Identification of three oligosaccharide binding sites in ricin.

The galactoside-binding sites of ricin B chain can be blocked by affinity-directed chemical modification using a reactive ligand derived from asialoglycopeptides containing triantennary N-linked oligosaccharides. The terminal galactosyl residue of one branch of the triantennary oligosaccharide is modified to contain a reactive dichlorotriazine moiety. Two separate galactoside-binding sites have been clearly established in the ricin B chain by X-ray crystallography [Rutenber, E., and Robertus, J. D. (1991) Proteins 10, 260-269], and it is necessary to covalently attach two such reactive ligands to the B chain to block its binding to galactoside affinity matrixes. A method was developed using thiol-specific labeling of the ligand combined with subsequent immunoaffinity chromatography which allowed the isolation of ricin B chain peptides covalently linked to the ligand from proteolytic digests of purified blocked ricin. The sites of covalent attachment of the two ligands in blocked ricin were inferred from sequence analysis to be Lys 62 in domain 1 of the B chain and Tyr 148 in domain 2. A minor species of blocked ricin contains a third covalently attached ligand. From the analysis of peptides derived from blocked ricin enriched in this species, it is inferred that Tyr 67 in domain 1 is the specific site on the ricin B chain where a third reactive ligand becomes covalently linked to the protein. These results are interpreted as providing support for the notion that the ricin B chain has three oligosaccharide binding sites.     

Design, synthesis and evaluation of biomimetic affinity ligands for elastases

A low-molecular-weight biomimetic affinity ligand selective for binding elastase has been designed and synthesized. The ligand was based on mimicking part of the interaction between a natural inhibitor, turkey ovomucoid inhibitor and elastase, and modelled from the X-ray crystallographic structure of the enzyme-inhibitor complex. Limited solid-phase combinatorial chemistry was used to synthesize 12 variants of the lead ligand using the triazine moiety as the scaffold for assembly. The ligand library was screened for its ability to bind elastase and trypsin, and two ligands were studied further. Ligand C4/6 [2-alanyl-alanyl-4-tryptamino-6-(alpha-lysyl)-s-triazine] was found to bind porcine pancreatic elastase, but not trypsin, with a dissociation constant of 6 x 10(-5) M and a binding capacity of 21 mg elastase per ml gel. The adsorbent was used to purify elastase from a crude extract of porcine pancreas. Immobilized ligand C4/5 6 [2-alanyl-alanyl-4-tyramino-6-(alpha-lysyl)-s-triazine] was similarly chosen for optimal binding of elastase from cod and used to purify the enzyme from a crude extract of cod pyloric caeca. Ligand C4/6 was subsequently synthesized in solution and its structure verified by 1H-NMR.     

Purification of human collagenases with a hydroxamic acid affinity column

Human collagenase has been isolated from skin fibroblasts and rheumatoid synovium by using an affinity matrix, prepared by coupling Pro-Leu-Gly-NHOH to agarose. Following the methodology described herein, the skin enzyme was isolated in two steps in 76% yield and the synovial enzyme was purified in three steps in 71% yield. Importantly, each enzyme hydrolyzed collagen into 3/4-1/4 cleavage fragments, indicating that a true collagenase had been isolated. The column was specific for the human enzyme since the collagenase from Clostridium histolyticum did not bind. The affinity ligand was designed according to the formalism proposed by Holmquist and Vallee [Holmquist, B., & Vallee, B. L. (1979) Proc. Natl. Acad. Sci. U.S.A. 76, 6216] that effective metalloenzyme inhibitors can be synthesized by coupling a suitable metal-coordinating group to a substrate analogue. In this case, the hydroxamic acid probably coordinates to the active-site metal and the Pro-Leu-Gly moiety is similar to the carboxyl side of the cleavage site of collagen, the enzyme's substrate. The IC50 for N-(benzyloxycarbonyl)-Pro-Leu-Gly-NHOH is 4 X 10(-5) M for both enzymes. The affinity chromatographic procedures described here should aid in future studies on vertebrate collagenases.     

Isolation and characterization of protein A24, a "histone-like" non-histone chromosomal protein.

