Melanins

MELANINS, a series of amorphous, nitrogen-containing black and dark-brown pigments found in the skin, hair, chorioidea, retina, wool, feathers, and other places in both vertebrates and invertebrates. Melanins are contained in melanotic tumors, as well as in the blood (melanemia) and urine (melanuria) of individuals suffering from these tumors; furthermore, it is frequently not melanin itself that is excreted in the urine, but a colorless melanogen, which turns into melanin in the air or under the influence of oxidizing agents (FeCl₃, concentrated HNO₃). The fluid secreted by the special glands of cuttlefish contains melanin (Chinese ink). Melanins are insoluble in water, alcohol, ether, chloroform, and other indifferent solvents, as well as in dilute acids. Concentrated sulfuric acid dissolves them upon heating. Alkalis easily dissolve some melanins, such as hair pigment, while other melanins are not dissolved even upon boiling with concentrated alkalis and pass into a soluble form only after fusion with KOH. The production and purification of melanins are so imperfect that it is difficult to vouch for the purity and individuality of the preparation in each individual case, which is why elemental analyses also yield fluctuating values: C - 48-60%, H - 3-8%, N - 8-14%. Some melanins contain sulfur, e.g., hair pigment (2-10%). There are indications of the presence of iron in some melanins, although this is a controversial question. The biological significance of melanin is not fully elucidated. Its deposition in the skin and the eye has the significance of a protective light filter. The substances producing melanin, or the so-called chromogens, are considered to be cyclic compounds of the protein molecule (especially tyrosine, tryptophan), the basis for which is their relation to tyrosine oxidases, which under the action of tyrosinase first assumes a reddish, then a violet color, and finally yields a substance colored black (see also Dioxyphenylalanine). The process of both physiological and pathological formation of melanin apparently proceeds in two phases: 1) the cleavage of the cyclic complex from the protein molecule, possibly under the influence of an autolytic enzyme, and 2) the transition of this complex into melanin under the influence of an oxidizing enzyme. Skin coloration in Addison's disease is connected with the formation of melanin. Melanoidin substances, formed during the boiling of proteins with mineral acids as a result of the interaction of carbohydrate and amino acid groups of the protein molecule, are close in properties to melanins.

O. Demyanovsky.