other foreign organisms and one of the weapons in their arsenal is the ability to use oxygen to damage foreign macromolecules. In some respects, one can say that they oxidize, “burn”, these intruders. Smaller particles are taken up by engulfing (the process called phagocytosis) by macrophages. The cells then destroy the particles inside the cell by attacking them with oxygen radicals and enzymes. If this battle is intense, sparks start to fly around. An extreme example is an asbestos fibre that has intruded into the lung. Macrophages try desperately to destroy it, even though the fibre is much bigger than the cells themselves. It is like a log which is too large to fit inside the stove. Then fire moves along the log and sends sparks flying around the room. Similarly, macrophages emit a number of oxygen radicals to the surrounding lung tissue.
Shooting at your own soldiers
Free oxygen radicals destroy also macromolecules within their own cells and this can damage the DNA molecules that contain the genes turning the strictly orderly coded sequences into nonsense. Therefore, there must be a very extensive and tight control mechanism of enzymes to ensure that oxygen radicals are kept in check and their numbers minimized. If very many radicals are formed, then the existence of the entire organism can be threatened.
Two American scientists Bruce Ames and Lois Gold once calculated that each rat cell can consume one million million (1012) oxygen molecules every day with about ten thousand million (1010) oxygen radicals being produced. These cause about 100,000 (105) points of DNA damage. Most of these, 99%, are repaired, but one thousand (103) errors are left unrepaired, i.e. in every cell there are about one thousand mutations created every day. In a rat’s lifetime this amounts to one million DNA errors or mutations. In man, the pace is slower but the lifetime longer, so the sum total is about the same – one million mutations per cell. The vast majority of these mutations are fortunately insignificant, because they happen at sites that are not crucial. But a small part of them may be in the genes important for the initiation of cancer and they may lead to the birth of a new cancer cell.
This explains why there is more and more damage accumulating in cells as they get older, and some of them may turn into cancer cells even without the input of any external “cancer chemical” or radiation. This is also the reason that the likelihood of cancer increases with age. Oxygen is an excellent example of the fact that there are no “good chemicals” and “bad chemicals”, even the good and essential chemicals have their adverse effects and they may even be life-threatening, if they are present at too high concentrations and in the wrong place at the wrong time. The abundance of “spontaneous” mutations challenges the hypothesis that a single molecule of a cancer-causing chemical could cause a cancer. Hypothetically yes, but what is the relevance of one molecule among millions of others? The important words are not “what if” but “how much”.
Under certain conditions, oxygen can become a very dangerous substance; in fact it confirms that in human metabolism the old adage holds true – fire is a good servant but a bad master.
One level up: Here a risk, there a risk, everywhere risks, risks!
Previous chapter: Is cancer more common today than it was in the past?
Next chapter: Why are animal experiments still needed?