In earlier studies, the nucleolar levels of protein A24 were found to be markedly decreased in the nucleolar hypertrophy induced by thioacetamide or during liver regeneration (Ballal, N.R., Goldknopf, I.L., Goldberg, D.A., and Busch, H. (1974) Life Sci. 14, 1835-1845; Ballal, N.R., Kang, Y.-J., Olson, M.O.-J., and Busch, H.J. Biol. Chem. 250, 5921-5925). To determine the role of protein A24, methods were developed for its isolation in highly purified form. Milligram quantities of highly purified protein A24 were isolated from the 0.4 N H2SO4-soluble proteins of calf thymus chromatin by exclusion chromatography on Sephadex G-100, followed by preparative polyacrylamide gel electrophoresis. Protein A24 was highly purified as shown by its migration as a single spot on two-dimensional polyacrylamide gel electrophoresis, its single NH2-terminal amino acid, methionine, and the production of approximately 50 peptides by tryptic digestion. Like histones 2A, 2B, 3, and 4. A24 was extractable from chromatin with 0.4 N H2SO4 or 3 M NaCl/7 M urea, but unlike most non-histone proteins or histone 1, protein A24 was not extracted with 0.35 M NaCl, 0.5 M HClO4, or 0.6 M NaCl. Protein A24 was present in only 1.9% of the total amount of histones 2A, 2B, 3 and 4; its molecular weight is 27,000.     

Characterization of monoclonal antibodies to the beta-adrenergic antagonist alprenolol as models of the receptor binding site

Antibodies to receptor ligands have been valuable in understanding the nature of receptor-ligand interactions. We have developed four monoclonal antibodies to the beta-adrenergic receptor antagonist alprenolol by immunizing A/J mice with (-)-alprenolol coupled to keyhole limpet hemocyanin. The antisera from these mice displayed specific [3H]dihydroalprenolol ([3H]DHA) binding that was inhibited by alprenolol, propranolol, and isoproterenol. Somatic cell fusion of spleen cells from the immunized mice to SP2/0 myeloma cells, followed by limited dilution subcloning, resulted in the isolation of four hybridomas (1B7, 5B7, 5D9, and 2G9) demonstrating three different classes of ligand binding characteristics. 1B7 had the highest binding affinity for antagonists based on Scatchard analysis (Kd [125I]- CYP = 1.4 X 10(-10) M; Kd [3H]DHA = 6.5 X 10(-9) M), and was the only antibody to demonstrate agonist-inhibition of [3H]DHA binding. Ki values computed from competitive inhibition curves of [3H]DHA binding to 1B7 resulted in a rank order of potency similar to that of beta-2-adrenergic receptors: (-)-propranolol greater than acebutolol amine greater than isoproterenol greater than (+)-propranolol greater than epinephrine greater than norepinephrine. 5B7 and 5D9 exemplified a second class of antibody. This pair had lower antagonist binding affinities (Kd [3H]DHA = 2 X 10(-8) M and 2.5 X 10(-7) M, respectively) and was stereoselective in binding receptor antagonists: (-)-propranolol greater than (+)-propranolol greater than acebutolol amine. Agonist inhibition of [3H]DHA binding to these antibodies could only be observed at very high concentrations (greater than 10(-4) M agonist), and was not dose-dependent. Finally, the class of anti-alprenolol monoclonal antibodies represented by 2G9 had the lowest antagonist binding affinity of all (IC50 alprenolol = 1 X 10(-5) M), did not demonstrate ligand stereoselectivity, and did not recognize agonists. We propose that antibodies raised against beta-adrenergic receptor ligands demonstrating stereoselective agonist binding will also demonstrate high affinity antagonist binding, and that they will closely parallel the binding characteristics of the receptor. According to this "agonist best-fit hypothesis," anti-idiotypic antibodies raised against the binding site of these idiotypes might contain true mirror images of the beta-adrenergic receptor binding site.     

Protein-ligand interaction. A calorimetric study of the interaction of oligosaccharides and hen ovalbumin glycopeptides with concanavalin A

