MELANINA (grech, melas, melanos black, dark) — the amorphous pigments of dark brown and black color which are contained normal in hair, skin, feathers and a retina of an eye of vertebrata at insects and some sea invertebrates, and also in plants. Strengthening of education and M.'s adjournment in skin happens under the influence of radiation solar and UV rays that causes emergence of suntan, formation of freckles etc. The increased M.'s maintenance is observed also in some types of warts and in some malignant tumors of skin (see. Melanoma ).
At excess accumulation of M. in an organism develops so-called. melanosis (see). M have properties of stable radicals and participate in reactions of electron transfer. Connect their protective functions concerning UF-radiation with it and ionizing radiation. Therefore extra care concerning solar radiation is necessary for persons with a weak xanthopathy.
At animal M. concentrate in special cells — melanophores.
Carbon, nitrogen and hydrogen are a part of a molecule M.; some M. contain also sulfur. M are water-insoluble, mineral to-takh and organic solvents, but some of them are easily dissolved in alkalis and drop out in a deposit at acidulation of solutions. It belongs, e.g., to melanin of hair. Use such property M. for their preparative receiving.
M are formed in cytoplasm of melanocytes by polymerization of oxidates tyrosine (see), or phenylalanine (see), formed under the influence of enzyme of a tyrosinase (KF 1 monophenol-monooxygenase. 14:18. 1), localized in mitochondrions of melanocytes. The majority of intermediate products of education M. as well as M., easily connect to amino acids and proteins. In fabrics M. usually are in a complex with proteins.
The mechanism of education of M. and their structure finally are not found out. Assume that the first two modes of formation of M. are enzymatic and are catalyzed about-difenoloksidazoy, and the subsequent are carried out in the non-enzymatic way in the presence of oxygen. Since about-difenoloksidaza its activity, and thereby is cupriferous enzyme, and M.'s education, is inhibited by various kompleksoobrazovatel connecting copper.
Process of education of M. at the person and animals is connected with activity of closed glands. At a number of hormonal frustration (an addisonov a disease, a hyper thyroidism, a hypopituitarism, etc.), and also at pregnancy disturbances of a xanthopathy, a hair etc. are observed. It is established that M.'s education is controlled by hormones of a hypophysis (alpha and beta melanotsitostimuliruyushchimi hormones), and also hormones of a thyroid gland, steroid and sex hormones. The mechanism of endocrine control of synthesis of M. is not found out yet: apparently, the main role in it belongs to melanotsitostimuliruyushchy hormones (see) which activity can be stimulated with other hormones (e.g., progesterone) or to be suppressed with them (e.g., adrenaline and noradrenaline).
M.'s education is stimulated also nek-ry physical (UF-and rentgenova beams) and chemical (compounds of arsenic, silver, bismuth, etc.) agents. The mechanism of action of ionizing radiation and the majority of chemical connections on M.'s synthesis comes down, apparently, to activation about-difenoloksidaznoy reaction, thanks to oxidation or blocking of SH-group of enzyme.
Lack of a normal xanthopathy, a retina of eyes etc. (see. Albinism ) at some people, apparently, it is caused by inborn insufficiency about-difenoloksidazy.
Definition of melanin in fabrics.
In fabrics M. it is necessary to differentiate from lipidogenny and gemoglobinogenny pigments. It is necessary to consider that M. are water-insoluble, mineral to-takh and organic solvents and collapse at effect of the concentrated alkalis. M «are easily bleached» and pass into a leykoforma at action of strong oxidizers (best of all for this purpose to use 10% hydrogen peroxide during not less than 24 hours).
M have reducing power. The basic gistokhy, the test for M.'s identification is argentaffin reaction, i.e. reaction of recovery of caustic silver (its ammoniac solution) to metal silver. For the first time this reaction was used by E. Bizzozero in 1908. Among the methods based on argentaffin reaction Masson's method — Fontana is most widespread. Reaction is carried out on fresh or fixed (in formalin, Buen's liquid, or liquid Carnoy) fabrics. The cuts exempted from paraffin carefully wash out a distilled water within 2 — 3 hour. For preparation of solution of Fontana for 5% to solution of caustic silver flow during the stirring ammonia on drops until the deposit which is formed at the same time is dissolved again. Then carefully add a drop behind a drop 5% solution of caustic silver before emergence of not disappearing dregs. Place well washed cuts in Fontana's solution and store 18 — 24 hours in the dark (in the closed vessel), then carefully wash a distilled water and fix in 5% solution of sodium thiosulphate of 3 — 5 min. Kernels dokrashivat karbol-safranin within 10 min. Then differentiate cuts in 70% ethanol, dehydrate, clarify in a xylol and conclude in Canada balsam.
M recover ferricyanide in ferrocyanide (Shmorl's reaction), form complexes with Fe2+ which can be revealed by means of ferricyanide of potassium (reaction with turnbullevy blue), and also give reaction of azocoupling in the recovered state. For identification of predecessors of M. it is used DOFA-reaction (see).
At radiation by UF-light M. fluoresce yellow light. In the polarized light of grain M. find simple refraction, in a dark field they brightly gleam whitish light.
Submicroscopic identification of M. is carried out on the basis morfol, criteria. M.'s granules have the rounded or rod form, contain the material having high electron density and gomogenno distributed in the form of grains or crystalloids. For M.'s identification in in ultrathin sections it is used also argentaffin and DOFA-reactions.
M.'s identification in tissues of an eye or in skin in practice is not accompanied by difficulties. Such difficulties arise at identification of brown pigments, napr, in a wall of a gut or in secondary tumors of an unknown origin. In such fabrics those few positive reactions which give M. can be a source of mistakes since these reactions are characteristic also of others pigments (see), napr, lipofuscin (see).
Bibliography: Braunstein A. E. Biochemistry of amino-acid exchange, page 284, etc., M., 1949; Lenindzher A. Biochemistry, the lane with English, M., 1976; Fitzpatrick T. Century of a. L er ner A. V. of Biochemical basis of human melanin pigmentation, Arch. Derm. Syph. (Chic.), v. 69, p. 133, 1954, bibliogr.; Lerner A. B. Melanin pigmentation, Amer. J. Med., v. 19, p. 902, 1955, bibliogr.; Rothman S. Physiolo gy and biochemistry of the skin, Chicago, 1955.
S. S. Debov, I. S. Derizhanova (stalemate. An.).