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A R Stafford

Publications and source records attributed to A R Stafford.

At least 19 recordsLinked to original sources
PubMed2001

Conformational changes in thrombin when complexed by serpins.

Thrombin possesses two positively charged surface domains, termed exosites, that orient substrates and inhibitors for reaction with the enzyme. Because the exosites also allosterically modulate thrombin's activity, we set out to determine whether the structure or function of the exosites changes when thrombin forms complexes with antithrombin, heparin cofactor II, or alpha(1)-antitrypsin (M358R), serpins that utilize both, one, or neither of the exosites, respectively. Using a hirudin-derived peptide to probe the integrity of exosite 1, no binding was detected when thrombin was complexed with heparin cofactor II or alpha(1)-antitrypsin (M358R), and the peptide exhibited a 55-fold lower affinity for the thrombin-antithrombin complex than for thrombin. Bound peptide or HD-1, an exosite 1-binding DNA aptamer, was displaced from thrombin by each of the three serpins. Thrombin binding to fibrin also was abrogated when the enzyme was complexed with serpins. These data reveal that, regardless of the initial mode of interaction, the function of exosite 1 is lost when thrombin is complexed by serpins. In contrast, the integrity of exosite 2 is largely retained when thrombin is complexed by serpins, because interaction with heparin or an exosite 2-directed DNA aptamer was only modestly altered. The disorganization of exosite 1 that occurs when thrombin is complexed by serpins is consistent with results of protease sensitivity studies and crystallographic analysis of a homologous enzyme-serpin complex.

Animals↗
PubMed2001

Molecular basis for the susceptibility of fibrin-bound thrombin to inactivation by heparin cofactor ii in the presence of dermatan sulfate but not heparin.

Although fibrin-bound thrombin is resistant to inactivation by heparin.antithrombin and heparin.heparin cofactor II complexes, indirect studies in plasma systems suggest that the dermatan sulfate.heparin cofactor II complex can inhibit fibrin-bound thrombin. Herein we demonstrate that fibrin monomer produces a 240-fold decrease in the heparin-catalyzed rate of thrombin inhibition by heparin cofactor II but reduces the dermatan sulfate-catalyzed rate only 3-fold. The protection of fibrin-bound thrombin from inhibition by heparin.heparin cofactor II reflects heparin-mediated bridging of thrombin to fibrin that results in the formation of a ternary heparin.thrombin.fibrin complex. This complex, formed as a result of three binary interactions (thrombin.fibrin, thrombin.heparin, and heparin.fibrin), limits accessibility of heparin-catalyzed inhibitors to thrombin and induces conformational changes at the active site of the enzyme. In contrast, dermatan sulfate binds to thrombin but does not bind to fibrin. Although a ternary dermatan sulfate. thrombin.fibrin complex forms, without dermatan sulfate-mediated bridging of thrombin to fibrin, only two binary interactions exist (thrombin.fibrin and thrombin. dermatan sulfate). Consequently, thrombin remains susceptible to inactivation by heparin cofactor II. This study explains why fibrin-bound thrombin is susceptible to inactivation by heparin cofactor II in the presence of dermatan sulfate but not heparin.

Binding Sites↗
PubMed2000

Hypersulfated low molecular weight heparin with reduced affinity for antithrombin acts as an anticoagulant by inhibiting intrinsic tenase and prothrombinase.

In buffer systems, heparin and low molecular weight heparin (LMWH) directly inhibit the intrinsic factor X-activating complex (intrinsic tenase) but have no effect on the prothrombin-activating complex (prothrombinase). Although chemical modification of LMWH, to lower its affinity for antithrombin (LA-LMWH) has no effect on its ability to inhibit intrinsic tenase, N-desulfation of LMWH reduces its activity 12-fold. To further explore the role of sulfation, hypersulfated LA-LMWH was synthesized (sLA-LMWH). sLA-LMWH is not only a 32-fold more potent inhibitor of intrinsic tenase than LA-LMWH; it also acquires prothrombinase inhibitory activity. A direct correlation between the extent of sulfation of LA-LMWH and its inhibitory activity against intrinsic tenase and prothrombinase is observed. In plasma-based assays of tenase and prothrombinase, sLA-LMWH produces similar prolongation of clotting times in plasma depleted of antithrombin and/or heparin cofactor II as it does in control plasma. In contrast, heparin has no effect in antithrombin-depleted plasma. When the effect of sLA-LMWH on various components of tenase and prothrombinase was examined, its inhibitory activity was found to be cofactor-dependent (factors Va and VIIIa) and phospholipid-independent. These studies reveal that sLA-LMWH acts as a potent antithrombin-independent inhibitor of coagulation by attenuating intrinsic tenase and prothrombinase.

Anticoagulants↗
PubMed1999

Comparison of heparin- and dermatan sulfate-mediated catalysis of thrombin inactivation by heparin cofactor II.

