Pterins

The pigments of butterflies were first identified by Heinrich Wieland in the 1930s. They belonged to a whole new class of nitrogen heterocycles, which he called pteridines (Greek pteros = wing). The structure of the white pigment of Pieris brassicae and P. rapae, leucopterin, and the yellow pigment xanthopterin (from Gonepteryx rhamni) were the first elucidated (Figure 8.19). Later it was found that compounds with the

Figure 8.19 The formation of the parent of the pterins from GTP, with the structures of some insect pterin pigments. Leucopterin is colourless, xanthopterin and chrysopterin are yellow, erythropterin is red. Biopterin is found generally in all cells. Note the similarity between the third and fourth steps in this sequence and the third step in the formation of tryptophane in Figure 8.5

Figure 8.19 The formation of the parent of the pterins from GTP, with the structures of some insect pterin pigments. Leucopterin is colourless, xanthopterin and chrysopterin are yellow, erythropterin is red. Biopterin is found generally in all cells. Note the similarity between the third and fourth steps in this sequence and the third step in the formation of tryptophane in Figure 8.5

pteridine structure are widely found in nature, e.g. tetrahydrofolic acid (Figure 2.15) and flavin (Figure 2.10). The compound biopterin (Figure 8.19) occurs in every animal cell or tissue as a co-factor of some enzyme reactions, e.g. the hydroxylation of phenylalanine to tyrosine and tyrosine to DOPA, but mammals are unable to make pterins. Biopterin is thought to be a growth factor for some insects.

As well as forming some butterfly wing pigments, pterins are also found as body pigments of Lepidoptera and Hymenoptera. Xanthopterin, as well as being present in the wings of Gonepteryx rhamni, is particularly widely distributed in insects and other animals. It provides the yellow colour of common wasps (Vespa vulgaris, Polistes dominulus (Plate 16), and V. crabro, the hornet). Little is known about pterin biosynthesis in insects, but it is probably as in mammals. In mammals it begins from the nucleotide guanosine triphosphate (GTP). The imidazole ring of guanine is opened up and loses one atom of carbon as formic acid (Figure 8.19), leaving a ribose triphosphate derivative of a diaminopyrimidine. The ribose undergoes ring opening to give an open-chain keto-sugar, which cyclizes with the free amine group to give dihydroneopterin triphosphate. Surprisingly all these steps are catalyzed by the one enzyme, GTP cyclohydrolase.

The black and orange warning colour of the milkweed bug Oncopeltus fasciatus (Plate 17) is due to part opaque black melanin and part transparent cuticle that allows the underlying pterins to show through. Five different pterins were identified in the red heteropteran fire bug Pyrrhocoris apterus (Plate 9), with erythropterin the most abundant.

0 0

Post a comment