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G A Homandberg

Publications and source records attributed to G A Homandberg.

69 records · Page 4Linked to original sources
PubMed1985

Heparin-binding fragments of fibronectin are potent inhibitors of endothelial cell growth.

Two heparin-binding proteolytic fragments of fibronectin--an amino-terminal 29-kd segment and a carboxyl-terminal 40-kd segment-are apparently specific, potent inhibitors of the growth of cultured bovine aortic endothelial cells and inhibit growth in a reversible, dose-dependent manner. In contrast, native fibronectin at higher dosages neither inhibits nor interferes with the effects of the 29-kd fragment. The data, therefore, suggest that fibronectin fragments may participate in the regulation of vascular growth.

Animals↗
PubMed1984

The isoinhibitors of chymotrypsin/elastase from Ascaris lumbricoides: the reactive site.

Five isoinhibitors of chymotrypsin/elastase present in aqueous extracts of Ascaris were isolated. The reactive site in each isoinhibitor, the peptide bond that during encounter is positioned over the catalytic site in chymotrypsin, is Leu-Met. This bond was hydrolyzed by incubating intact isoinhibitors with 5-25 mol% chymotrypsin at pH 3.2 for 4-6 days (isoinhibitor 1) or 2.5-5 weeks (isoinhibitors 2-5). The reaction under these conditions did not proceed beyond 60% modified isoinhibitor (peptide bond hydrolyzed) and 40% intact inhibitor. The Leu-Met bond, hydrolyzed in modified isoinhibitor, can be resynthesized at pH 7.6 by incubating modified inhibitor with a stoichiometric amount of chymotrypsin bound to Sepharose CL-4B and then dissociating the complex in a kinetically controlled fashion with 5% trichloroacetic acid. The product, intact inhibitor, was obtained in greater than 80% yield. The site in the isoinhibitor that is positioned over the catalytic site in elastase during encounter is the same as for encounter with chymotrypsin. The Leu-Met bond hydrolyzed during encounter with elastase can be resynthesized by chymotrypsin. Chymotrypsin and elastase bind to the inhibitor at the same site.

Animals↗
PubMed1984

Human platelet fibrinogen gamma chain structure.

Human plasma fibrinogen is produced by liver parenchymal cells. Such molecules contain two classes of gamma-chains (gamma A, gamma'), which differ with respect to their COOH-terminal sequences. When fibrin is crosslinked in the presence of factor XIIIa and Ca2+, three types of gamma-dimer are formed (gamma A-gamma A; gamma A-gamma'; gamma'-gamma'). A separate fibrinogen pool is located in platelet alpha-granules. We analyzed this fibrinogen to determine whether gamma'-chains were present to the same extent (7%) that they are found in plasma fibrinogen. Electrophoretic analysis (Laemmli system) of reduced samples of the clot that formed subsequent to release of fibrinogen from thrombin-stimulated washed platelets, revealed a single crosslinked gamma-dimer band in the gamma A-gamma A position. Material collected into EDTA-containing buffer and subsequently crosslinked in the presence of added factor XIII and Ca2+ also revealed a gamma A-gamma A dimer band. This finding was further investigated by Western blotting of reduced gel specimens that had been reacted with an anti-gamma-chain antibody followed by 125I-labeled protein A. A single type of gamma-chain, gamma A, was present in the fibrinogen from a Triton X-100 or 10 M urea platelet lysate, or in noncrosslinked fibrin obtained from thrombin-treated platelets. Crosslinked reduced fibrin from thrombin-treated platelets or that prepared from the Triton lysate revealed a single type of gamma-dimer, gamma A-gamma A. We conclude that there are no gamma'-chains (less than 1%) in platelet fibrinogen. This structural difference from hepatic fibrinogen probably results from differences in the processing and/or regulation of the fibrinogen gamma-chain gene in megakaryocytes.

Afibrinogenemia↗
PubMed1983

Separation and analysis of the major forms of plasma fibronectin.

Human plasma fibronectin exists in circulation in multiple molecular forms that are distinguishable by SDS-polyacrylamide gel electrophoresis (zone I, approx. 450 kDa dimers; zone II, 190-235 kDa; Zone III, 146-175 kDa). (Chen, A.B., Amrani, D.L. and Mosesson, M.W. (1977) Biochim. Biophys. Acta 493, 310-322). We report here on investigations of plasma fibronectin that had been purified from the 'heparin-precipitable fraction' of plasma by DEAE-cellulose chromatography using buffers containing a chaotropic salt (KSCN). Zone I fibronectin and zone II fibronectin were subsequently separated by Sepharose CL-6B chromatography in the presence of 0.3 M KSCN. Electrophoresis of reduced zone I fibronectin dimers showed the presence of three types of subunits (i.e., 220 kDa, 215 kDa, 207 kDa), evidently all having the same NH2-terminal sequence. Subunits of this size were also found in reduced zone II fibronectin, as well as another polypeptide of 190 kDa, the latter amounting to under 5% of the total. Unreduced zone I fibronectin was resolved by gel electrophoresis into a doublet. The upper component amounted to approx. 90% of the total and was comprised of 220 kDa and/or 215 kDa subunits; the lower component contained 207 kDa plus a 220 kDa or 215 kDa subunit. Scanning transmission electron microscopy indicated that under physiologic conditions zone II fibronectin molecules, like those in zone I, exist as pleiomorphic, loosely folded structures (approx. 16 X 8-12 nm) that are somewhat smaller than dimeric zone I molecules (approx. 24 X 16 nm). Circular dichroic spectral analyses suggests that both types have similarly folded local domains. Affinity chromatography experiments revealed a relative decrease in the binding of zone II fibronectin to gelatin but no difference from zone I fibronectin with respect to heparin or fibrin binding.

