Evaluation of refolding conditions for a recombinant human interleukin-3 variant (daniplestim).

The refolding of daniplestim, a human interleukin-3 variant (SC-55494) from Escherichia coli inclusion bodies, was optimized using a reversed-phase HPLC method developed to permit quantification of the reduced and oxidized forms of daniplestim. The presence of cysteine or dithiothreitol accelerated refolding of daniplestim from E. coli inclusion body slurries dissolved in urea or guanidine solutions and was complete in 4-6 h. Regardless of the dissolution and refolding protocol used to renature daniplestim, equivalently bioactive protein was produced. Under refolding conditions, no covalent modification of daniplestim by cysteine or cyanate was observed. The folding process was characterized further by following the unfolding of purified daniplestim by far-UV CD and fluorescence spectroscopies under both oxidizing and reducing conditions at pH values between 7 and 11. Formation of the single disulphide bond had a large stabilizing effect on daniplestim structure ( approximately 4-5 kCal at pH 9.5). This thermodynamic stabilization drove the refolding process towards the native form, even under conditions where the reduced protein was largely unfolded. From these data, scaleable refolding conditions for daniplestim were established.

Determination of the disulfide bond pairings in human tissue factor pathway inhibitor purified from Escherichia coli.

The disulfide bond assignments of human alanyl tissue factor pathway inhibitor purified from Escherichia coli have been determined. This inhibitor of the extrinsic blood coagulation pathway possesses three Kunitz-type inhibitor domains, each containing three disulfide bonds. The disulfide bond pairings in domains 1 and 3 were determined by amino acid sequencing and mass spectrometry of peptides derived from a thermolysin digest. However, thermolysin digestion did not cleave any peptide bonds within domain 2. The disulfide bond pairings in domain 2 were determined by isolating it from the thermolysin treatment and subsequently cleaving it with pepsin and trypsin into peptides which yielded the three disulfide bond pairings in this domain. These results demonstrate that the disulfide pairings in each of the three domains of human tissue factor pathway inhibitor purified from Escherichia coli are homologous to each other and also to those in bovine pancreatic trypsin inhibitor.

Isolation of Escherichia coli synthesized recombinant eukaryotic proteins that contain epsilon-N-acetyllysine.

Recombinant porcine (rpST) and bovine somatotropins (rbST) synthesized in Escherichia coli contain the amino acid, epsilon-N-acetyllysine. This amino acid was initially discovered in place of the normal lysine144 in a modified reversed-phase HPLC (RP-HPLC) species of rpST. Mass spectrometry and amino acid sequencing of a tryptic peptide isolated from this RP-HPLC purified protein were used to identify this altered residue as epsilon-N-acetyllysine. Ion-exchange chromatography was utilized to prepare low isoelectric point (pI) forms of rpST and rbST, which are enriched in epsilon-N-acetyllysine. Electrospray mass spectrometry demonstrated that the majority of the protein in these low pI fractions contained species 42 Da larger than normal. Immobilized pH gradient electrophoresis (IPG) of the ion-exchange purified low pI proteins was used to isolate several monoacetylated species of rpST and rbST. The location of the acetylated lysine in each IPG-purified protein was determined by tryptic peptide mapping and amino acid sequencing of the altered tryptic peptides. Amino acid analyses of enzymatic digests of rpST and rbST were also used to confirm the presence of epsilon-N-acetyllysine in these recombinant proteins. These data demonstrate that a significant portion of rpST and rbST produced in E. coli contain this unusual amino acid.

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.

Novel in-frame two codon translational hop during synthesis of bovine placental lactogen in a recombinant strain of Escherichia coli.

