The long term objective is the isolation, structural analysis and mechanism of formation of post-translational modifications of growth hormone and prolactin. These forms include low molecular weight fragments, two-chain forms and the disulfide dimer. The hypothesis to be examined is that the intact hormones are prohormones from which physiologically active products are formed. The first priority is to determine the structure of a low molecular weight peptide (Mr about 5,000) with diabetogenic properties. The peptide can be dissociated from partially purified growth hormone and purified by HPLC. The peptide produces glucose intolerance in dogs in microgram amounts whereas milligrams of GH are required to produce a similar hyperglycemia. We have only indirect evidence that the factor is a fragment of GH. Because the peptide does not react with antisera to GH, its presence in blood and its involvement in diabetes has never been determined.
The second aim i s to extend our work on proteolytically modified (two-chain forms) of GH. The positions and number of residues that must be removed for potentiation of biological activity must be identified in greater detail. The possibility that proteolysis produces biologically active fragments that remain bound hydrophobically to the cleaved hGH must be examined.
The third aim i s to determine how limited proteoloysis alters the biological activities of two new forms of prolactin. Whether new activities are generated or known physiologic actions potentiated must be determined. The fourth goal is to determine how proteolytic modification of the interchain disulfide dimers of GH and PRL affects their physiologic properties. The fifth goal is to examine the hypothesis that deamidation, NH2 terminus acylation and disulfide bridging direct proteolytic attack of GH and PRL to produce a variety of modifications and fragments. The work will provide material for development of RIAs to enable us to determine if the modifications circulate in blood and if they are involved in endocrine disoders.
