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J W Tweedie

Publications and source records attributed to J W Tweedie.

At least 19 recordsLinked to original sources
PubMed2004

Gamma-glutamyl transpeptidase inhibition suppresses milk protein synthesis in isolated ovine mammary cells.

The membrane spanning enzyme gamma-glutamyl transpeptidase (gamma-GT; EC 2.3.2.2) catalyses the breakdown of the tripeptide glutathione and uses free amino acids (AA) to form gamma-glutamyl transpeptidase (GT) AA that become transported into cells and converted back into free AA. gamma-Glutamyl transpeptidase activity has been shown to be important for mammary AA uptake in rodent systems, and while gamma-GT activity is high in lactating bovine mammary tissue, the role of this enzyme in milk protein synthesis of the ruminant has not been defined. The present study shows that gamma-GT activity in the ovine mammary gland, like that of rodents, increases during pregnancy and peaks early in lactation. Acivicin, a well-known inhibitor of gamma-GT, decreased gamma-GT activity in acini isolated from the ovine mammary gland and did not have secondary toxicity effects on cell viability or the uptake of radiolabeled amino-isobutyric acid. Isolated ovine acini were incubated in the presence of radiolabeled leucine, and incorporation of label into secreted protein increased during incubation. Incubation of acini with acivicin decreased milk protein secretion by 75%, indicating that gamma-GT plays an important role in milk protein production in the ruminant. Acivicin did not inhibit secretion of specific caseins but caused a global decrease in individual proteins suggesting that gamma-GT may be responsible for providing a complement of AA for milk protein synthesis.

Animals↗
PubMed2000

Crystal structure and iron-binding properties of the R210K mutant of the N-lobe of human lactoferrin: implications for iron release from transferrins.

Lactoferrin (Lf) and serum transferrin (Tf) combine high-affinity iron binding with an ability to release this iron at reduced pH. Lf, however, retains iron to significantly lower pH than Tf, giving the two proteins distinct functional roles. In this paper, we compared the iron-release profiles for human Lf, Tf, and their N-lobe half-molecules Lf(N) and Tf(N) and showed that half of the difference in iron retention at low pH ( approximately 1.3 pH units) results from interlobe interactions in Lf. To probe factors intrinsic to the N-lobes, we further examined the specific role of two basic residues that are proposed to form a pH-sensitive dilysine trigger for iron release in the N-lobe of Tf [Dewan, J. C., Mikami, B., Hirose, M., and Sacchettini, J. C. (1993) Biochemistry 32, 11963-11968] by mutating Arg 210 to Lys in the N-lobe half-molecule Lf(N). The R210K mutant was expressed, purified, and crystallized, and its crystal structure was determined and refined at 2.0-A resolution to a final R factor (R(free)) of 19.8% (25.0%). The structure showed that Lys 210 and Lys 301 in R210K do not form a dilysine interaction like that between Lys 206 and Lys 296 in human Tf. The R210K mutant retained iron to lower pH than Tf(N), consistent with the absence of the dilysine interaction but released iron at approximately 0.7 pH units higher than Lf(N). We conclude that (i) the ability of Lf to retain iron to significantly lower pH than Tf is due equally to interlobe interactions and to the absence in Lfs of an interaction analogous to the dilysine pair in Tfs, even when two lysines are present at the corresponding sequence positions, and (ii) an appropriately positioned basic residue (Arg 210 in human Lf) modulates iron release by inhibiting protonation of the N-lobe iron ligands, specifically His 253.

Binding Sites↗
PubMed1997

Mutagenesis of the histidine ligand in human lactoferrin: iron binding properties and crystal structure of the histidine-253-->methionine mutant.

The contribution of the conserved His ligand to iron binding in transferrins has been addressed by site-directed mutagenesis and X-ray crystallographic analysis. His 253 in the N-terminal half-molecule of human lactoferrin, LfN (residues 1-333), has been changed to Gly, Ala, Pro, Thr, Leu, Phe, Met, Tyr, Glu, Gln, and Cys by oligonucleotide-directed mutagenesis. The proteins have been expressed in baby hamster kidney cells, at high levels, and purified. The results show that the His ligand is essential for the stability of the iron binding site. All of the substitutions destabilized iron binding irrespective of whether the replacements were potential iron ligands or not. Iron was lost below pH approximately 6 for the Cys, Glu, and Tyr mutants and below pH 7 or higher for the others, compared with pH 5.0 for LfN. The destabilization is attributed to both steric and electronic effects. The importance of electronic effects has been shown by the crystal structure of the H253M mutant, which has been determined at an effective resolution of 2.5 A and refined to a final R factor of 0.173. The iron atom is changed from six-coordinate to five-coordinate; the Met 253 side chain is not bound to iron even though there appears to be no steric barrier. This is attributed to the poorer affinity of the thioether ligand for Fe(III) compared with imidazole nitrogen. The decreased stability of the iron binding is attributed solely to the loss of the His ligand as the protein conformation and interdomain interactions are unchanged.

