Syntheses of glycine and L-serine by their interconversion in the posterior silkgland of the silkworm, Bombyx mori

It was observed by solution-state 13C NMR spectroscopy that a great portion of the 13C of [1-13C]L-serine fed to the 5th instar larvae of the silkworm, Bombyx mori was incorporated into C1 of glycine in silk fibroin. [1-13C]Glycine was detected along with [1-13C]serine in fibroin of the posterior silkgland cultured in a medium containing [1-13C]serine. This formation of [1-13C]glycine was inhibited by addition of aminopterin to the culture medium. These findings suggest that an active conversion from serine to glycine, which needs tetrahydrofolate, occurs in the posterior silkgland for fibroin synthesis. Moreover, the solid-state 13C CP/MAS spectrum of the fibroin prepared from cocoons spun by larvae fed with [13C]formate revealed that serine C3 was labelled specifically with 13C, suggesting that the reverse conversion from glycine to serine took place in the silkworm. The posterior silkgland has the ability to synthesize not only fibroin but also its major materials, glycine and serine.