Indole pigments
Melanins
These pigments produce buff, red-brown, brown, and black colours. Melanins occur widely in the feathers of birds; in hair, eyes, and skin of mammals, including humans; in skin or scales or both of many fishes, amphibians, and reptiles; in the ink of cephalopods (octopus, squid); and in various tissues of many invertebrates.
Melanins are polymers (compounds consisting of repeating units) of variable mass and complexity. They are synthesized from the amino acid tyrosine by progressive oxidation, a process catalyzed by the copper-containing enzyme tyrosinase. Extractable in very dilute alkali, melanins are also soluble when fresh and undried in very dilute acid solutions; they are bleached by hydrogen peroxide, which is sometimes applied to growing hair to create a blond effect, and by chlorine, chromate, and permanganate.
Pale-yellow, tawny, buff, reddish, brown, and black colours of hair and some feathers can arise from the presence of melanins in various phases of formation or subdivision in granules. The dark, light-absorbing sublayers of melanin that intensify reflected structural (Tyndall) blues or iridescent displays in feathers were mentioned above. Black melanins and brown melanoproteins occur in many invertebrate animals. Certain worms and many crustaceans and mollusks exhibit melanism in the skin.
The degree of natural melanization depends upon relative concentrations of copper and of the copper-containing enzyme tyrosinase. Dark hairs contain higher traces of copper than pale hairs do; should the intake of copper fall substantially below a fraction of a milligram per day, new fur emerges successively less dark. This trend is reversed by restoring sufficient copper to the diet.
All human skin except that of albinos contains greater or lesser amounts of melanin. In fair-skinned persons the epidermis, or outermost layer of the skin, contains little of the pigment; in the dark-skinned races epidermal deposits of melanin are heavy. On exposure to sunlight, human epidermis undergoes gradual tanning with increases in the melanin content, which helps to protect underlying tissues from injurious sun rays.
Indigoids
Like melanins, the indigo compounds are excretory metabolic breakdown products in certain animals. But, in contrast to the melanins, their distribution as conspicuous pigmentary compounds is very limited, and they are not dark but red, green, blue, or purple.
One of the most common members of this group is indigo, or indigotin, which occurs as a glucoside (i.e., chemically combined with glucose) in many plants of Asia, the East Indies, Africa, and South America. It has long been used as a blue dye.
Phenoxazones and sclerotins
Once confused with melanins, biochromes such as phenoxazones and sclerotins show a similar colour series (yellow, ruddy, brown, or black). Genetic research, notably with reference to eye pigments of the fruitfly, Drosophila melanogaster, has resulted in the description of a class of so-called ommochromes, which are phenoxazones. The ommochromes not only are conspicuous in the eyes of insects and crustaceans but have also been detected in the eggs of the echiurid worm Urechis caupo and in the changeable chromatophores in the skin of cephalopods. In addition to being responsible for the brown, vermilion, cinnabar, and other colours of insect eyes, ommochromes are also sometimes present in the molting fluid and integument. They are distinguished from the melanins by solubility in formic acid and in dilute mineral acids, by manifestation of violet colours in concentrated sulfuric acid, and by showing reversible colour changes with oxidizing and reducing agents. The ommochromes, which are derived from breakdown of the amino acid tryptophan, include ommatins and ommins. The ommatins, although complex in chemical structure, are relatively small molecules. The ommins are large molecules, in which the chromogenic moiety is seemingly condensed with longer chains, such as peptides (amino acids linked together).
Sclerotins arise as a result of an enzyme-catalyzed tanning of protein. Certain roaches secrete a phenolase enzyme, the glucoside of a dihydroxyphenol, and a glycosidase. Mixing of these substances results in the release of the phenolic compound from glucose and its combination, via a reaction catalyzed by the phenolase, with protein; the products are pink, ruddy, and ultimately dark-brown polymers that are incorporated into the insect’s body cuticle and egg cases. Similar reactions take place in the carapace (the shell covering the body) of certain crustaceans.
Purines and pterins
Although the purine compounds cannot be classed as true pigments—they characteristically occur as white crystals—they often contribute to the general colour patterns in lower vertebrates and invertebrates. That purines are excretory materials is illustrated by the uric acid (or urates) and guanine found in the excrement of birds and of uric acid found in that of reptiles. Uric acid has also been detected in the mucus excreted by sea anemones, and urates are present in small amounts in the urine of higher apes and humans.
The white, silvery, or iridescent chromatophores, both stationary iridocytes and changeable leucophores, of some fishes, amphibians, lizards, and cephalopods contain microcrystalline aggregates of the purine guanine; a layer of white skin on the underside of many fishes, called the stratum arginatum, is particularly rich in guanine.
Closely related to the purines and formerly classed among them are the pterins, so named from their notable appearance in and first chemical isolation from the wings of certain butterflies. Both purines and pterins contain a six-atom pyrimidine ring; in purines this ring is chemically condensed with an imidazole ring; pterins contain the pyrazine ring. Pterins occur as white, yellow, orange, or red granules in association with insect wing material.
Flavins (lyochromes)
Flavins constitute a class of pale-yellow, greenly fluorescent, water-soluble biochromes widely distributed in small quantities in plant and animal tissues. The most prevalent member of the class is riboflavin (vitamin B2).
Flavins are synthesized by bacteria, yeasts, and green plants; riboflavin is not manufactured by animals, which therefore are dependent upon plant sources. Riboflavin is a component of an enzyme capable of combining with molecular oxygen; the product, which is yellow, releases the oxygen in the cell with simultaneous loss of colour.
Miscellaneous pigments
The chemical constitution of many pigments remains imperfectly known. Only a few of the more conspicuous examples are mentioned below.
Hemocyanins
Copper-containing proteins called hemocyanins occur notably in the blood of larger crustaceans and of gastropod and cephalopod mollusks. Hemocyanins are colourless in the reduced, or deoxygenated, state and blue when exposed to air or to oxygen dissolved in the blood. Hemocyanins serve as respiratory pigments in many animals, although it has not been established that they perform this function wherever they occur.
Hemerythins
Iron-containing, proteinaceous pigments, hemerythrins are present in the blood of certain bottom-dwelling marine worms (notably burrowing sipunculids) and of the brachiopod Lingula; the pigments serve as oxygen-carriers.
Hemovanadin
Pale-green pigment, hemovanadin, is found within the blood cells (vanadocytes) of sea squirts (Tunicata) belonging to the families Ascidiidae and Perophoridae. The biochemical function of hemovanadin, a strong reducing agent, is unknown.
Actiniochrome
A relatively rare pigment, actiniochrome occurs in red or violet tentacle tips and in the stomodeum (oral region) of various sea anemones. The pigment plays no recognized physiological role.
Adenochrome
Adenochrome is a nonproteinaceous pigment that occurs as garnet-red inclusions at high concentrations in the glandular, branchial heart tissues of Octopus bimaculatus. The compound contains small amounts of ferric iron and some nitrogen and gives a positive reaction for pyrroles. It is believed to be an excretory product.
Denis Llewellyn Fox Edward Howland Burtt