Amino Acids↗
PubMed1983

Minimum information content and formation of interacting ribonuclease fragment complexes.

The degree to which amino acid sequence can be simplified with retention of conformational and functional properties has been investigated by semisynthesis using non-covalent fragment complexes of bovine pancreatic ribonuclease as test cases. Based on the ribonuclease S system, a set of synthetic model sequences was defined for the S-peptide (1-20) region which interacted productively with native S-protein (21-124). The most simple sequence, an eicosapeptide containing helix-favoring Ala residues at all positions except Glu 1 and 14, Phe 8, His 12, and Met 13, effected at least 15% of ribonuclease catalytic activity (versus native ribonuclease S) when added to S-protein in saturating amounts. The data for model S-peptides define an alpha-helical framework and specific side chains at positions 8, 12, and 13 as the core of sequence information necessary for S-peptide to effect a productive non-covalent complex with S-protein. Previous ribonuclease fragment studies also were used as a basis for making the productive, non-overlapping complex, (1-15):(21-111):(116-124). Addition of synthetic (1-15) and (116-124) to (21-111) led to a 3 degrees increase in Tm and 4% (versus ribonuclease A) catalytic activity. The three-fragment complex, with the beta-bend residues 112-115 deleted, exhibited significantly lower stability to thermal denaturation than did related two-fragment complexes. The potential use of three-fragment complexes related to the above is discussed for semi-synthetic sequence modeling concomitantly in the N- and C-terminal regions of ribonuclease.

Amino Acid Sequence↗
PubMed1982

Enzymatic condensation of nonassociated peptide fragments using a molecular trap.

We have tested the feasibility of achieving protease-catalyzed condensation between nonassociating peptide fragments through mediation of a molecular trap. In this study, two subfragments of bovine pancreatic ribonuclease S-peptide, containing residues 1-10 and 11-15, were rejoined by clostripain catalysis to form the 1-15 peptide. The extent of this stereospecific condensation was enhanced by adding ribonuclease S-protein (residues 21-124), which acts as a trap in binding 1-15 but not 1-10 or 11-15 and which thus shifts the equilibrium to favor 1-15 formation. The resultant (1-15) X (21-124) noncovalent complex, defined as [des-16-20]ribonuclease S, was detected by the enzymatic activity characteristic of the naturally derived ribonuclease S complex. Reaction of 1 mM S-protein and 20 mM fragments leads to 80% of the ribonuclease activity expected from the amount of 21-124 present. This indicates that 4% of the fragments 1-10 and 11-15 were condensed, compared to a maximal condensation of 5% based on the amount of trap. The less than theoretical yield is due largely to slow proteolytic degradation of 21-124 to a form which is no longer able to bind the condensation product 1-15. Yields were increased to 15% by addition of further trap. The successful synthesis of 1-15 emphasizes the usefulness of molecular traps to promote stereospecific fragment condensation between nonassociating peptide fragments for the synthesis and semisynthesis of polypeptides.

Amino Acids↗
PubMed1981

Clostripain-catalyzed re-formation of a peptide bond in a cytochrome C fragment complex.

Enzymatically-catalyzed condensation of cytochrome c fragments, ferrous heme fragment (1-38) and apofragment (39-104), has allowed the back-conversion of cytochrome c complex to native cytochrome c. The conversion was accomplished in 90% (v/v) glycerol, a solvent which has been shown to decrease the ionization of the terminal alpha-carboxyl group liberated during hydrolysis of a peptide bond. The effect on the pK is probably the main reason the thermodynamic obstacle to re-synthesis is minimized. A 30% conversion to cytochrome c was obtained. The cytochrome c product was distinguished from the non-covalent complex and separated fragments by molecular weight analysis with sodium dodecyl sulfate polyacrylamide gel electrophoresis, by elution from Sephadex G-50 and sulfopropyl-Sephadex in the presence of denaturant, by amino acid analysis of the product purified under complex-dissociation conditions, and by spectral analysis of the absorption bands of the heme. This method provides an opportunity to study the covalent rather than the complex form of cytochrome c analogs.

