Molecular topology and dimerization of recombinant bovine somatotropin

Extraordinarily stable disulfide-linked homodimer of human growth hormone

Although a 22-kDa human growth hormone (hGH) is the predicted protein product of the hGH-N gene, a pleiotropic collection of uncharacterized molecular weight and charge isoforms is also produced. Using chromatography and preparative SDS-PAGE under reducing conditions we isolated an unusually stable mercaptoethanol-resistant (MER) 45-kDa hGH. A 5-h incubation at 100 degrees C in the presence of 2-mercaptoethanol was required to convert approximately 90% of MER-45-kDa hGH into a 22-kDa hGH. Other reductants were not as effective in splitting MER-45-kDa hGH. After fracturing MER-45-kDa hGH, the 22-kDa hGH fragments would spontaneously reassociate if the reductant was removed; however, alkylation of cysteine residues prevented their reassociation. Identical amino acid sequences for the first six N-terminal residues were obtained for MER-45-kDa hGH and its 22-kDa hGH cleavage product. Structural identity of MER-45-kDa hGH and 22-kDa hGH was demonstrated by MALDI-TOF mass spectrometry of tryptic digests. MER-45-kDa hGH did not break up upon incubation with EDTA and EGTA. The significance of this work to our understanding of the structure of hGH isoforms is that it demonstrates that MER-45-kDa hGH is not a single chain polypeptide but is instead a homodimer of 22-kDa hGH monomers. The MER-45-kDa hGH dimer is held together by interchain disulfide bonds and not by divalent metal cation bridges. Additionally, MER-45-kDa hGH's interchain disulfide links are exceptionally resistant to reducing agents and thus confer extreme stability to the homodimer.

Structural characterization of the two refold dimers of recombinant bovine somatotropin (bST)

Two major dimers are generated during the folding/oxidation of inclusion bodies of recombinant bovine somatotropin (bST). These dimers represent the major part of the inactive high molecular weight species that are formed in this process. The structures of the two dimers are unambiguously determined by peptide mapping using trypsin, thrombin cleavage, and selective DTT reduction experiments. Results indicate that the formation of both dimers involves the large disulfide loop cysteines. The latter-eluting dimer from RP-HPLC, previously reported as a large loop concatenated dimer, was revised to be an antiparallel disulfide-linked dimer. On the other hand, the first eluting dimer is a concatenane in which two monomers are held together by the interlocking of the two large disulfide loops.