New Polar Constituents of the Pupae of the Silkworm Bombyx mori L. I. Isolation and Identification of Methionine Sulfoxide, Methionine Sulfone, and γ-Cyclic di-L-Glutamate

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In addition to serine (L:D = 68:32), methionine sulfoxide (MSO), L-methionine sulfone (L-MSO₂), and disodium γ-cyclic di-L-glutamate were identified in a methanol extract of Bombyx mori L. pupae. MSO was isolated in a diastereomeric mixture of L(+)- and D(+)-MSO in a ratio of 99:1. The presence of these compounds in other developmental stages, including eggs, larvae (1st, 4th, 5th, and mature 5th instar), adults, and excrement (feces and urine) was investigated. The L(+)-isomer of MSO was present in extracts of the 1st and 5th instar larvae, adults, and eggs, but was not detected in feces or urine. The D(+)-isomer was found only in pupal stage extracts, and was excreted into the meconium with L(+)-isomer. L-MSO₂ and γ-cyclic di-L-glutamate were not detected at other insect life stages or in the insect excrement. γ-Cyclic di-L-glutamate is thought be produced due to blockage of the glutamate synthetic pathway (glutamine synthetase) by L-MSO₂ and Mg²⁺. The biochemical role of L-MSO₂ during the pupal life stage remains unknown, but importantly, the stage-specific expression suggests that it is a candidate molecule for the induction of diapause.