A calorimetric study is reported concerning the interaction between concanavalin A (Con A) and some oligosaccharides and glycopeptides hydrolyzed from hen ovalbumin. The measurements were carried out in acetate buffer, pH 4.5, where, by far, the prevailing form of the protein is the dimeric one [Kalb, A.J., & Lustig, A. (1968) Biochim. Biophys. Acta 168, 366; Dani, M., Manca, F., & Rialdi, G. (1981) Biochim. Biophys. Acta 667, 108]. The calorimetric technique allows the direct determination of the binding enthalpy delta H, degrees B, the evaluation of the apparent association constant K'B, and then the evaluation of the apparent free energy and entropy, delta G degrees' B and delta S degrees' B. Three groups of data have been collected in the present study. The first one concerns the interaction between concanavalin A and some mono- and disaccharides [methyl alpha-glucopyranoside (alpha MGlup), methyl alpha-mannopyranoside (alpha MManp), D-maltose, D-trehalose, and D-cellobiose]. The analysis of the data indicates that in these cases there are small favorable entropic and enthalpic contributions to the affinity. The stoichiometry of the reaction is 2 mol of ligand/mol of Con A dimer, the sites resulting being equivalent and noninteracting. Melezitose, the only trisaccharide studied, shows a different behavior: its affinity for Con A is higher as compared to the other oligosaccharides containing alpha-glucosyl residues and closer to that of methyl alpha-mannopyranoside. However, the stoichiometry is different, namely, 1 mol of ligand/dimer of Con A.(ABSTRACT TRUNCATED AT 250 WORDS)     

Antigen binding thermodynamics and antiproliferative effects of chimeric and humanized anti-p185HER2 antibody Fab fragments.

The murine monoclonal antibody 4D5 (anti-p185HER2) inhibits the proliferation of human tumor cells overexpressing p185HER2 in vitro and has been "humanized" [Carter, P., Presta, L., Gorman, C. M., Ridgway, J. B. B., Henner, D., Wong, W.-L. T., Rowland, A. M., Kotts, C., Carver, M. E., & Shepard, H. M. (1992) Proc. Natl. Acad. Sci. U.S.A. (in press)] for use in human cancer therapy. We have determined the antigen binding thermodynamics and the antiproliferative activities of chimeric 4D5 Fab (ch4D5 Fab) fragment and a series of eight humanized Fab (hu4D5 Fab) fragments differing by amino acid substitutions in the framework regions of the variable domains. Fab fragments were expressed by secretion from Escherichia coli and purified from fermentation supernatants by using affinity chromatography on immobilized streptococcal protein G or staphylococcal protein A for ch4D5 and hu4D5, respectively. Circular dichroism spectroscopy indicates correct folding of the E. coli produced Fab, and scanning calorimetry shows a greater stability for hu4D5 (Tm = 82 degrees C) as compared with ch4D5 Fab (Tm = 72 degrees C). KD values for binding to the extracellular domain (ECD) of p185HER2 were determined by using a radioimmunoassay; the delta H and delta Cp for binding were determined by using isothermal titration calorimetry. ch4D5 Fab and one of the humanized variants (hu4D5-8 Fab) bind p185HER2-ECD with comparable affinity (delta G degrees = -13.6 kcal mol-1).(ABSTRACT TRUNCATED AT 250 WORDS)     

Isolation and structural characterization of an insulin-related molecule, a predominant neuropeptide from Locusta migratoria

Neurohaemal lobes of corpora cardiaca of Locusta migratoria are an established storage site for neurohormones produced by the neurosecretory cells of the brain. As previously reported [Hietter, H., Van Dorsselaer, A., Green, B., Denoroy, L., Hoffmann, J.A. & Luu, B. (1990) Eur. J. Biochem. 187, 241-247], the isolation and characterization of a novel 5-kDa peptide from these lobes served as the basis for oligonucleotide screening of cDNA libraries prepared from poly(A) RNA from neurosecretory cells of the central nervous system. From subsequent cDNA cloning studies [Lagueux, M., Lwoff, L., Meister, M., Goltzené, F. & Hoffmann, J.A. (1990) Eur. J. Biochem. 187, 249-254], the existence of a 145-residue precursor protein was deduced, which contained, in addition to the 5-kDa peptide, amino-acid sequences with homology to the A and B chains of an insulin-related peptide. In the present study we have isolated the native molecule from corpora cardiaca of Locusta and characterized, by Edman degradation and plasma-desorption mass spectrometry, the two chains as follows: A chain, Gly-Val-Phe-Asp-Glu-Cys-Cys-Arg-Lys-Ser-Cys-Ser-Ile-Ser-Glu-Leu-Gln-Thr- Tyr-Cys - Gly (Ile, isoleucine); B chain, Ser-Gly-Ala-Pro-Gln-Pro-Val-Ala-Arg-Tyr-Cys-Gly-Glu-Lys-Leu-Ser-Asn-Ala- Leu-Lys - Leu-Val-Cys-Arg-Gly-Asn-Tyr-Asn-Thr-Met-Phe. Taken in conjunction with the previous cloning studies, our data lead to a clear picture of the processing of Locusta preproinsulin. They indicate that locusta corpora cardiaca contain remarkably large amounts of one single insulin form, in contrast to multiple insulin isoforms of Bombyx mori, the only other insect species from which insulin-related peptides have been isolated and characterized [Nagasawa, H., Kataoka, H., Isogai, A., Tamura, S., Suzuki, A., Mizoguchi, A., Fujiwara, Y., Suzuki, A., Takahashi, S. & Ishizaki, H. (1986) Proc. Natl Acad. Sci. USA 83, 5840-5843].     

