Where's North? Ask Superoxide!
report, offers a very intriguing answer. In sensing the Earth's magnetic field, cryoptochrome relies on so-called redox reactions which exchange electrons between molecules. Such reactions are crucial for life, but can also be damaging; antioxidants are used by organisms, but also in pharmacology and as dietary supplements to keep the reactions in check. Apparently, cryptochrome recruits as a reaction partner in its magnetosensitive behavior a special form of molecular oxygen, namely its negatively charged brethren superoxide. For this purpose cryptochrome requires superoxide in low doses, which is good since superoxide, though arising in organisms and used in signaling elsewhere in the body, is actually toxic. The human body has an extremely efficient enzyme, superoxid dismutase, that keeps superoxid at a very low concentration level, apparently too low for human cryptochromes to capture it and tell North from South. Humans, somewhere in evolution, might have lost the magnetic sense, but gained longevity. More on our cryptochrome web site.
Magnetoreception through cryptochrome may involve superoxide. Ilia A. Solov'yov and Klaus Schulten. Biophysical Journal, 96:4804-4813, 2009.