Structural studies on ovine growth hormone. Cyanogen bromide fragments: N- and C-terminal sequences

Properties of cryptic epitopes and their corresponding antibodies as indicated by the study of human and ovine growth hormones

Antibodies (Ab) directed to hidden antigenic determinants (cryptotopes) are undesirable because they are not neutralizing. Additionally, we have previously demonstrated a close association between the extent of Ab to cryptic determinants and the expression of autoantibodies (autoAb) under some experimental conditions. Thus, the first objective of this work was to establish the physicochemical characteristics of Ab to cryptotopes and the second one was to examine the structural features of cryptic epitopes themselves. Using human and ovine growth hormones (hGH and oGH) as antigenic models and competition ELISA under different conditions of temperature, pH or ionic strength, we did not find any difference between the binding properties of anti-cryptic epitope antibodies (Ab) and anti-native epitope Ab. Then, using synthetic peptides and tryptic digests and direct and competition ELISAs we studied the structures of cryptic hGH and oGH epitopes. Isolated peptides either in solution or adsorbed on microplates failed to react. Partially digested hGH was recognized only when insolubilized on microplates, and anti-oGH Ab only reacted with a large fragment of the hormone either in solution or insolubilized. These results indicate that, at least in the case of hGH and oGH, cryptic epitopes are not simple linear sequences, as commonly referred without any evidence, but new exposed conformational structures different from those found in the native antigen.

Oxidation of methionine residues in equine growth hormone by Chloramine-T

1. Reactivity of methionine residues towards Chloramine-T was studied in the equine growth hormone. 2. With a 20.0-fold molar excess of reagent over methionine, full oxidation of the four residues of the protein is achieved. 3. Methionine 4 is the most reactive group, followed by methionines 72 and 178--methionine 123 being the less reactive residue. 4. As judged by circular dichroism spectra and binding assays, protein conformation and binding capacity to specific receptors remains unchanged even after full oxidation of all four methionine residues. 5. Results agree with data previously obtained with bovine growth hormone.

An equine growth hormone molecular species lacking the 76-92 peptidic fragment

A new eGH molecular species was isolated and purified by reverse phase HPLC. SDS-polyacrylamide gel electrophoresis, amino acid composition, and C- and N-terminal determinations support a primary structure identical to that described by Zakin et al. (1976), except for the lack of the 76-92 peptidic fragment and the maintaining of 30% of its biological activity.

Equine growth hormone. Detection of immunoreactive sequences using poly- and monoclonal antibodies

The immunochemical behavior of several fragments of equine growth hormone (eGH) was examined using competitive binding assays with antibodies (Abs) to eGH obtained from different sources. Antigenicity was detected within the sequences 5-72 and 73-123 by rabbit Abs to eGH and by three mouse monoclonal antibodies (MAbs) produced by using bovine growth hormone as immunogen, but showing heteroclitic properties towards eGH. The polyclonal Abs to eGH also recognized as immunoreactive two smaller peptides corresponding to the amino acid residues 52-72 and 110-123. By contrast, the heteroclitic Abs to eGH developed by hypopituitary patients therapeutically injected with human growth hormone failed to react with any eGH-derived fragment. The rabbit polyclonal Abs and the mouse MAbs scarely discriminated between native and S-carbamidomethylated eGH, while the heteroclitic human Abs detected a clear difference between the native and the modified hormone.

Modification of lysine 69 reactivity in bovine growth hormone by carbamylation of its N-terminal group

Bovine growth hormone was carbamylated under conditions that assure full reaction of the N-terminal residue. Approximately 28% of lysine 179 and 7% of lysine 143 were also carbamylated. The modified hormone retained an important growth promoting activity and was as effective as the native hormone in competition assays in vivo for the receptors in rat liver. However, a change in its conformation must occur since lysine 69, which is resistant to trinitrophenylation in the native hormone, reacted easily and under mild conditions, in the carbamylated protein, The growth promoting activity and binding properties of the carbamylated and trinitrophenylated hormone were practically nil.

Oxidation of methionine residues in bovine growth hormone by chloramine-T

The methionine residues in bovine growth hormone were chemically modified wih chloramine-T at pH 7.4 using different chloramine-T methionine molar ratios. Methionine 4 was shown to be the most reactive followed in decreasing order by methionines 148, 123 and 178. With a 50-fold molar excess of chloramine-T over methionine almost full oxidation of all the methionine residues was achieved. The oxidation of the four residues of methionine does not change the growth promoting activity of the hormone.

Detection of immunologically active zones in equine growth hormone

Peptide fragments, obtained from equine growth hormone by cyanogen bromide cleavage and further chemical treatment, were isolated and identified. Their immunological reactivities were tested by hemagglutination and complement fixation methods using rabbit antisera against native hormone. Antigenic determinants were detected in the fragments comprising amino acid sequences 5-72 and 73-123, this last one being predominant. Fragment 124-178 had very low reactivity. Nitration of peptide 73-123 did not modify its immunological properties,but oxidation diminished them. Comparison of the antigenicity of equine growth hormone fragment 73-123 with that of the homologous ovine growth hormone fragment 86-123 lent support to the hypothesis that at least one antigenic determinant area in the horse hormone fragment is dependent on sequence 86-123.