A recombinant Escherichia coli strain was constructed for the overexpression of bovine placental lactogen (bPL), using a bPL structural gene containing 9 of the rare arginine codons AGA and AGG. When high level bPL synthesis was induced in this strain, cell growth was inhibited and bPL accumulated to less than 10% of total cell protein. In addition, about 2% of the recombinant bPL produced from this strain exhibited an altered trypsin digestion pattern. Amino acid residues 74 through 109 normally produce 2 tryptic peptides, but the altered form of bPL lacked these two peptides and instead had a new peptide which was missing arginine residue 86 and one of the two flanking leucine residues. The codon for arginine residue 86 was AGG and the codons for the flanking leucine residues 85 and 87 were TTG. When 5 of the 9 AGA and AGG codons in the bPL structural gene were changed to more preferred arginine codons, cell growth was not inhibited and bPL accumulated to about 30% of total cell protein. When bPL was purified from this modified strain, which included changing the arginine codon at position 86 from AGG to CGT, none of the altered form of bPL was produced. These observations are consistent with a model in which translational pausing occurs at the arginine residue 86 AGG codon because the corresponding arginyl-tRNA species is reduced by the high level of bPL synthesis, and a translational hop occurs from the leucine residue 85 TTG codon to the leucine residue 87 TTG codon. This observation represents the first report of an error in protein synthesis due to an in-frame translational hop within an open reading frame.

Isolation and characterization of porcine somatotropin containing a succinimide residue in place of aspartate129.

Aspartate129 in porcine somatotropin was converted into a cyclic imide residue (succinimide) under acidic solution conditions. Reversed-phase high performance liquid chromatography was utilized to isolate and quantitate this altered species, which accounted for approximately 30% of the total protein. The molecular mass of this modified species was determined by electrospray mass spectrometry to be 18 Da less than normal porcine somatotropin, indicative of a loss of 1 H2O molecule. Tryptic peptide mapping demonstrated that the peptide composed of residues 126-133 was altered in this modified protein. Amino acid analysis, amino acid sequencing, mass spectrometry, and capillary zone electrophoresis were used to demonstrate that aspartate129 in this peptide had been converted into a succinimide residue. Further confirmation that this peptide contained a succinimide was obtained by hydrolyzing the modified peptide at pH 9.0, which yielded both the aspartate and isoaspartate peptides.

Comparison of the galactopoietic response to pituitary-derived and recombinant-derived variants of bovine growth hormone.

Two studies were designed to examine the differences in galactopoietic potency of molecular variants of pituitary- and recombinant-derived bovine GH (bGH). The recombinant bGH molecules included amino-terminal and position-127 amino acid substitutions which are representative of two of the four natural pituitary variants or of partially degraded bGH molecules. Amino-terminal variants of bGH included methionine (Met1), alanine (Ala1), serine (Ser1) or deletion of four amino acids (delta 1-4). The delta 1-4 variants were representative of degradation products previously isolated in pituitary bGH preparations. In the first study, 54 lactating Holstein cows received i.m. injections of a buffer solution (control), pituitary-derived bGH, or recombinant-derived [Met1,Leu127]-bGH, [Met1,Val127]-bGH, [Ala1,Leu127]-bGH, or [Ala1,Val127]-bGH. Cows received 25 mg bGH/day for 21 days. Substitution of the amino-terminal alanyl residue with methionine did not affect milk response. GH variants with Val127 elicited a greater milk response (8.5 kg/day) than Leu127 bGH variants (6.5 kg/day). The average milk response to the four recombinant bGH variants was 7.5 kg/day greater than controls compared with 4.4 kg/day for pituitary-derived bGH. In contrast, blood bGH concentrations were equivalent for pituitary and recombinant bGH treatments, approximately 20 micrograms/l more than control levels at 3 h after injection. Blood free fatty acid concentrations were increased, but insulin and glucose levels were unaffected by bGH treatment. In the second study, 54 lactating Holstein cows received i.m. injections of a buffer control solution or recombinant-derived [Met1,Leu127]-bGH, [Ser1,Leu127]-bGH, [Ser1,Val127]-bGH, [delta 1-4,Leu127]-bGH or [delta 1-4,Val127]-bGH. Cows received 25 mg bGH/day for 28 days. The milk response to full-length bGH variants was 6.6 kg/day greater than the response to the amino-terminal deletion variants (P less than 0.05). Substitution of valine for leucine did not affect milk response to either the deletion (delta 1-4) or full-length (Met1 or Ser1) bGH molecules. In conclusion, the lowered galactopoietic potency of pituitary bGH preparations was demonstrated, at least in part, to be due to the presence of amino-terminal amino acid deletions rather than differences in amino acid sequences of recombinant bGH. Ala1 bGH variants with valine at position 127 elicited a greater milk response than Leu127 variants.

