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Every color, every flash, every sunray exacts a toll 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.
Thankfully, the pigment responsible for darkening our hair, skin, and eyes moonlights as a clean-up crew, mopping up one such dangerous compound before it accumulates into damaging clumps.
An investigation by researchers from the University of Tübingen in Germany and Yale University has revealed the removal process is somewhat unusual as far as biochemistry goes, relying upon a strange quirk of quantum-like behavior.
Lining the back wall of our eyeball's inner surface is a shag-pile rug of light-reactive cells called the retina. Every fiber in this carpet is packed with pancake-like stacks of discs containing a crucial substance that catches photons of light, starting a chain of reactions that results in a nervous impulse the brain interprets as sight.
The very first step in this conversion process is a surprisingly dangerous one. The substance, called retinal, contorts into a shape that interferes with the cell's functions, effectively becoming a toxin.
Evolution has prepared us for this inconvenience, providing enzymes that flip the twisted form of retinal back into a safe and practical shape. What's more, the eye constantly recycles the stacks of discs, dismantling from one end and shuffling fresh light-sensitive packages into place from the other.
As efficient as this process is, it's far from perfect. In people with a rare condition called Stargardt disease, a single deficient enzyme causes a build-up of toxic products that lead to a loss of clear vision in the focal area of the retina.
Even in individuals with a functional set of enzymes carrying out work as efficiently as possible, a gap in the breakdown process risks another potentially dangerous compound called lipofuscin building up and accumulating into dangerous clumps.
Again, evolution has an answer, apparently in the form of the dark pigment melanin, which has been seen combining with lipofuscin granules in the retinas of older individuals.
"It's beginning to look like melanin is nature's solution to a variety of biology's challenges," says Yale therapeutic radiologist Douglas E. Brash.
Melanin's effect can wane as we age. Over time, these aggregates can cause the tissue to deteriorate, this time leading to a far more common form of sight impairment, age related macular degeneration (AMD).
While previous studies by other members of the research group support the pigment's role in clearing lipofuscin, the mechanism behind the breakdown has remained a mystery.
A clue could be found in research revealing lipofuscin breaks apart following the introduction of reagents that produce highly reactive forms of oxygen called radicals.
On their own, melanin's electrons aren't in a high enough energy state to perform such a task, being blocked by laws of quantum physics that keep them relatively grounded.
But there is a rather curious loophole. Called chemexcitation, it involves the quantum fine-print of additional materials combining in a way that boosts electrons beyond levels that would typically be prevented, allowing melanin to get a little excited and produce oxygen radicals where needed.
"These quantum chemistry reactions excite a melanin electron to a high energy state and flip its spin, allowing unusual chemistry afterward," says Brash.
The process itself isn't unknown in biology, though usually reserved as a way to kick electrons up high enough to generate light once they come screaming back down. Bioluminescence aside, its role in other pathways – including those involving melanin – is only just now being understood.
Combining high-resolution electron microscopy, genetics, and pharmacology, Brash and his colleagues traced the origins of the melanin and lipofuscin granules and demonstrated melanin's place in the pathway of removing dangerous compounds – but they also showed melanin used its quantum-boosted state to degrade lipofuscin.
Ideally, the knowledge might be applied to a search for pharmaceuticals that could serve as an alternative to melanin in aging individuals, breaking down lipofuscin before it can cause havoc in retinal tissues.
"For 30 years I was convinced that melanosomes – the organelles in cells that create melanin – degrade the lipofuscin, but couldn't identify a mechanism," says the study's senior author Ulrich Schraermeyer, an experimental ophthalmologist at the University of Tübingen.
"Chemiexcitation is the missing link, and it should let us bypass the problem that AMD begins when the eye's melanin declines with age. A drug that is chemiexcited directly may be a breakthrough for our patients."
This research was published in PNAS.
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