Isolation of full-length putative rat lysophospholipase cDNA using improved methods for mRNA isolation and cDNA cloning

We have cloned a full-length putative rat pancreatic lysophospholipase cDNA by an improved mRNA isolation method and cDNA cloning strategy. These new methods allow the construction of a cDNA library from the adult rat pancreas in which the majority of recombinant clones contained complete sequences for the corresponding mRNAs. A previously recognized but unidentified long and relatively rare cDNA clone containing the entire sequence from the cap site at the 5' end to the poly(A) tail at the 3' end of the mRNA was isolated by single-step screening of the library. The size, amino acid composition, and the activity of the protein expressed in heterologous cells strongly suggest this mRNA codes for lysophospholipase [Van den Bosch, H., Aarsman, A. J., DeJong, G. N., & Van Deenen, L. M. (1973) Biochim. Biophys. Acta 296, 94-104].     

Structure-activity studies of the inhibition of FabI,the enoyl reductase from Escherichia coli,by triclosan: kinetic analysis of mutant FabIs

Triclosan, a common antibacterial additive used in consumer products, is an inhibitor of FabI, the enoyl reductase enzyme from type II bacterial fatty acid biosynthesis. In agreement with previous studies [Ward, W. H., Holdgate, G. A., Rowsell, S., McLean, E. G., Pauptit, R. A., Clayton, E., Nichols, W. W., Colls, J. G., Minshull, C. A., Jude, D. A., Mistry, A., Timms, D., Camble, R., Hales, N. J., Britton, C. J., and Taylor, I. W. (1999) Biochemistry 38, 12514-12525], we report here that triclosan is a slow, reversible, tight binding inhibitor of the FabI from Escherichia coli. Triclosan binds preferentially to the E.NAD(+) form of the wild-type enzyme with a K(1) value of 23 pM. In agreement with genetic selection experiments [McMurry, L. M., Oethinger, M., and Levy, S. B. (1998) Nature 394, 531-532], the affinity of triclosan for the FabI mutants G93V, M159T, and F203L is substantially reduced, binding preferentially to the E.NAD(+) forms of G93V, M159T, and F203L with K(1) values of 0.2 microM, 4 nM, and 0.9 nM, respectively. Triclosan binding to the E.NADH form of F203L can also be detected and is defined by a K(2) value of 51 nM. We have also characterized the Y156F and A197M mutants to compare and contrast the binding of triclosan to InhA, the homologous enoyl reductase from Mycobacterium tuberculosis. As observed for InhA, Y156F FabI has a decreased affinity for triclosan and the inhibitor binds to both E.NAD(+) and E.NADH forms of the enzyme with K(1) and K(2) values of 3 and 30 nM, respectively. The replacement of A197 with Met has no impact on triclosan affinity, indicating that differences in the sequence of the conserved active site loop cannot explain the 10000-fold difference in affinities of FabI and InhA for triclosan.     

Atypical beta-adrenergic receptor in 3T3-F442A adipocytes. Pharmacological and molecular relationship with the human beta 3-adrenergic receptor.

