IMMUNOLOGICAL EVIDENCE FOR 2 COMMON PRECURSORS TO CORTICOTROPINS, ENDORPHINS, AND MELANOTROPIN IN THE NEUROINTERMEDIATE LOBE OF THE TOAD PITUITARY

The biosynthesis of corticotropin (ACTH(1-39)), β-endorphin[β(61-91)-lipotropin] and α-melanotropin in the toad intermediate lobe was studied by using immunoprecipitation procedures with antisera specific for these peptides. Intermediate lobes were pulse-incubated with [(3)H] phenylalanine and then chase-incubated for varying periods; the radioactive proteins were immunoprecipitated. Immunoprecipitates were separated by acidic urea or sodium dodecyl sulfate polyacrylamide gel electrophoresis. Evidence from the pulse-chase and sequential immunoprecipitation studies using antisera to ACTH and β-endorphin suggests that the toad intermediate lobe synthesizes two common precursors (apparent M(r) 32,000 and 29,500) containing both the ACTH and β-endorphin sequences. These precursors are processed to yield several forms of immunoreactive corticotropin (apparent M(r)23,000, 21,000, 13,000, and 4300), immunoreactive endorphin (apparent M(r) 11,700 and 3500), and immunoreactive α-melanotropin. The 4300 M(r) form of corticotropin and the 11,700 and 3500 M(r) forms of endorphins were found to comigrate with synthetic ACTH(1-39,)β-lipotropin and β-endorphin, respectively, on both acidic urea and sodium dodecyl sulfate gels.