Heparin and dermatan sulfate activate heparin cofactor II (HCII) comparably, presumably by liberating the amino terminus of HCII to bind to exosite I of thrombin. To explore this model of activation, we systematically substituted basic residues in the glycosaminoglycan-binding domain of HCII with neutral amino acids and measured the rates of thrombin inactivation by the mutants. Mutant D, with changes at Arg(184), Lys(185), Arg(189), Arg(192), Arg(193), demonstrated a approximately 130-fold increased rate of thrombin inactivation that was unaffected by the presence of glycosaminoglycans. The increased rate reflects displacement of the amino terminus of mutant D because (a) mutant D inactivates gamma-thrombin at a 65-fold slower rate than alpha-thrombin, (b) hirudin-(54-65) decreases the rate of thrombin inactivation, and (c) deletion of the amino terminus of mutant D reduces the rate of thrombin inactivation approximately 100-fold. We also examined the contribution of glycosaminoglycan-mediated bridging of thrombin to HCII to the inhibitory process. Whereas activation of HCII by heparin was chain-length dependent, stimulation by dermatan sulfate was not, suggesting that dermatan sulfate does not utilize a template mechanism to accelerate the inhibitory process. Fluorescence spectroscopy revealed that dermatan sulfate evokes greater conformational changes in HCII than heparin, suggesting that dermatan sulfate stimulates HCII by producing more effective displacement of the amino terminus.

Amino Acid Sequence↗
PubMed1999

Exosites 1 and 2 are essential for protection of fibrin-bound thrombin from heparin-catalyzed inhibition by antithrombin and heparin cofactor II.

Assembly of ternary thrombin-heparin-fibrin complexes, formed when fibrin binds to exosite 1 on thrombin and fibrin-bound heparin binds to exosite 2, produces a 58- and 247-fold reduction in the heparin-catalyzed rate of thrombin inhibition by antithrombin and heparin cofactor II, respectively. The greater reduction for heparin cofactor II reflects its requirement for access to exosite 1 during the inhibitory process. Protection from inhibition by antithrombin and heparin cofactor II requires ligation of both exosites 1 and 2 because minimal protection is seen when exosite 1 variants (gamma-thrombin and thrombin Quick 1) or an exosite 2 variant (Arg93 --> Ala, Arg97 --> Ala, and Arg101 --> Ala thrombin) is substituted for thrombin. Likewise, the rate of thrombin inhibition by the heparin-independent inhibitor, alpha1-antitrypsin Met358 --> Arg, is decreased less than 2-fold in the presence of soluble fibrin and heparin. In contrast, thrombin is protected from inhibition by a covalent antithrombin-heparin complex, suggesting that access of heparin to exosite 2 of thrombin is hampered when ternary complex formation occurs. These results reveal the importance of exosites 1 and 2 of thrombin in assembly of the ternary complex and the subsequent protection of thrombin from inhibition by heparin-catalyzed inhibitors.

Anticoagulants↗
PubMed1999

Vasoflux, a new anticoagulant with a novel mechanism of action.

BACKGROUND: Heparin and direct thrombin inhibitors, such as hirudin, have limitations in the treatment of acute coronary syndromes. Heparin does not inactivate fibrin-bound thrombin, whereas hirudin fails to block thrombin generation. In contrast, Vasoflux is a novel anticoagulant that inactivates fibrin-bound thrombin and attenuates factor Xa generation. METHODS AND RESULTS: Vasoflux is prepared by depolymerization of heparin, restricting molecular size to between 3000 and 8000 Da, and reducing antithrombin affinity by periodate oxidation. Vasoflux catalyzes fibrin-bound thrombin inactivation by heparin cofactor II (HCII) and inhibits factor IXa activation of factor X independently of antithrombin and HCII. Compared with other anticoagulants in a thrombogenic extracorporeal circuit, Vasoflux maintains filter patency at concentrations that produce an activated clotting time (ACT) of 220 seconds. In contrast, to maintain filter patency, heparin, low-molecular-weight heparin (LMWH), and hirudin require concentrations that produced an ACT of 720, 415, and >1500 seconds, respectively, whereas dermatan sulfate was ineffective at concentrations that produced an ACT of 360 seconds. CONCLUSIONS: Vasoflux is more effective than heparin and LMWH because it inactivates fibrin-bound thrombin and is superior to hirudin and dermatan sulfate because it also blocks factor Xa generation.

Anticoagulants↗
PubMed1998

Localization of the thrombin-binding domain on prothrombin fragment 2.