Expression of ligand binding properties for an atypical beta-adrenergic receptor (beta-AR) subtype was studied during the adipose differentiation of murine 3T3-F442A cells and compared with that of the human beta 3-AR expressed in Chinese hamster ovary cells stably transfected with the human beta 3-AR gene (CHO-beta 3 cells) Emorine, L. J., Marullo, S., Briend-Sutren, M. M., Patey, G., Tate, K., Delavier-Klutchko, C., and Strosberg, A. D. (1989) Science 245, 1118-1121). 3T3-F442A adipocytes exhibited high and low affinity binding sites for (-)-4-(3-t-butylamino-2-hydroxypropoxy) [5,7-3H]benzimidazole-2-one ((-)-[3H]CGP-12177) (KD = 1.2 and 38.3 nM) and (-)-[125I]iodocyanopindolol ([125I]CYP) (KD = 47 and 1,510 pM). The high affinity sites corresponded to the classical beta 1- and beta 2-AR subtypes whereas the KD values of the low affinity sites for the radioligands were similar to those measured in CHO-beta 3 cells (KD = 28 nM and 1,890 pM for (-)-[3H]CGP12177 and [125I]CYP, respectively). These low affinity sites were undetectable in preadipocytes but represented about 90% of total beta-ARs in adipocytes. The atypical beta-AR and the human beta 3-AR add similarly low affinities (Ki = 3-5 microM) for (+/-)-(2-(3-carbamoyl-4-hydroxyphenoxy)ethylamino-3)-(4-(1-methyl- 4- trifluormethyl-2-imidazolyl)-phenoxy)-2-propanol methane sulfonate (CGP20712A) or erythro-(+/-)-1-(7-methylindan-4-yloxy)-3-isopropylaminob utan-2-ol (ICI118551), highly selective beta 1- and beta 2-AR antagonists, respectively, in agreement with the poor inhibitory effect of the compounds on (-)-isoproterenol (IPR)-stimulated adenylate cyclase activity. Atypical beta-AR and beta 3-AR had an affinity about 10-50 times higher for sodium-4-(2-[2-hydroxy-2-(3-chlorophenyl)ethylamino]propyl)phenoxyace tate sesquihydrate (BRL37344) than the beta 1-AR subtype. This correlates with the potent lipolytic effect of BRL37344 in adipocytes. The rank order of potency of agonists in functional and binding studies was BRL37344 greater than IPR less than (-)-norepinephrine greater than (-)-epinephrine both in 3T3 adipocytes and CHO-beta 3 cells. As in CHO-beta 3 cells, the classical beta 1- and beta 2-antagonists CGP12177, oxprenolol, and pindolol were partial agonists in adipocytes. Although undetectable in preadipocytes, a major mRNA species of 2.3 kilobases (kb) and a minor one of 2.8 kb were observed in adipocytes by hybridization to a human beta 3-AR specific probe.(ABSTRACT TRUNCATED AT 400 WORDS)     

The kangaroo cation-independent mannose 6-phosphate receptor binds insulin-like growth factor II with low affinity.

The mammalian cation-independent mannose 6-phosphate receptor (CI-MPR) binds mannose 6-phosphate-bearing glycoproteins and insulin-like growth factor (IGF)-II. However, the CI-MPR from the opossum has been reported to bind bovine IGF-II with low affinity (Dahms, N. M., Brzycki-Wessell, M. A., Ramanujam, K. S., and Seetharam, B. (1993) Endocrinology 133, 440-446). This may reflect the use of a heterologous ligand, or it may represent the intrinsic binding affinity of this receptor. To examine the binding of IGF-II to a marsupial CI-MPR in a homologous system, we have previously purified kangaroo IGF-II (Yandell, C. A., Francis, G. L., Wheldrake, J. F., and Upton, Z. (1998) J. Endocrinol. 156, 195-204), and we now report the purification and characterization of the CI-MPR from kangaroo liver. The interaction of the kangaroo CI-MPR with IGF-II has been examined by ligand blotting, radioreceptor assay, and real-time biomolecular interaction analysis. Using both a heterologous and homologous approach, we have demonstrated that the kangaroo CI-MPR has a lower binding affinity for IGF-II than its eutherian (placental mammal) counterparts. Furthermore, real-time biomolecular interaction analysis revealed that the kangaroo CI-MPR has a higher affinity for kangaroo IGF-II than for human IGF-II. The cDNA sequence of the kangaroo CI-MPR indicates that there is considerable divergence in the area corresponding to the IGF-II binding site of the eutherian receptor. Thus, the acquisition of a high-affinity binding site for regulating IGF-II appears to be a recent event specific to the eutherian lineage.     

An oligodeoxynucleotide affinity column for the isolation of sequence specific DNA binding proteins.