Animals↗
PubMed1996

Mutation of arginine 121 in lactoferrin destabilizes iron binding by disruption of anion binding: crystal structures of R121S and R121E mutants.

A conserved arginine residue helps to form the synergistic anion binding site in transferrins. To probe the importance of this residue for anion binding and iron binding, Arg 121 has been mutated to Ser and Glu in N-terminal half-molecule of human lactoferrin. The two mutants, R121S and R121E, have been expressed, purified, and crystallized. Their three-dimensional structures have been determined by X-ray diffraction at 2.3 and 2.5 A resolution, respectively. The structures were determined by molecular replacement and were refined by restrained least squares methods to final R values of 0.185 and 0.204. Both mutants still bind iron but with decreased stability. The crystal structures show that destabilization of iron binding probably results from disruption of the anion binding site; mutation of Arg 121 removes one wall of the anion binding pocket and causes the synergistic carbonate ion to be displaced 0.5 A from its position in the wild-type protein. In the process it becomes partially detached from the helix N-terminus that forms the rest of the anion binding site.

Anions↗
PubMed1996

Altered domain closure and iron binding in transferrins: the crystal structure of the Asp60Ser mutant of the amino-terminal half-molecule of human lactoferrin.

The crystal structure of a site-specific mutant of the N-terminal half-molecule of human lactoferrin, Lf(N), in which the iron ligand Asp60 has been mutated to Ser, has been determined at 2.05 A resolution in order to determine the effects of the mutation on iron binding and domain closure. Yellow monoclinic crystals of the D60S mutant, in its iron-bound form, were prepared, and have unit cell dimensions a = 110.2 A, b = 57.0 A, c = 55.2 A, beta = 97.6 degrees, space group C2, with one molecule of 333 residues in the asymmetric unit. The structure was determined by molecular replacement, using the wild-type Lf(N) as search model, and was refined by restrained least-squares methods. The final model, comprising 2451 protein atoms (from residues 2 to 315) one Fe3+ and one CO2-(3), and 107 water molecules, gives an R-factor of 0.175 for all data in the resolution range 20.0 to 2.05 A. The model conforms well with standard geometry, having root-mean-square deviations of 0.014 A and 1.2 degrees from standard bond lengths and angles. The structure of the D60S mutant deviates in two important respects from the parent Lf(N) molecule. At the mutation site the Ser side-chain neither binds to the iron atom nor makes any interdomain contact as the substituted Asp does; instead a water molecule fills the iron coordination site and participates in interdomain hydrogen bonding. The domain closure is also changed, with the D60S mutant having a more closed conformation. Consideration of crystal packing suggests that the altered domain closure is a genuine molecular property but both the iron coordination and interdomain contacts are consistent with weakened iron binding in the mutant. The implications for iron binding in transferrins generally are discussed.

Animals↗
PubMed1996

Elevation of lactoferrin gene expression in developing, ductal, resting, and regressing parenchymal epithelium of the ruminant mammary gland.

Accumulation of lactoferrin mRNA in mammary tissue from virgin, pregnant, lactating, and involuting ewes and cows was localized using 35S-labeled cRNA probes. Expression of lactoferrin was low in the glands of virgin animals. In the glands of animals in early pregnancy, very high expression occurred in the ducts and immature alveoli, but expression tended to decrease as the alveoli matured. In the lactating and involuting gland, expression was generally low or absent in actively secreting alveoli and high in alveoli that had an accumulation of vesicles in the lumen and secretory epithelium, which was indicative of stasis. Occasionally, expression of lactoferrin was seen in cells that appeared to be secretory, particularly in involuting glands. Lactoferrin mRNA was expressed not only at different sites from other milk protein genes, such as alpha-lactalbumin and alpha s1-casein, but also during different stages of mammary development, supporting the view that the expression of lactoferrin is regulated differently from that of other milk proteins. For all ewes and cows, lactoferrin mRNA was detected in the epithelial ducts of the mammary parenchyma and the teat in a gradient that increased in ducts nearer the teats. The expression of lactoferrin in the ductal epithelium close to the teat was consistent with the antibacterial role of lactoferrin.

