Chemistry of Animal Pigments

CHEMISTRY OF ANIMAL PIGMENTS Chlorophyll.—To begin with, something must be said about chlorophyll (q.v.), the green colouring matter of plants. Not only is it the most important pigment in the world, being vitally connected with photosynthesis, but it often occurs in a few animals, it is the source of some other animal pigments, and it presents an interesting analogy with haemoglobin, the red pig ment of the blood. What is called chlorophyll seems to be a complex of four pigments, two chlorophyll-greens and two chloro phyll-yellows, the latter belonging to a different group. In the two chorophyll-greens, which differ from one another only in the proportion of oxygen they contain, the molecule can be split into two parts by the action of an alkali. One of these is a complex colourless alcohol called phytol. The other has for its foundation ring with one atom of nitrogen. In the chlorophyll molecule there are four of these rings joined together, and linked to these in some way is an atom of magnesium. The molecule of haemoglobin, which is even larger than that of chlorophyll, may also be split into two parts:—(i) a colourless portion, not an alcohol, but a protein called globin; and (2) a coloured portion, haem. This haem, like chlorophyll, consists of four "pyrrol rings" linked to gether with an atom of metal, which in this case is iron. Very different chemically and physiologically from the two true chloro phylls, yet similarly related to one another, are the two chloro phyll-yellow pigments, carotin and xanthophyll, which have also their analogues among animal pigments, belonging as they do to the series of lipochromes or fatty pigments, such as the reddish colour-substance of shrimps and prawns.

Green Animals.

There are five ways in which animals may have a green colour: (a) The greenish freshwater sponges (spon gillids), the common green freshwater polyp (Hydra viridis), some sea-anemones, many Alcyonarian corals, and the littoral worm, Convoluta roscoffensis, are illustrations of a green appear ance due to the presence of numerous symbiotic algae, often of the genus Zoochlorella. The green colour is due to the included partner-plants which have the usual chlorophyll. (b) There is an appreciable greenish tinge on the rough shaggy hair of the tree-sloths (Bradypodidae) of the South American forests. But the green colour is quite adventitious, being due to the presence of numerous unicellular Algae on the rough hairs. (c) While most greenish Protozoa, such as Stentor polymorphus, many marine Ciliates, and occasional green Amoebae, owe their colour to symbiotic unicellular Algae, there are a few which are believed to have chlorophyll of their own. This is probably the case in the common Euglena viridis and in some other green Flagellata, and in the green bell-animalcule, Vorticella viridis. It is possible that the primitive Protists, from which Protophyta and Protozoa diverged, may have had a photosynthetic pigment allied to chloro phyll, and that some Protozoa have inherited this. (d) A few notable green animals have a green pigment different from chloro phyll, a good example being the Gephyrean worm Bonellia, whose pigment is called bonellein. An allied green pigment occurs in viridis, one of the Palolo worms. (e) Finally, the green colour may be a structural effect, without there being any green pigment, as in some green beetles and green birds. It is in structive to find that the same colour-result may be brought about in these different ways.

Blood Pigments.

These form a group the most important member of which is the red pigment haemoglobin, characteristic of Vertebrates, but also occurring in some Invertebrates. Earth worms and many marine Annelida have a pigment practically the same as the haemoglobin of Vertebrates, but it is important to notice that haemoglobin varies a little from one type to another, the differences having to do with the attached protein (globin), and not with the essential nucleus or haem. This is a good in stance of specificity, for even in nearly related species, such as horse and ass, or dog and fox, there is a difference in the details of the haemoglobin crystals. This hints at the chemical basis or chemical individuality of species.

An interesting instance of the occurrence of haemoglobin among Invertebrates is to be found in "blood-worms," the red aquatic larvae of some species of harlequin-fly (Chironomus), which live in foul or in very deep water where there is less oxygen than usual. One species lives as a larvae at a depth of i,000ft. in Lake Superior and only comes to the surface occasionally. The haemo globin helps the blood-worm to capture, and perhaps store, the sparse oxygen.

In many Invertebrates, especially in crustaceans and molluscs, the blood pigment is haemocyanin, allied to haemoglobin, but with copper instead of iron. It has often a bluish colour, but may be so pale that the blood appears colourless, though it is far from being pigmentless. As in the case of haemoglobin, there seem to be many varieties of haemocyanin. It does not seem to occur in insects, where the gas-carrying function of the blood is less important, owing to the system of air-tubes or tracheae which carry air to every hole and corner of the body. Haemo cyanin appears to have between a quarter and a third of the oxygen-carrying power of haemoglobin.

This is an appropriate place for a reference to an interesting series of pigments, called cytochromes, discovered by D. Keilin in 1925. Cytochrome contains the "haem" nucleus, and is there fore allied to haemoglobin and haemocyanin. It is perhaps almost universal in its distribution, for it occurs in both Vertebrates and Invertebrates, as well as in plants. The probable function of cytochromes is to control the distribution of oxygen within the cell.

Melanins.—A third group of pigments is the melanin series, occurring in the dark skin of the negro, the black feathers of the crow, the black choroid of the eye, and the ink-sac of cuttle fishes. Melanins are typically dark pigments, always occurring in minute granules, almost defying solution, and very hard to purify since they will not crystallize. Thus relatively little is known of their chemical constitution. According to the general view, however, they are derivable from an important amino-acid called tyrosin, or some similar substance. When tyrosin is treated in a test-tube with a common enzyme, tyrosinase, and then exposed to air, it forms a pigment, first reddish.and then black, which seems identical with a natural melanin. But as amino acids readily arise from the breaking-down of proteins, we reach a provisional interpretation of melanin as the outcome of the everyday disintegrative or katabolic changes in the proteins which form the universal building-materials of protoplasm.

Chromolipoids or Lipochromes.

A fourth group of pig ments, often called "fatty pigments," is widely represented among both plants and animals. They show no great resemblance to fats beyond their solubility in ether. The two yellowish pigments, carotin and xanthophyll, which accompany chlorophyll-green in ordinary plants, are characteristic chromolipoids, and they occur in animals as well. Carotin gives the yellowish colour to butter and xanthophyll occurs in the yolk of the bird's egg. Very common and allied to carotin is the reddish zoonerythrin ("animal rest") familiar in the Norway lobster (Nephrops norvegicus) and the red wattle above the eye of the grouse. The blue colour of the common lobster is due to a compound of zoonerythrin with a protein. When the protein is destroyed by heating, the free pig ment produces the red colour, which is well seen in the living rock lobster (Palinurus).

Other Pigments.—There are many other animal pigments which cannot be included in any of these four main groups, such as the uric acid pigments of the wings of some butterflies, the purple of Murex and related gasteropods, the carmine of the female cochineal insect, and so on. The last mentioned is a gluco side, yielding sugar when treated with dilute acid, and may per haps be interpreted as a reserve product, at the opposite pole from the uric acid pigments which are of the nature of waste. Of great importance is such a step as D. L. Thomson's tracking of the "flavone" or "flavonol" of the wings of the marble-white butterfly (Melanargia galatea) to a similar pigment obtained by the caterpillar from the grass on which it feeds.