The Excretory Role of Pteridines in Insects

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Abstract
Previous work on the storage excretion theory of pteridines is summarized briefly, and the controversial aspects of this theory are discussed. A general model for all nitrogen-containing substances in an animal body is presented. The possible pathways for pteridines in this model are discussed, and the storage excretion hypothesis is related to the model. Storage and excretion of nitrogen-containing substances in Pieris brassicae were compared chromatographically on a quantitative basis. Only traces of the pteridines are excreted, the majority being stored, mostly in the adult cuticular scales. Only 20% of the uric acid is excreted; the xanthine is quantitatively stored in the fatbody. Removal of part of the wing storage potential increases excretion and fatbody storage only very slightly. There is a reduction in overall pteridine synthesis. A partly scaleless mutant, cerula, of P. brassicae also shows a slight increase in pteridine excretion coupled with an overall decrease in pteridine synthesis. Vanessa io, a related species not containing large quantities of pteridines, was closely compared with P. brassicae. In Vanessa nearly half the pupal pteridines are excreted with the meconium, and no storage is found in the cuticular scales. Several insects of other orders were investigated quantitatively. Only in some Hemiptera were pteridines found, stored in quantities comparable to those in Pieridae. Using an ultra-micro Kjeldahl technique for total soluble nitrogen, the relative role of purines, pteridines, and some other minor substances, both in storage and excretion, was established. It is obvious that in Pieridae the pteridines account for a much higher percentage of the total highly oxidized nitrogen produced (14%) than they account for in most other insects. The Pieridae differ further in storing most of these pteridines irreversibly in the cuticular scales of the adult. The evidence strongly suggests that the Pieridae have evolved a metabolic mechanism of which pteridines are the general end-product as well as a mechanism of dry storage excretion for these end-products. The pigmentation effect of cuticular pteridine deposition is considered a sideeffect.