Animals↗
PubMed1994

Lactoferrin cDNA. Expression and in vitro mutagenesis.

The full length copy DNA (cDNA) for human lactoferrin has been synthesised by the polymerase chain reaction (PCR) using sequence specific primers. The template was first strand cDNA, synthesised from human bone marrow RNA using oligo(dT) to prime DNA synthesis by MMLV reverse transcriptase. The full-length human lactoferrin cDNA has been expressed in baby hamster kidney (BHK) cells using the expression vector pNUT. The protein expressed from the cloned cDNA is secreted into the culture medium and yields of up to 40 mg per litre have been obtained. A mutant protein corresponding to the N-lobe of human lactoferrin (LfN) has also been expressed in BHK cells. The cDNA coding for this protein was produced by the introduction of stop codons into the region of the cDNA corresponding to the helix linking the N- and C-lobes of the native protein. LfN is also expressed as a secreted protein and has been obtained in high yield. LfN binds iron and has UV/Vis and ESR spectra which are virtually identical to the native protein. However, the pH at which iron is released from LfN is quite different to the pH of iron release from the native and the full-length recombinant protein. A number of mutations have been introduced into LfN by site-directed mutagenesis and the mutant proteins expressed in BHK cells. These mutations involve the iron binding ligands and have been designed to introduce some of the changes found in the C-lobe of melanotransferrin into LfN. An attempt has been made to express a protein corresponding to the C-lobe of human lactoferrin (LfC) by attaching the sequence for the signal peptide of lactoferrin to the cDNA sequences coding for the C-lobe.

Amino Acid Sequence↗
PubMed1993

Structure of the recombinant N-terminal lobe of human lactoferrin at 2.0 A resolution.

The three-dimensional structure of the N-terminal half-molecule of human lactoferrin, LfN, prepared by recombinant DNA methods, has been determined by X-ray crystallography at 2.0 A resolution. The protein is in its iron-bound form and is deglycosylated. X-ray diffraction data were obtained by diffractometry to 3.2 A resolution and synchrotron data collection, using Weissenberg photography with imaging plates, to 1.8 A resolution. The structure was solved by molecular replacement, using the N-lobe of native diferric human lactoferrin (Lf) as search model. Restrained least squares refinement (program TNT) has resulted in a model structure with an R-factor of 0.184 for all data 34,180 (reflections) in the resolution range 8.0 to 2.0 A. The model comprises 2490 protein atoms (residues 4 to 327), 1 Fe3+, 1 CO3(2-) and 180 solvent molecules, all regarded as water. The structure of LfN is essentially the same as that of the N-lobe of intact Lf, being folded into two similar alpha/beta domains, with the Fe3+ and CO3(2-) bound in a specific site in the interdomain cleft. These details are not affected by either deglycosylation or expression in a non-native system. At the C terminus, however, the conformation of residues 321 to 333 is changed. Whereas in Lf residues 321 to 332 form a helix crossing between the domains at the back of the iron site, in LfN residues 321 to 326 have an extended conformation, forming a third interdomain beta-strand, and residues 328 to 333 appear disordered. The conformational change is attributed to the loss of stabilizing interactions from the C-lobe and is mediated by two Gly residues, at positions 321 and 323. It is further proposed that the conformational change is responsible for the more facile iron release properties of LfN, by its effect on the hinge mechanism and increased solvent exposure of residues near the back of the iron site. Other details of the polypeptide chain conformation and the binding site have also been analysed. Two cis-proline residues are found at positions 71 and 142. The bidentate binding of the CO3(2-) to the metal ion is unambiguous, and a network of hydrogen bonds in and around the binding site links the two domains. Clearly-defined amino-aromatic hydrogen bonds are found for Arg210, near the metal site, and some 31 internal water molecules have been identified, 15 of them in essentially discrete sites, and 16 in a cluster filling a cavity in the interdomain cleft.

Anions↗
PubMed1992

Preliminary crystallographic studies of the amino terminal half of human lactoferrin in its iron-saturated and iron-free forms.