Determination of the disulfide bond pairings in bovine transforming growth factor-alpha.

A rapid method for determining the three disulfide bond pairings in bovine transforming growth factor-alpha (bTGF-alpha) was developed by digesting bTGF-alpha with thermolysin followed by separation of the generated peptides by reversed-phase HPLC. The disulfide-bonded peptides were identified by amino acid sequencing and fast atom bombardment mass spectrometry. The disulfide bond pairings in bTGF-alpha were determined to be homologous to those in the human and mouse TGF-alpha molecules. A species of low bioactivity isolated from the folding/oxidation mixture of chemically synthesized bTGF-alpha was demonstrated to contain two incorrect disulfide bonds. These results indicate that mispairing of disulfide bonds in bTGF-alpha significantly reduces the activity of this molecule.

Chemical synthesis of bovine transforming growth factor-alpha: synthesis, characterization and biological activity.

Bovine transforming growth factor-alpha (bTGF-alpha) is a 50 amino acid polypeptide with three disulfide linkages. In order to evaluate the biological function of this peptide, bTGF-alpha was synthesized via an automatic synthesizer and purified to homogeneity in high yield. The integrity of this synthetic peptide was confirmed by chemical analyses and bioassays. In a bovine liver radioreceptor assay, bTGF-alpha competes with radiolabeled EGF and has activity comparable to mEGF and hTGF-alpha. Compared to hEGF, bTGF-alpha elicits a greater response in a bovine mammary cell proliferation.

Formation of isoaspartate 99 in bovine and porcine somatotropins.

Asparagine 99 in bovine (BST) and porcine somatotropins (PST) was converted to an isoaspartate residue during incubation at neutral or alkaline pH. Isoaspartate 99 BST or isoaspartate 99 PST was resolved from the normal somatotropin by reversed-phase high-performance liquid chromatography (HPLC). The altered peptide of residues 96-108 which contains isoaspartate 99 was detected by tryptic peptide mapping of the modified BST or PST. Amino acid sequencing, amino acid analysis, mass spectrometry, and co-elution with a chemically synthesized peptide containing isoaspartate 99 were used to demonstrate the existence of isoaspartate in the modified peptides. Peptide bond cleavage between Asn 99 and Ser 100 also occurred during incubation of BST and PST at neutral or alkaline pH. This chemically cleaved product was resolved on reversed-phase HPLC from both the isoaspartate 99 and normal somatotropin molecules.

A novel concatenated dimer of recombinant bovine somatotropin.

A novel protein concatenated dimer structure was generated during the folding/oxidation of inclusion bodies of recombinant bovine somatotropin synthesized in Escherichia coli. The structure of this dimeric molecule was determined by peptide mapping with trypsin, and limited proteolysis by thrombin. Peptide mapping demonstrated that the two disulfide pairs in bovine somatotropin dimer were identical to those in monomer. Limited proteolysis with thrombin resulted in the cleavage of only a single peptide bond between arginine-132 and alanine-133 in bovine somatotropin dimer. This single peptide bond cleavage was sufficient to convert this dimer to a monomeric molecular weight species as analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis and HPLC. Since the single cleaved peptide bond is present in the large disulfide loop of bovine somatotropin, these data demonstrate that the dimeric molecule exists as a novel concatenated structure through the interlocking of the disulfide loops of this protein.

Purification and characterization of pituitary bovine somatotropin.