A nucleic acid affinity matrix containing a short oligodeoxynucleotide ligand has been prepared as an example of a material which can be used for the rapid and effective isolation of sequence specific DNA binding proteins. Two complementary oligodeoxynucleotides have been employed, one of which contains a small 5'-spacer arm with a terminal thiol group. Using this terminal thiol group, the ligand can be covalently coupled to Tresyl-activated Sepharose 4B or Epoxy-activated Sepharose 6B via a thioether linkage. This approach allows the specific attachment of the nucleic acid ligand via its 5'-terminus to the insoluble matrix. The double stranded affinity material was obtained by annealing of the complementary DNA fragment. As an example, we have used an eicosomer affinity column containing the sequence d(GAATTC) for the isolation of the Eco RI restriction endonuclease. Using a single column, the enzyme could be isolated by eluting the column with a single step or multistep gradient of increasing salt concentration. The enzyme was purified to 75%-85% homogeneity with yields of 0.1 mg to 0.2 mg from 0.5 g of cell paste.     

Analysis of ribonuclease-nucleotide interactions by quantitative affinity chromatography.

Quantitative affinity chromatography on uridine-5'-(Sepharose-4-aminophenylphosphoryl)-2'(3')-phosphate was developed for the study of binding of ribonuclease species to nucleotide ligands. Elution of the native species ribonuclease-A and -S on the afffinity matrix in 0.4 M ammonium acetate, pH 5.2, containing various amounts of the soluble competing ligand 2'-cytidine monophosphate, reveals an inverse response of elution volume to concentration of soluble ligand. This response conforms to behavior expected for the competing binding equilibria enzyme-soluble ligand and enzyme-insoluble ligand. A-NALYSIS OF ELUTION DATA ALLOWS CALCULATION OF KI and KIM, the dissociation constants, respectively, for the soluble and insoluble protein-ligand complexes. The values of these chromatographically derived constants are similar to values of dissocation constants determined in solution by kinetics of inhibition by 2'-cytidine monophosphate and uridine-5'-(j-aminophenylphosphoryl)-2'(3')-phosphate. Successful competitive elution experiments with [p-F-Phe8]semisynthetic ribonuclease-S' and individual elution trials for [4-F-His12]semisynthetic ribonuclease-S' indicate the utility of the quantitative affinity chromatographic technique for determination of ligand binding properties of ribonuclease derivatives, including inactive species. Nonbiospecific aspects of the interaction of ribonuclease with the affinity matrix in ammonium acetate buffers of concentrations 0.1 M and below were noted, delinating limits of conditions allowing the biospecificity needed for ligand-binding analyses by competitive elution. The dependence of ribonuclease competitive elution behavior on the amount of protein eluted also was examined and related to theoretical considerations in the quantitative application of affinity chromatography.     

One-step affinity purification of amidase from mutant strains of Pseudomonas aeruginosa

Amidases (acylamide amidohydrolase EC 3.5.1.4) from mutant strains (i.e., B6, AI3, AIU1N, OUCH 4 and L10) of Pseudomonas aeruginosa were purified in one-step by ligand affinity chromatography using Epoxy-activated Sepharose 4B-acetamide. The yields of the purified enzymes were about 90% for all mutant strains with purification factors of about 10 and were apparently homogeneous when analysed by SDS-PAGE and native PAGE. The protein bands on native PAGE coincided with the stained band of enzyme activity for all amidase preparations. Affinity columns had a maximum binding capacity of 0.5 mg amidase protein/ml of sedimented gel and could be regenerated and reused several times without any loss of binding capacity and resolution. Affinity gels containing either semicarbazide or urea were also found useful for the isolation of amidase. The differences in substrate specificity of these amidases reported previously were also observed in the elution behaviour of these enzymes from the affinity columns.     