The amino terminal half of human lactoferrin (LfN) produced from transfected baby hamster kidney cells has been crystallized in its iron-saturated and iron-free forms. The crystals of glycosylated LfN and deglycosylated LfN are monoclinic, space group C2, with cell dimensions a = 133.0 A, b = 58.3 A, c = 58.3 A, alpha = 90.0 degrees, beta = 114.7 degrees, gamma = 90.0 degrees, and one molecule per asymmetric unit. Crystals of apo LfN have also been prepared using deglycosylated protein. These crystals are tetragonal, space group P4(1)2(1)2 (or P4(3)2(1)2), with cell dimensions of a = b = 58.4 A and c = 217.2 A and one molecule per asymmetric unit. Both the iron-saturated and the iron-free crystals are suitable for high resolution X-ray analysis.

Apoproteins↗
PubMed1992

Studies of the N-terminal half of human lactoferrin produced from the cloned cDNA demonstrate that interlobe interactions modulate iron release.

The factors influencing iron binding and release by lactoferrin have been addressed by comparison of the native full length molecule (Lf) with the N-terminal half of human lactoferrin (LfN) produced from the cloned cDNA expressed in baby hamster kidney (BHK) cells. The coding sequences for LfN were inserted into the expression vector pNUT between the metallothionein promoter and the human growth hormone transcription termination sequences. Transformed BHK cells were grown in roller bottles where concentrations of LfN as high as 35 mg/liter were obtained. The pure protein, produced by the transformed BHK cells, was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, protein blotting and immunodetection, N-terminal sequence analysis, UV-visible spectroscopy, electron spin resonance spectroscopy, and measurements of metal binding and release. By these criteria LfN was found to be correctly processed, glycosylated, and able to bind iron reversibly. Both UV-visible and electron spin resonance spectra of the half molecule were very similar to those of native lactoferrin and the full length lactoferrin produced in BHK cells, but there were marked differences in the pH at which iron release occurred. Iron release from LfN occurs in the pH range 6.0-4.0, compared with 4.0-2.5 for native lactoferrin and 6.2-4.0 for transferrin. These results suggest that the more facile release of iron from LfN compared with native lactoferrin results from the absence of stabilizing contacts between the N- and C-terminal halves and that the characteristic difference in pH stability between lactoferrins and transferrins is due primarily to differences in these interactions.

Amino Acid Sequence↗
PubMed1991

Expression of cloned human lactoferrin in baby-hamster kidney cells.

Human lactoferrin was expressed from a cloned cDNA introduced into mammalian cells in tissue culture. Total RNA was extracted from human bone marrow, and lactoferrin cDNA was synthesized by primer-specific polymerase chain reaction after oligo(dT)-primed first-strand synthesis. The cDNA was sequenced to confirm its identity with previously published human lactoferrin sequences and cloned into the eukaryotic expression vector pNUT. Recombinant vector DNA containing the human lactoferrin sequence was introduced into baby-hamster kidney (BHK) cells in culture, and stable transfectants were produced by dominant marker selection. Human lactoferrin was expressed from the metallothionein promoter of pNUT by Zn2+ induction. The protein was secreted into the tissue-culture medium and was subsequently purified to homogeneity in a single step. Initial characterization suggests that the protein expressed by BHK cells is identical with native human lactoferrin.

Amino Acid Sequence↗
PubMed1983

The immunological and catalytically active form of L-type pyruvate kinase in rat liver cytosol.

The protein species precipitated from rat liver cytosol by rabbit antisera raised to pure L-type pyruvate kinase were investigated by sodium dodecyl sulphate gel electrophoresis. The primary antisera (anti-L-type pyruvate kinase) precipitated protein species with mol. wts 56,000, 41,000 and 39,000. The 41,000 mol. wt protein was identified as fructose-bis-phosphatase. Double diffusion and immunotitration experiments established that L-type pyruvate kinase and fructose-bis-phosphatase shared common antigenic determinants. This information enabled an improved antiserum (anti-LPK) to be obtained. The use of anti-LPK showed that the 56,000 mol. wt subunit was the only catalytically and immunologically active form of L-type pyruvate kinase in liver. This was confirmed by biosynthetic experiments with cultured hepatocytes. The specific activity of the enzyme in liver extracts was also determined by quantitative immunotitration with anti-LPK. Despite changes in dietary status which varied the concentration of enzyme protein, the maximum specific activity of the enzyme remained constant and essentially the same as that of pure enzyme.

Animals↗