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Crude extracts of Bombyx mori brains can provoke adult development when injected into brain-removed dormant pupae of Bombyx mori and Samia Cynthia ricini. From this fact the prothoracicotropic hormone (PTTH) of Bombyx has long been thought to be species-nonspecifically active on Samia. Chemical fractionation of Bombyx brain or head extracts by fractional precipitation with acetone, Sephadex G-50 gel-filtration, and DEAE-Sepharose CL-6B chromatography, however, separated the fractions which activated Bombyx brainless pupae from those which activated Samia. Those results reveal the existence of two species-specific PTTHs.

Blockage of adult development by brain removal and its resumption by application of the prothoracicotropic hormone (PTTH) were studied using pupae of a racial hybrid J-122 × C-115 of Bombyx mori. A log-linear dose-response relationship was obrained after injection of a PTTH solution. The Bombyx-unit of PTTH has been defined from this dose-response curve.

The prothoracicotropic hormone (PTTH) was isolated from adult heads of Bombyx mori . Fifty micrograms of pure PTTH was obtained from 648,000 heads through a 15-step purification procedure with a 2 × 10 6 -fold purification and an 8% recovery. Chemical analyses of this PTTH have shown that it is a single-chain peptide consisting of 40–43 amino acid residues (MW, 4330–4740), the N-terminus of which is glycine. As little as 0.1 ng of PTTH elicited adult development in a debrained pupa of Samia cynthia ricini . Five picograms of PTTH directly stimulated the prothoracic glands in vitro so as to enhance ecdysone release. The hemolymph ecdysteroids of brainless Samia pupae that were developed by PTTH injection increased with essentially the same pattern as in developing normal pupae.