PTERINE: PIGMENTE UND WIRKSTOFFE IM TIERREICH
Authors: ZIEGLER‐GÜNDER VON IRMGARD 
Published: January 2008

Xanthopterin, leucopterin and erythropterin have long been known as crystalline deposits in the wings of butterflies and in the abdomen of other insects. The chemical composition of these pterins has been established, but as yet little is known of the detailed structure of the numerous pterins showing blue, violet or yellow fluorescence in ultraviolet light which occur in arthropods, either together with the crystalline pterins or alone. These pterins are found especially in eggs, in eyes and in luminous organs, where they accompany the closely related substance luciferin.

Similar non‐crystalline pterins with blue, violet or yellow fluorescence occur in poikilothermal vertebrates, chiefly in skin and eyes. They were formerly thought to be a single substance, known as ‘fluorescyanin’ or ‘ichthyopterin’, but they turn out to be a complex mixture. Among the numerous pterins that can be separated by chromatography, isoxanthopterin and pterincarbonic‐acid(8) regularly appear. The second of these substances occurs as a product of photolysis of the yellow‐fluorescing eye pigment of a certain Drosophila race, of the yellow pterin from the eyes and skin of the frog and of a light‐blue‐fluorescing pterin in the toad. The extremely photolabile precursor has been identified in Drosophila as a lactyl derivative of pterincarbonic‐acid(8), but the nature of the others is still unknown although they are probably closely related.

Among the pterin pigments in the skin and eyes of fishes there is, sometimes at least, one photolabile component, the nature of which is likewise unknown.

The crystalline pterin pigments of insects occur in wasps in hypodermis cells overlying metabolicaliy inactive tissues and in pierid butterflies on the walls of canals in the scales. The pigments are deposited in these situations shortly before or after the emergence of the insect from the pupal stage; they are derived from unknown precursors, probably brought by the blood.

Pterins with blue or yellow fluorescence, on the other hand, do not occur in the crystalline state, but are bound to nucleoprotein, protein or polypeptide. There is evidence that these pterins are synthesized in melanophores, both in fishes and amphibians. Since there are indications that pterins play a part in melanogenesis, this should be taken into account in relation to the physiology of melanophores, at least in arthropods and poikilothermal vertebrates.

In a few cases pterins remain in the melanophores of adult animals, but in amphibians they are usually found in particular sulphur‐yellow cells (hitherto wrongly called ‘lipophores’), in iridiocytes together with purins, or in a special pterin layer between the stratum spongiosum and the stratum compactum of the corium. There the pterins are tightly bound to ribonucleoprotein granules. In the scales of fishes they occur in the hyalodentine layer of the growth rings. Yellow pterinophores change in many cases into red ‘erythrophores’: the composition of this red pterin (until now also known as ‘lipochrome’) is not yet known.

The structural framework of the pterin molecule is found also in folic acid derivatives which are metabolicaliy active as formyl transporters, yet there seems to be no direct replacement between the pigmentary pterins and these active substances. The pigmentary pterins, on the other hand, are probably all very nearly related; they show great variation in their manifestation, according to the genetic constitution of the animals, and can replace one another. The biochemical and genetical analysis of these phenomena is very promising.

Pterins are nearly related to purins and to riboflavin. The biosynthesis of these three substances follows the same path up to a certain point. A change of purin into pterin, however, only takes place to a very limited extent.

Whereas in insects pterins with blue and yellow fluorescence occur from the embryonic stages onwards, in the Anura they first appear several days after hatching and show a characteristic rate of increase during larval life. This is closely bound up with the transformation of ventral pterinophores into iridiocytes.

Many naturally occurring pterins resemble riboflavin in redox behaviour and in other characteristics; they accompany riboflavin and in part replace it. With their redox capacity they appear to function in tissues as hydrogen transporters. At all events, the respiration of skin containing pterin, and of tissues in solutions of photolabile amphibian pterin, is temporarily increased by illumination. The presence of very similar pterins in the eyes of arthropods and of lower vertebrates must be taken into account in studies of the chemistry of vision.