Peptides related to the NH2-terminal end of proopiocortin in man

Peptides related to the NH2-terminus of proopiocortin in man were studied with three different RIAs directed toward gamma 3MSH, human 16K, and mouse 16K. The culture medium derived from a human corticotropic adenoma (SCH medium), which had previously been used as a human reference standard, generated competitive binding curves parallel to that of purified human 16K in all three RIA systems. Gel exclusion chromatography performed with pituitary-derived materials (adenoma extract and medium, and plasma from patients with Nelson's syndrome) showed that the overall immunoreactive gamma 3MSH eluted as one major peak at the position of human 16K. Its molecular weight estimated under denaturing conditions was 11,000. Gel exclusion chromatography performed with nonpituitary-derived materials (tumor extract and plasma from patient with the ectopic ACTH syndrome) showed that a major peak eluted at the position of human 16K, and a smaller molecular weight peptide eluted in a position intermediary between that of human 16K and synthetic gamma 3MSH. These data show that immunoreactive gamma 3MSH is indeed identical to human 16K in pituitary-derived materials. A different processing of the proopiocortin molecule is likely to occur in nonpituitary tumors and will result in the release of a smaller molecular weight peptide. The exact nature of this peptide is not known. It is speculated that it may serve as a nonpituitary tumor marker.