The distribution and molecular properties of the tanning hormone bursicon were studied using an isolated wing biological assay. Wing cuticle tanning activity was measured in saline homogenates of the central nervous system and selected muscles. Activity was detected in all ganglia of the nervous system in both the prepupal and pharate adult developmental stages. In both stages, this activity was predominantly located in the abdominal portion of the nervous system. The titres in all ganglia, except the suboesophageal, increased during metamorphosis. Of the various non-neural tissues examined, only the closer muscle of the spiracle contained detectable levels of activity.

All activity in the nervous tissue was sensitive to proteolytic digestion; it could not be mimicked by any of the 17 putative transmitter and/or hormonal substances tested. Partial purification of the tanning activity indicated an apparent molecular weight of 20-30 K.

The partially purified material (from gel filtration) resembled the hormone bursicon in the following three ways: its titre in the tissue declined following normal bursicon release; it could be recovered from the haemolymph during normal bursicon release; it could be released from isolated nervous tissue following high potassium stimulation in a calcium-dependent manner.

It was concluded that the hormone bursicon represents most if not all the tanning activity present in the central nervous system, that it is widely distributed in that tissue and that it is present as a peptide (or class of peptides) with homogeneous size and charge.

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