Tanned silks

Abstract

The silks of the large cocoons of saturniid moths range in colour from pale to dark brown. They are invariably accompanied by phenolic compounds. The nature of these phenols and their interaction with silk form the subject of this paper. Silk, though fluid when secreted, dries out almost instantly. It is pure white; and if artificially kept dry remains white. If the larva is allowed (as is in normal) to drench it with a watery secretion from the anus, or if artificially moistened, it darkens. Saturniid silks are here shown to consist, not only of structural fibroin and sericin, but also of enzymes and phenols. The presence of moisture allows these to interact. The phenols in the silk gland are in the form of glucosides. They are cleaved by a glucosidase and the liberated phenols are then oxidized by an oxidase. The oxidation products act as tanning agents supposedly by introducing exogenous cross-links between protein chains. Tanning brings about considerable physical and chemical changes in the properties of the silk. These are described. Two of the tanning phenols have been identified. The O -glucoside of 3-hydroxyanthranilic acid has not been previously recorded as a natural metabolite of animals or plants (although it is said to appear in the excreta of Bombyx larvae following enforced administration of 3-hydroxyanthranilic acid). The 5- O -glucoside of gentisic acid has not been recorded in animals but has been found in boron-deficient plants. The identity of both has been confirmed by synthesis. 3-Hydroxyanthranilic acid glucoside is derived from dietary tryptophan, but gentisic acid does not arise from an aromatic amino acid. Our evidence is consistent with its being taken up from the food-plant as a conjugate, and with the phenolic moiety being used, without change, by the silk glands. The biological and industrial implications of tanning of the cocoon silk are briefly considered.