Immunological evidence for two common precursors to corticotropins, endorphins, and melanotropin in the neurointermediate lobe of the toad pituitary.

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Abstract
The biosynthesis of corticotropin (ACTH1-39), .beta.-endorphin [.beta.(61-91)-lipotropin] and .alpha.-melanotropin in the toad intermediate lobe was studied by using immunoprecipitation procedures with antisera specific for these peptides. Intermediate lobes were pulse-incubated with [3H]phenylalanine and then chase-incubated for varying periods. The radioactive proteins were immunoprecipitated. Immunoprecipitates were separated by acidic urea or sodium dodecyl sulfate [SDS] polyacrylamide gel electrophoresis. Evidence from the pulse-chase and sequential immunoprecipitation studies using antisera to ACTH and .beta.-endorphin suggested that the toad intermediate lobe synthesizes 2 common precursors (apparent MW 32,000 and 29,500) containing the ACTH and .beta.-endorphin sequences. These precursors are processed to yield several forms of immunoreactive corticotropin (apparent MW 23,000, 21,000, 13,000 and 4300), immunoreactive endorphin (apparent MW 11,700 and 3500), and immunoreactive .alpha.-melanotropin. The 4300 MW form of corticotropin and the 11,700 and 3500 MW forms of endorphins comigrated with synthetic ACTH1-39, .beta.-lipotropin and .beta.-endorphin, respectively, on acidic urea and SDS gels.