Bovine somatotropin (bST) has been isolated from pituitary glands and compared in a variety of chemical analyses and bioassays with somatotropin derived from recombinant Escherichia coli. Comparison of pituitary extracts and purified bST by Western blot analysis of two-dimensional gels suggested that the immunoreactive somatotropin species present in the extract were also present in the purified material, with no significant losses or degradation as a result of the purification method. NH2-terminal sequence analysis indicated the presence of equal quantities of Ala-Phe-Pro-Ala-Met-Ser-Leu-Ser- and Phe-Pro-Ala-Met-Ser-Leu-Ser- sequences. The Met-Ser-Leu-Ser-NH2-terminal sequence, a degradation product observed in NIH standard lots, was not detected. Assay of bioactivity in a bovine liver receptor-binding assay and in a female rat growth assay showed pituitary bST and recombinant methionyl-bovine somatotropin to be equipotent. Tryptic maps and sequence analysis of pituitary-derived somatotropin suggest the presence of isoaspartate derivatization at Asp128.

Biosynthesis and incorporation into protein of norleucine by Escherichia coli.

The methionine analog norleucine was produced during the synthesis of bovine somatotropin by Escherichia coli strain W3110G containing the recombinant plasmid pBGH1. Norleucine was generated by the leucine biosynthetic pathway from pyruvate or alpha-ketobutyrate in place of alpha-ketoisovalerate as the initial substrate. The intracellular level of norleucine was high enough to permit the analog to compete successfully with methionine for incorporation into protein. Two ways were found to prevent either the formation of norleucine or its incorporation into protein. The endogenous synthesis of norleucine was eliminated by deleting the leucine operon. The addition of sufficient methionine or 2-hydroxy-4-methylthiobutanoic acid, a precursor of methionine, to the culture medium prevented any norleucine from being incorporated into protein.

Three-dimensional structure of a genetically engineered variant of porcine growth hormone.

The three-dimensional structure of a genetically engineered variant of porcine growth hormone, methionyl porcine somatotropin (MPS), has been determined at 2.8-A resolution, using single crystal x-ray diffraction techniques. Phases were obtained by use of a single isomorphous K2OsCl6 derivative and were improved by use of the density modification procedure. The MPS structure is predominantly helical. It consists mainly of four antiparallel alpha-helices arranged in a left twisted helical bundle, a structural motif observed in a number of other unrelated proteins. However, the way the four helices are connected in the bundle is unusual and, to our knowledge, has never been reported before. Alignment of the amino acid sequence of MPS with that of other growth hormones reveals that residues within the alpha-helices are predominantly invariant and thus these invariant residues are necessary to maintain the structural integrity of these proteins.

Crystallization of methionyl porcine somatotropin, a genetically engineered variant of porcine growth hormone.

Crystals of methionyl porcine somatotropin have been grown out of ammonium sulfate, by the hanging drop method of vapor diffusion. The crystals belong to the trigonal space group P3121 or P3221, with a = 87.7 A and c = 58.7 A, and diffract beyond 2.1 A resolution.

Isolation and characterization of native and lower molecular weight forms of sheep plasminogen.

Two major forms of native sheep plasminogen (SPg-a) have been isolated from plasma by affinity chromatography. These forms differ in molecular weight, charge characteristics, affinity for epsilon-aminocaproic acid (epsilon-Ahx), and carbohydrate content. Upon treatment of SPg-a with plasmin, lower molecular weight plasminogens can be isolated. A plasminogen (SPg-b) of molecular weight approximately 8,000 less than native plasminogen is rapidly produced when either major plasminogen form is treated with plasmin. The molecular weight differences found in the major SPg-a forms are retained in the SPg-b forms, derived from each SPg-a. Upon protracted treatment of either major form of SPg-a or SPg-b with plasmin, a plasminogen (SPg-c) or molecular weight approximately 32,000 less than SPg-b is produced. A single peptide (P) is also produced in this step. The SPg-c species produced from each original SPg-a major form possess essentially the same molecular weights and carbohydrate compositions; but the P cleaved retains the molecular weight and carbohydrate differences found in each major SPg-a or SPg-b form. A large decrease in the S20,w of SPg-a is observed upon the binding of epsilon-Ahx to this protein. A much smaller alteration in the S20,w of SPg-b and SPg-c is observed upon binding of epsilon-Ahx to these proteins.