Hair
color - biochemistry
In humans, all the different hair colors are due to just two types
of pigment (melanin) called eumelanins and pheomelanins (European
spelling, phaeomelanin). Eumelanins are the dark brown and black
pigments while pheomelanins are the red and blonde pigments. The
different colors of hair in different people are due to a combination
of these two different basic biochemical structures. By mixing the
two types together in different concentrations the many different
shades of hair color are made.
Eumelanins are very strong, stable proteins made from tyrosine.
The large eumelanin biochemical structure is formed by processing
the amino acid tyrosine into dopa and dopamine and connecting several
of these molecules together to form eumelanin. The key enzyme in
this process is tyrosinase. The more tyrosinase activity the more
eumelanin is formed. This is one method by which different people
have different shades of brown to black hair color. More tyrosinase
activity results in more pigment production and so a darker hair
color. As we get older, tyrosinase activity increases. It is most
active in middle age and thereafter tyrosinase activity decreases.
There are also other biochemical mechanisms by which the shade of
hair color is regulated. Several factors interact with tyrosinase
to help regulate eumelanin production. In addition, another key
limiting factor in hair color is the availability of the raw tyrosine
ingredient. A lack of tyrosine availability means the tyrosinase
enzyme make eumelanin at full capacity.
Pheomelanins are also made from the same tyrosine as eumelanins
and the process is much the same with tyrosinase playing a key role.
Pheomelanins are produced when an intermediate product in the eumelanin
production pathway interacts with the amino acid cysteine. This
results in the formation of a pheomelanin molecule which contains
sulfur from the cysteine. These molecules are yellow to orange in
color. So this is another way by which different shades of hair
color can be produced. The more interaction there is between dopaquinone
and cysteine the more yellow and orange pigments are produced.
Thus those people with darker hair have relatively more eumelanin
production. People with true red hair produce more pheomelanin.
The pathway to eumelanin formation is largely inhibited. Because
people with red hair are less able to make the dark eumelanin pigment
their skin is generally quite pale and burns easily with sun exposure.
A study that analyzed the amount of eumelanin and pheomelanin in
human hair suggested that; black hair contains approximately 99%
eumelanin and 1% pheomelanin, brown and blond hair contain 95% eumelanin
and 5% pheomelanin; and red hair contains 67% eumelanin and 33%
pheomelanin (Borges 2001). Although people with dark hair may still
produce the yellow - orange pheomelanin, it is largely masked by
the dark eumelanin pigment and we cannot see much of it. However,
the red - yellow pheomelanin is believed to cause the warm, golden,
or auburn tones found in some types of brown hair.
Hair
color - biochemistry references
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