- In a finding that may offer clues about Parkinson's disease, a
team led by Duke University researchers used a sophisticated laser
system to gain evidence that a dark brown pigment that accumulates
in people's brains consists of layers of two other pigments commonly
found in hair.
Other scientists previously had determined via chemical analysis
that the dark pigment, called neuromelanin, is composed of the two
pigments: eumelanin, found in black-haired people, and pheomelanin,
found in redheads. But how those pigments are arranged structurally
remained unknown -- and this structuring may prove to be of critical
importance, according to the researchers.

In addition, in 2005 a Duke team that included some of the same
scientists involved in the current study reported using the laser
system to establish that pheomelanin is chemically disposed to
activate oxygen while eumelanin is not. Oxygen activation is
suspected to play a role in the neurogenic cascade of events behind
Parkinson's disease.

In the new report, investigators from Duke, North Carolina State
University and the Institute of Biomedical Technologies in Segrate,
Italy, outlined evidence that neuromelanins isolated from human
brains have cores of oxygen-activating pheomelanin covered by a
protective surface of eumelanin.

"This is the first piece of morphological data about how these
pigments are constructed," said study leader John Simon, the George
B. Geller Professor of chemistry at Duke.

The team published the findings online during the week of Sept. 25
in the journal Proceedings of the National Academy of Sciences. The
research was funded by the U.S. Air Force Office of Scientific
Research, through grants to the Duke University Free Electron Laser
Laboratory, and by the Italian Fund for Basic Science.

The findings "should stimulate renewed interest in the roles of
neuromelanin in the pathogenesis of Parkinson's disease, the second
most prevalent neurodegenerative disorder," Shosuke Ito, a chemist
at Japan's Fujita Health University School of Health Sciences, wrote
in a companion commentary published in the journal.

According to the team's report, whose first author is Simon's
graduate student, William Bush, neuromelanin granules begin
appearing in human brains between ages 3 and 5, and their
concentrations increase steadily thereafter.

However, neuromelanin levels drop precipitously in the brains of
Parkinson's patients, who also experience a death of brain cells
that are darkly pigmented and an increase in brain tissue
concentrations of the metal iron.

Brain cells that produce dopamine, a key neurotransmitter disrupted
in Parkinson's disease, experience high levels of oxidation as that
dopamine is made, the researchers noted.

Scientists have hypothesized that brain cells synthesize
neuromelanin to serve as a defense mechanism against high oxidation
stress, the team's report said.

Neuromelanin's layered granular structure could help protect brain
cells from damage in several ways, Ito wrote in his commentary.

Having eumelanin at their surfaces would protect the granules with a
pigment known to efficiently bind iron and other molecules that
could otherwise play a role in oxidative damage. If the underlying
core of pheomelanin were instead positioned at the surface, "the
neuro-protective role of neuromelanin would not be expected," Ito
added.

However, eumelanin is limited in how much iron it can take up, and
other scientists have proposed that iron over-saturation at the
granules' surfaces could contribute to the high levels of the metal
in the brains of Parkinson's victims.

"Increased oxidative stress under such conditions could result in
degradation of the eumelanic surface of neuromelanin," Ito wrote.
That could expose a pheomelanin core "that is not only ineffective
in iron-binding, but also behaves as a pro-oxidant itself," he
added.

"Once these neuromelanin granules start getting chewed into, an
environment is created that is much more pro-oxidation," Simon
said. "As pigment starts to get eroded, you can imagine how
oxidative stress could be increased in multiple ways."

In the study, which Ito called "sophisticated," the researchers used
a special laser device that makes light with electrons that have
been freed from their usual bondage to atoms. Housed in a large bay
in the Duke University Free Electron Laser Laboratory, the device
can be "tuned" step-by-step to produce light at a variety of
different wavelengths, with each wavelength probing different energy
regions in target molecules.

The team also used a device called a photoelectron emission
microscope to resolve individual neuromelanin granules and
distinguish between the two pigment types.

Using these devices in combination, the researchers could pinpoint
the "oxidation potentials" of molecules coating the surfaces of
neuromelanin granules. Oxidation potentials measure how likely given
chemicals are to activate oxygen by giving up electrons. Activated
oxygen can produce compounds called radicals that can stress cells.

The team found that oxidation potentials of molecules at the
surfaces approximated those found in black hair pigments in the 2005
study. "That meant it was eumelanin, which is pretty antioxidant,"
Simon said.

The laser beams could not penetrate beneath the granules' surfaces
to record oxygen potentials nearer their cores. But previous
chemical analyses by other researchers had established that
neuromelanin is a mixture of both red and black hair pigments. So,
the new finding suggests "a structural motif, with pheomelanin at
the core and eumelanin at the surface," the team reported.

"Something special is happening, where the red pigment is getting
encased in the black," Simon said. "So the red, being fairly pro-
oxidant, is being encased in this antioxidant pigment."

Simon's group could only deduce the probable structure of
neuromelanin, rather than measure it directly, because scientists
have so far been unable to synthesize the pigment from chemical
building blocks in a form that duplicates the natural version, he
said.

"No one knew how to test or probe these things," Simon said. "And I
can't overestimate how difficult it was to get materials to test."
His group worked with small amounts of autopsied brain tissues
provided by a research group led by Luigi Zecca at the Italian
Institute of Biomedical Technologies.

Other researchers in the study were Glenn Edwards, director of the
Duke University Free Electron Laser Laboratory; Robert Nemanich and
Jacob Garguilo of N.C. State; and Fabio Zucca and Alberto Albertini
of the Italian Institute of Biomedical Technologies.

Duke University, 27 Sep 2006 (Medical News)