Amino Acids↗
PubMed1980

Modification of the carboxypeptidase A active site residue Glu-270 prevents interaction with a protein protease inhibitor from Ascaris.

The involvement of two active site residues of carboxypeptidase A in binding a protein inhibitor from Ascaris was studied. Glu-270 was modified with N-ethyl-5-phenylisoxazolium-3'-sulfonate and Tyr-248 was modified with tetranitromethane or diazotized arsanilic acid. Modification of Glu-270 abolished protein inhibitor binding and Glu-270 was protected from modification when the enzyme was bound to the protein inhibitor. In contrast, modification of Tyr-248 did not abolish protein inhibitor binding, nor did such binding protect Tyr-248 from modification. The absorption isosbestic point of arsanilazocarboxypeptidase A (Tyr-248 chemically modified) underwent a blue shift from 428 to 416 nm when the modified enzyme was bound to the protein inhibitor between pH 7.7 and 9.0. The 416 nm isosbestic point is characteristic of the loss of interaction between modified Tyr-248 and the active site zinc ion. These results with a protein inhibitor can be compared to substrate catalysis in which Tyr-248 moves away from the active site zinc ion of carboxypeptidase A when substrate binds. The close association of Glu-270 with Ascaris inhibitor interaction is consistent with other results which show that of the active site residues, only the modification of Glu-270 completely abolishes catalysis.

Animals↗
PubMed1980

Enzyme-catalyzed formation of semisynthetic staphylococcal nuclease using a new synthetic fragment, [48-glycine]synthetic-(6-49).

While trypsin can catalyze resynthesis of the peptide bond between fragments in the noncovalent complex of nuclease-T-(6-48) and nuclease-T-(49-149), this reaction leads to excision of Lys 49 and formation of inactive [des Lys 49]-nuclease-(6-149). To provide a method for making active active covalent semisynthetic nuclease, we chemically synthesized the fragment of residues 6 to 49 in which lysine 48 was replaced by glycine. This peptide was made using the recently described solid phase support, 4-(oxymethyl)phenylacetamidomethyl-polystyrene. The resultant crude polypeptide exhibited 30-50% of native nuclease-T enzymatic activity when added to native nuclease-T-(50-149). When the non-covalent complex formed by native nuclease-T-(50-149) and a 10-fold molar excess of [Gly 48]synthetic-(6-49) was equilibrated with trypsin in 90% glycerol, an increase in enzymatic activity from 8 to 32% (versus nuclease) was observed. Simultaneously, approximately 20% conversion of nuclease-T-(50-149) to nuclease-molecular weight material was observed by gel electrophoretic analysis. These data indicate that a covalent semisynthetic species is formed with activity about equal to that of native nuclease. The results confirm the importance of loop integrity on catalytic site organization. The Gly-48-containing fragment system defined above can allow preparation of semisynthetic nuclease sequence analogs.

Micrococcal Nuclease↗
PubMed1976

Characterization of proteins from Ascaris lumbricoides which bind specifically to carboxypeptidase.

Inhibitors of the peptidase and esterase activities of carboxypeptidases A and B have been isolated from extracts of Ascaris lumbricoides var suis. These proteins were obtained by treatment of the aqueous extracts at low pH, precipitation with ammonium sulfate, molecular sieving on Bio-Gel P-4, and chromatography on DEAE-cellulose. The inhibitors were resolved into three homogeneous peaks on CM-cellulose. These components, CM-A, CM-B, and CM-C, have constant specific activity and were recovered in a 41% yield. They moved as single bands when subjected to electrophoresis at high or low pH on polyacrylamide gels and they have similar amino acid compositions. Methionine, tyrosine, and cysteine are absent from each of the inhibitors. The 65 residues of CM-B suggest a minimum molecular weight of 7530, in close agreement to the value of 7600 +/- 200 determined on a Bio-Gel P-100 column. Each of the proteins has the same NH2-terminal residues, NH2-Asx-Glx-Val-Glx- and the same COOH-terminal residue, leucine. A plot of per cent acrylamide versus log relative mobility suggests that the three proteins are charge isomers. CM-B appears to be stable to high NaCl concentrations, extremes of pH, high temperatures, and digestion by intestinal proteases. Carboxypeptidase C, carboxypeptidase N, and yeast protease C are not inhibited by CM-B. However, the exopeptidase and esterase activities of human carboxypeptidase A are inhibited. The inhibitors appear to bind to bovine carboxypeptidase A with an atypical stoichiometry. Two moles of CM-B inhibitor bind to 1 mol of enzyme. The evidence is: (a) a demonstrated purity of bovine carboxypeptidase A, (b) minimal and maximal inhibitor molecular weights by different methods, of 7600 and 8300, and (c) a maximum specific activity of apparently homogeneous inhibitors which is 50% of that predicted for unit stoichiometry.

Amino Acids↗