Mélanine

The mélanines are among the Pigment S principal persons in charge of the coloring of the Tégument S in the animal kingdom. One speaks wrongly about " the mélanine" in the singular. The Color of the skin, the Hair X and the eyes at the man depend mainly on his type and its concentration. The plants, the birds for their Feather S, the Protozoaire S produce some too.

The biological mélanines are Macromolécule S produced for the majority by the Mélanocyte S, addition or condensation of Monomère S formed starting from the Tyrosine (Eumélanine) or of tyrosin and the Cystéine (Phéomélanine), with the assistance of the Enzyme Tyrosinase. A small proportion, as the Neuromélanine of the Substantia will nigra human brain, is not manufactured in the mélanocytes but probably comes from the metabolism of a simple amine like the Dopamine.

There exist also artificial mélanines: polycetylene S, polyaniline S, and polypyrrole S of black or brown color, and their Copolymer S; they have an industrial or biotechnological application.

The main role of the mélanine is pigmentary protection against radiations UV. In certain animals, the mélanophores, Chromatophore S carriers of mélanine, contribute to the camouflage or announce by their transformations the emotion, the stress or a change of the environment (temperature etc). In certain invertebrates, the disease-causing agents are encapsulated in mélanine, which would thus contribute on their premises to defense against the infections. It also seems that its groups carboxylate and hydroxyl-phenol can isolate from the medium certain detrimental elements when they are in excess, like heavy metals.

Via the Psychology, the mélanine can exert a great influence on the human society (choice of a partner, Discrimination).

Types

One knows two principal types of mélanine comprising each one two sub-groups, and a less known minor type:

the Eumélanine black or brown
the Phaéomélanine red or yellow
the Neuromélanine

Synthesis

The mechanism of formation of the mélanine is very complex and the precise mechanisms for all the varieties are not yet completely elucidated. The mélanine is produced starting from tyrosin thanks to the enzyme tyrosinase. Tyrosin is first of all converted into DOPED, then in DOPAquinone. In the absence of cystein, indol 5-6 quinone is formed, giving the eumélanine. In the presence of cystein, cystéinyl-DOPA is formed, giving phaeomélanine. These two pigments polymerize with a variable degree according to their nature.

common basic Stage:

Tyrosine (tyrosin oxydase) → DOPED (3,4-dihydroxyphénylalanine) (doped exydase) → dopaquinone

Two possible ways towards the eumélanine:

Dopaquinone → leucodopachrome → dopachrome → DHI (5,6-dihydroxyindole or DHI) → indol 5,6 quinone
Dopaquinone → leucodopachrome → dopachrome →DHICA (acid 5,6-dihydroxyindole-2-carboxylic)

In general the two ways exist simultaneously. If the mixture is rich in DHI, the mélanine is black, if it is rich in DHICA, it is brown.

Two possible ways towards the phéomélanine:

Dopaquinone + cystein → 5-S-cysteinyldopa → benzothiazine → phéomelanine
Dopaquinone + cystein → 2-S-cysteinyldopa → precursory of benzothiazine → phéomelanine

At the man, under the effect of UV, the Kératinocyte S secrete the hormone alpha-MSH by scission of the pro-opiomélanocortine (POMC), thanks to the action of a protein called p53. Alpha MSH sticks to the mélanocytes and stimulates the production of mélanine. P53 being also a protein produced in the event of stress, that could particularly explain the hyperpigmentation which appears sometimes, in the elderly, in the event of prolonged irritation of the skin.

It is on the level of the regulation of the activity of tyrosinase that intervene the products dépigmentants like the Hydroquinone and the Acide Kojique. Others as the Antioxydant S (Ascorbic acid e.g.) intervene on the level of formed quinones. Any genetic mutation making tyrosinase less active or goes away is at the origin of the Albinisme, disorder which is translated in its extreme form by the complete absence of mélanine.

It was proposed that the capacity of the mélanine to be behaved as an antioxydant would be all the more strong as its Molecular weight is high. Adverse conditions with polymerization (some pH, presence of poisons…) would release from the radicals and would make the mélanine carcinogenic or destroying, supporting the formation of the Mélanome S or the macular Dégénérescence.

Aspect under the microscope

Under the microscope, the mélanine must be distinguished from the pigments produced by the destruction of the Hématie S (Hémosidérine). It appears in the form of granules brown non-réfractiles of less than 800 Nm whereas the granules of blood pigments are larger, réfractiles and greens, yellows or brown-red a diluted solution of Permanganate of potassium makes disappear the mélanine.

At the man

One finds the mélanine in the Peau, the Cheveu X and hairs, the pigmentary epithelium of the iris, the Surrénale, the vascular scratch of the Internal ear, the brain (Corps striated and black substance). Except for brain, it is contained in intracellular Organelle S of round form where lengthened called Mélanosome S, produced by the mélanocytes.

It is the mélanine which makes us bronze, thus protecting ARN from the cells of the skin of the rays Ultraviolet S (especially UVB) of the sun. During an exposure prolonged to the sun, one bronzes because the body manufactures more mélanine, one is thus protected better from UV. The mélanine acts by absorbing them and by restoring in the form of heat received energy. The eumélanine is much more effective in this role than the phéomélanine which protects very little, and breaks up easily under the effect of UV, releasing from the carcinogenic free radicals; for this reason the red-headed people are more fragile than the brown ones. The fair , and a fortiori the Albino, are also very fragile vis-a-vis the sun. But even for the matt or dark skins, the protection offered by the mélanine is far from being total, and of too important degradations of DNA the canceration cells of the skin can involve.

To note that the creams Autobronzant have nothing to do with the mélanine and do not protect absolutely from the sun.

The mélanine is present in quantity proportional to the intensity of the color. Thus, a person with the black skin in factory much more than one person with the white skin, a brown iris contains some more than one blue iris, a brown hair more than one blond hair. The albinos can not of it produce whole. The quantity of mélanine produced by a person determines her Phototype.

In fact the genes control its production, of which respective proportions of eumélanine and phéomélanine. Under the effect of UV, the kératinocytes secrete the hormone alpha-MSH by scission of the pro-opiomélanocortine (POMC), thanks to the action of a protein called p53. Alpha MSH sticks to the mélanocytes and stimulates the production of mélanine. P53 being also a protein produced in the event of stress, that could particularly explain the hyperpigmentation which appears sometimes, in the elderly, in the event of prolonged irritation of the skin. The Mélatonine plays also a part in bronzing by increasing the concentration in mélanine mélanosomes.

In addition, the formation of alpha-MSH starting from producing POMC of the endorphins B supposed to contribute to a feeling of wellbeing, David E Fisher, director of research having highlighted the role of p53 in bronzing, proposes that the endorphin release during this process could encourage people to be exposed to the sun. The whole of the processes would support protection against cutaneous cancers. Indeed, if an exposure to the sun too long or too brutal is dangerous, a moderate and regular exposure can on the contrary constitute a protection, except at the people deprived of the capacity to bronze.

The appearance of grey hair (Canitie) comes from a stop from production from mélanine on the level from their bulb which one does not know yet very well the mechanism; one knows at least that it is hereditary (if the parents had the grey hair early, then the children will early have also grey hair) and dependant on the age (the older one is, the more one has grey hair).

The Freckles and old age contain widened mélanocytes which locally produce more mélanosomes that the neighbouring skin, from where dyes to them darker. The beauty spots are local concentrations of mélanocytes.

It is the mélanine present in the hairs which makes it possible to carry out final depilations with the laser.

References

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