CYSTINE IN SILK FIBROIN, BOMBYX MORI

Silkworms of Bombyx mori in their fifth instar were fed mulberry leaves coated with 35S cysteine and the fibroin extracted from their glands. The 35S fibroin was oxidized and a sample, analysed, when the 35S was found exclusively in 35S cysteic acid. The oxidized 35S fibroin yielded a soluble fraction on digestion with trypsin: that which remained insoluble was then treated with chymotrypsin which solubilized part of it. Peptides containing 35S cysteic acid were separated from these two soluble fractions by a combination of the techniques of resin chromatography, Sephadex chromatography, and paper electrophoresis. The sequences of four peptides were examined and were shown to form part of a ‘parent sequence’, Gly-Ala-Gly-Ala-Gly-Cya-Asx-Ser-Ala-Val-Cya(Pro, Leu), which accounted for approximately three-quarters of the cysteic acid in silk fibroin. It is suggested that the fibroin molecule is a single chain of molecular weight 103,000 with an intrachain disulphide bond between the two half-cystine residues indicated in the above sequence.