Every color, every flash, every ray of sunlight wreaks havoc on the light-sensitive tissues at the back of our eyes, producing toxic materials that risk damaging the very cells that allow us to see.
Luckily, the pigment responsible for darkening our hair, skin, and eyes is lighting up the moon like a cleanup crew, mopping up any of these dangerous compounds before they build up into harmful clumps.
An investigation by researchers from the University of Tübingen in Germany and Yale University found that the process of elimination is somewhat unusual as far as biochemistry is concerned, relying on a strange behavioral quirk of quantum type.
The back wall of the inner surface of our eyeball is lined with a shaggy carpet of light-reactive cells called retina. Each fiber of this mat is filled with stacks of pancake-like discs containing a crucial substance that captures photons of light, setting off a chain of reactions that results in a nerve impulse that the brain interprets as sight.
The very first step in this conversion process is surprisingly dangerous. The substance, called retinal, contorts into a shape that interferes with cell functionseffectively becoming a toxin.
Evolution has prepared us for this inconvenience, by providing enzymes that flip the twisted shape of the retina into a safe and convenient form. In addition, the eye constantly recycles stacks of discs, dismantling them on one side and replacing new light-sensitive packaging on the other.
As efficient as this process is, it is far from perfect. In people with a rare condition called Stargardt diseasea single deficient enzyme causes a buildup of toxic products that lead to loss of clear vision in the focal zone of the retina.
Even in individuals with a working set of enzymes doing as efficient a job as possible, a gap in the degradation process risks creating another potentially dangerous compound called lipofuscin accumulate and form dangerous clumps.
Again, evolution has an answer, apparently in the form of black pigment. melaninwhich has been seen to combine with lipofuscin granules in the retinas of older individuals.
“It’s starting to look like melanin is nature’s solution to a variety of challenges in biology,” said Yale therapeutic radiologist Douglas E. Brash.
The effect of melanin may decrease with age. Over time, these aggregates can deteriorate tissues, this time in mind to a much more common form of visual impairment, Macular degeneration (AMD).
While previous studies by other members of the research group support the role of the pigment in the elimination of lipofuscin, the mechanism behind the degradation has remained a mystery.
A clue could be found in search revealing that lipofuscin breaks down upon the introduction of reagents that produce highly reactive forms of oxygen called radicals.
On their own, melanin’s electrons are not in a high enough energy state to perform such a task, being blocked by the laws of quantum physics that keep them relatively grounded.
But there is a rather curious loophole. Called chemexcitationit involves the quantum imprint of additional materials combining in such a way as to stimulate electrons beyond levels that would typically be inhibited, allowing melanin to get a little excited and produce oxygen radicals if needed.
“These quantum chemistry reactions excite a melanin electron to a high-energy state and flip its spin, allowing for unusual chemistry afterwards,” said Impetuous.
The process itself is not unknown in biology, though generally reserved for a means of making electrons soar high enough to generate light once they screech back down. Aside from bioluminescence, its role in other pathways – including those involving melanin – is only now understood.
By combining high-resolution electron microscopy, genetics and pharmacology, Brash and his colleagues traced the origins of melanin and lipofuscin granules and demonstrated melanin’s place in the elimination pathway for dangerous compounds – but they also showed that melanin uses its quantum boosted state to degrade lipofuscin.
Ideally, the knowledge could be applied to finding pharmaceuticals that could serve as an alternative to melanin in aging people, breaking down lipofuscin before it wreaks havoc in retinal tissue.
“For 30 years I was convinced that melanosomes – the organelles of cells that create melanin – degrade lipofuscin, but I could not identify a mechanism”, said the study’s lead author, Ulrich Schraermeyer, an experimental ophthalmologist at the University of Tübingen.
“Chemiexcitation is the missing link, and it should allow us to circumvent the problem that AMD starts when melanin in the eye decreases with age. A drug that is directly chemexcited can be a breakthrough for our patients.”
This research was published in PNAS.