Purification and characterization of high-molecular-weight forms of adrenocorticotropic hormone of ovine pituitary glands

Isolation of corticotropin-beta-lipotropin common precursor from ovine pituitary glands

Ovine corticotropin-beta-lipotropin common precursor was purified to homogeneity from commercial frozen ovine pituitary glands. A crude preparation was obtained following a procedure published elsewhere (Lee, T.H. and Lee, M.S. (1977) Biochemistry 16, 2824-2829) and was purified by gel filtration on Sephadex G-200 in the presence of 0.5% SDS and 0.1% 2-mercaptoethanol, and under an atmosphere of nitrogen. The gel filtration was repeated once. The partially purified preparation obtained from the second Sephadex G-200 gel filtration was further fractionated by preparative SDS-acrylamide gel electrophoresis, using immunoprecipitated and electrophoretically purified [125I]corticotropin-beta-lipotropin common precursor as a marker. The preparation was judged homogeneous by the appearance of a single protein band in analytical SDS-acrylamide gel electrophoresis, which exhibited both corticotropin and beta-lipotropin immunoreactivities, and a single symmetrical peak in high-pressure liquid chromatography on a reverse phase C18 column. The isolated ovine corticotropin-beta-lipotropin common precursor possessed specific activities of 116 micrograms of immunoreactive corticotropin and 210 micrograms of immunoreactive beta-lipotropin per mg of protein, equivalent to 89 and 62% of theoretical values, respectively. The amino acid composition of the homogeneous preparation was determined.

"Joining peptide" of pro-opiomelanocortin. I. Radioimmunoassay and extraction of related peptides from pituitary glands

In order to immunoassay the specific region of bovine pituitary pro-opiomelanocortin (POMC) between ACTH and gamma-MSH, referred to as "joining peptide," antisera were prepared against the synthetic amidated decapeptide Val-Ala-Val-Gly-Glu-Gly-Pro-Gly-Pro-Arg-NH2. The non-amidated peptide represents residues -23 to -14 of bovine POMC. An NH2-terminal tyrosine analog of the decapeptide was used as the radioligand. Under optimal conditions, immunoassay with selected antisera exhibited a sensitivity (50% displacement of the radioligand) toward the decapeptide in the range of 31-55 pg. Immunoreactivity found in extracts of fresh or lyophilized bovine pituitary glands displaced the iodinated Tyr-decapeptide in the RIA in a parallel manner. The amount of immunoreactive (ir)-material was dependent upon the state of preservation of the tissue, the method of extraction, and the particular antiserum used. Extractable immunoreactivity was separated into low (Mr 1,500) and high (Mr 17,000) molecular weight peptides using gel chromatography (G-75). Additional ir-material appeared in the void volume (Mr greater than 22,500). Thus, these antisera have the capacity to interact not only with a region of the joining peptide but also with its larger, and apparent precursor forms. The immunoassay developed should be valuable in understanding the disposition and processing in this specific region of POMC.

Characterization of a macromolecular aggregate of ovine pituitary corticotropin-beta-lipotropin common precursor

A macromolecular aggregate of corticotropin-beta-lipotropin common precursor had been observed in ovine pituitary preparations as an excluded fraction of Sephadex G-200 gel filtration. This fraction could not penetrate a 10% gel during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, when 2-mercaptoethanol or other disulfide-cleaving agents were not present in the buffer used to solubilize the protein preparation prior to the electrophoresis. On a 4.6% gel (acrylamide:bisacrylamide, 20:1), the material migrated as a diffuse band to a position between those of beta-galactosidase (Mr 130 000) and myosin (Mr 200 000). Both observations were consistent with an apparent Mr greatly in excess of that of the corticotropin-beta-lipotropin common precursor reported by many investigators. Neither 5% SDS nor 1% Triton X-100 could dissociate the macromolecular aggregate, but 2-mercaptoethanol and urea, either alone or in combination, were able to dissociate it to two main protein components, one of which was identified as corticotropin-beta-lipotropin with an apparent Mr of 34 000. The fact that urea alone could dissociate this macromolecular aggregate led us to believe that it might be a non-covalent aggregate and that 2-mercaptoethanol probably did not achieve the dissociation through the cleavage of an interchain disulfide bond but by bringing about conformational changes as a result of reduction of intrachain disulfide bonds so that aggregation became unfavorable. Moreover, the dissociation by urea or by 2-mercaptoethanol was found to be irreversible. The origin of the macromolecular aggregate of corticotropin-beta-lipotropin common precursor remains obscure.