The sites of thyroid hormone formation in rabbit thyroglobulin

Rabbit thyroglobulin (Tg) was labeled in vivo with 125I and purified by gel filtration. Separation by high performance liquid chromatography (HPLC) of tryptic digests of S-cyanoethylated Tg yielded four major iodothyronine-containing peaks, designated A, B, C, and D. These were further purified on HPLC and sequenced for identification of amino acid residues and for location of the iodothyronine by 125I counting. The published primary structure for bovine Tg, derived from cDNA sequencing of the Tg gene (Mercken, L., Simons, M.J., Swillens, S., Massaer, M., and Vassart, G. (1985) Nature 316, 647-651), permitted tentative location of the rabbit hormonogenic peptides within the Tg polypeptide chain. Site A, corresponding to bovine residue 5, contained 44% of Tgs [125I]T4 (thyroxine) and 25% of its [125I]T3 (triiodothyronine); its specific activity of iodine was higher than that for other sites, indicating priority of iodination. Site B, containing 24% of Tgs [125I]T4 and 18% of its [125I]T3, corresponded to bovine residue 2555. Site C, at the third residue from the C terminus (bovine residue 2748), was the major T3 site, accounting for over 50% of Tgs [125I]T3. The amino acid sequence around this site shows less homology among different animal species than do those flanking the other hormonogenic sites. Site D accounted for 17% of Tgs [125I]T4 and corresponded to bovine Tyr-1291, in the midportion of Tgs polypeptide chain. The three major T4-forming sites had the sequence Asp-Tyr (sites B and D) or Glu-Tyr (site A), while the sequence Ser-Tyr-Ser appeared to favor T3 synthesis (site C), suggesting an important influence of primary structure on hormonogenesis. We conclude that site A is the major T4-forming site and site C the major T3-forming one, but others are available and offer the opportunity for flexibility in meeting different demands for hormone formation.     

Conformational substates of the oxyheme centers in alpha and beta subunits of hemoglobin as disclosed by EPR and ENDOR studies of cryoreduced protein

Exposure of frozen solutions of oxyhemoglobin to gamma-irradiation at 77 K yields EPR- and ENDOR-active, one-electron-reduced oxyheme centers which retain the conformation of the diamagnetic precursor. EPR spectra have been collected for the centers produced in human HbO(2) and isolated alphaO(2) and betaO(2) chains, as well as alphaO(2)beta(Zn), alpha(Zn)betaO(2), and alphaO(2)beta(Fe(3+)) hybrids, each in frozen buffer and in frozen glasses that form in the presence of glycols and sugars and also in the presence of IHP. These reveal two spectroscopically distinct classes of such ferriheme centers (g(1) 相似文献   

Membrane binding characteristics of two forms of transforming growth factor-beta

Cartilage-inducing factors A and B (CIF-A and CIF-B) from bovine bone have recently been identified as transforming growth factor-beta (TGF-beta) (Seyedin, S.M., Thompson, A. Y., Bentz, H., Rosen, D. M., McPherson, J. M., Conti, A., Siegel, N. R., Galluppi, G. R., and Piez, K. A. (1986) J. Biol. Chem., 261, 5693-5695) and a unique protein homologous to TGF-beta (Seyedin S. M., Segarini, P. R., Rosen, D. M., Thompson, A. Y., Bentz, H., and Graycar, J. (1987) J. Biol. Chem., 262, 1946-1949), respectively. Although the biological activities of TGF-beta and CIF-B are similar, the divergence of CIF-B from the highly conserved amino acid sequence of TGF-beta prompted an investigation of its receptor binding properties. Three classes of cell surface binding components were identified. Class A has exclusive affinity for TGF-beta; class B has greater affinity for CIF-B; and class C has equal affinity for both proteins. A high molecular weight component, the predominant binding species, was further characterized and shown to consist of two components that are either class B or class C. The differential binding properties of TGF-beta and CIF-B to cell surface components suggest that there are biological activities unique to each of the proteins.     

Inhibition of sequestration of human B2 bradykinin receptor by phenylarsine oxide or sucrose allows determination of a receptor affinity shift and ligand dissociation in intact cells

Depending on their interaction with intracellular proteins, G protein-coupled receptors (GPCR) often display different affinities for agonists at 37 degrees C. Determining the affinity at that temperature is often difficult in intact cells as most GPCRs are internalized after activation. When sequestration of the B2 bradykinin receptor (B2R) was inhibited by either 0.5 M sucrose or phenylarsine oxide (PAO), a shift in the affinity was detected when the incubation temperature was raised from 4 degrees C to 37 degrees C or lowered from 37 degrees C to 4 degrees C. In contrast, binding of the antagonist [3H]NPC 17731 was temperature-independent. B2R mutants displayed different affinity shifts allowing conclusions on the role of the involved amino acids. By inhibiting receptor sequestration it was possible to determine also dissociation of [3H]BK and of [3H]NPC 17731 from intact cells at 37 degrees C. Surprisingly, both dissociation rates were markedly enhanced by the addition of unlabeled ligand, most likely via prevention of reassociation of dissociated [3H]ligand. This suggests that dissociated [3H]ligand cannot move freely away from the receptor. In summary, our data demonstrate that inhibition of receptor internalization either by PAO or sucrose provides an excellent method to study receptor function and the effects of mutations in intact cells.