Co-crystallographic studies have shown that the interaction of human prothrombin fragment 2 (F2) with thrombin involves the formation of salt bridges between the kringle inner loop of F2 and anion-binding exosite II of thrombin. When F2 binds to thrombin, it has been shown to evoke conformational changes at the active site and at exosite I of the enzyme. Using plasma, recombinant, and synthetic F2 peptides (F2, rF2, and sF2, respectively) we have further localized the thrombin-binding domain on F2. F2, rF2-(1-116), rF2-(55-116), and sF2-(63-116), all of which contain the kringle inner loop (residues 64-93) and the acidic COOH-terminal connecting peptide (residues 94-116), bind to thrombin-agarose. In contrast, analogues of the kringle inner loop, sF2-(63-90), or the COOH-terminal connecting peptide, sF2-(92-116), do not bind. Thus, contrary to predictions from the crystal structure, the COOH-terminal connecting peptide as well as the kringle inner loop are involved in the interaction of F2 with thrombin. F2 and sF2-(63-116) bind saturably to fluorescently labeled active site-blocked thrombin with Kd values of 4.1 and 51.3 microM, respectively. The affinity of sF2-(63-116) for thrombin increases about 5-fold (Kd = 10 microM) when Val at position 78 is substituted with Glu. F2 and sF2-(63-116) bind to exosite II on thrombin because both reduce the heparin-catalyzed rate of thrombin inhibition by antithrombin approximately 4-fold. In contrast, only F2 slows the uncatalyzed rate of thrombin inactivation by antithrombin. Like F2, sF2-(63-116) induces allosteric changes in the active site and exosite I of thrombin because it alters the rates of thrombin-mediated hydrolysis of chromogenic substrates and displaces fluorescently labeled hirudin54-65 from active site-blocked thrombin, respectively. Both peptides also prolong the thrombin clotting time of fibrinogen in a concentration-dependent fashion, reflecting their effects on the active site and/or exosite I. These studies provide further insight into the regions of F2 that evoke functional changes in thrombin.

Amino Acid Sequence↗
PubMed1997

Evidence for allosteric linkage between exosites 1 and 2 of thrombin.

Investigations to date have demonstrated that ligand binding to exosites 1 or 2 on thrombin produces conformational changes at the active site. In this study, we directly compared the effect of ligand binding to exosites 1 and 2 on the structure and function of the active site of thrombin and investigated functional linkage between the two exosites. Binding studies were performed in solution with fluorescein-Phe-Pro-Arg-CH2Cl (FPR)-thrombin. Hirudin-(54-65) and sF2, a synthetic peptide corresponding to residues 63-116 of prothrombin fragment 2, were used as ligands for exosites 1 and 2 of thrombin, respectively. The two ligands produce diametric changes in the fluorescence of fluorescein-FPR-thrombin and also have opposing effects on the rate of thrombin hydrolysis of a number of chromogenic substrates. These results indicate that sF2 and hirudin-(54-65) differentially affect the conformation of the active site. Experiments then were performed to investigate whether both ligands can bind to thrombin simultaneously. When thrombin-bound fluorescein-sF2 is titrated with hirudin-(54-65), complete displacement of fluorescein-sF2 is observed. Likewise, when thrombin-bound fluorescein-hirudin-(54-65) is titrated with sF2, complete displacement occurs. Additional support for reciprocal binding was obtained in fluorescence experiments where both probes were labeled and in experiments monitoring ligand binding to agarose-immobilized thrombin. This mutually exclusive binding of either ligand can be explained by reciprocal, allosteric modulation of ligand affinity between the two exosites. Thus, not only do the two exosites differentially influence the active site, they also affect the binding properties of the opposing exosite.

Allosteric Regulation↗
PubMed1992

Lipid vesicles which can bind to protein kinase C and activate the enzyme in the presence of EGTA.

Maximal protein kinase C activity with vesicles of phosphatidic acid and 1,2-dioleoyl-sn-glycerol is observed in the absence of added Ca2+. Addition of phosphatidylcholine to these vesicles restores some calcium dependence of enzyme activity. 1,2-Dioleoyl-sn-glycerol eliminates the Ca(2+)-dependence of protein kinase C activity found with phosphatidic acid alone. Phorbol esters do not mimic the action of 1,2-dioleoyl-sn-glycerol in this respect. This suggests that the 1,2-dioleoyl-sn-glycerol effect is a result of changes it causes in the physical properties of the membrane rather than to specific binding to the enzyme. The effect of 1,2-dioleoyl-sn-glycerol on the phosphatidic-acid-stimulated protein kinase C activity is dependent on the molar fraction of 1,2-dioleoyl-sn-glycerol used and results in a gradual shift from Ca2+ stimulation at low 1,2-dioleoyl-sn-glycerol concentrations to calcium inhibition at higher concentrations of 1,2-dioleoyl-sn-glycerol. Phosphatidylserine-stimulated activity is also shown to be largely independent of the calcium concentration at higher molar fractions of 1,2-dioleoyl-sn-glycerol. Thus, with certain lipid compositions, protein kinase C activity becomes independent of the calcium concentration or requires only very low, stoichiometric binding of Ca2+ to high affinity sites on the enzyme. Protein kinase C can bind to phosphatidic acid vesicles more readily than it can bind to phosphatidylserine vesicles in the absence of calcium. Addition of 1,2-dioleoyl-sn-glycerol to phosphatidylserine vesicles promotes the partitioning of protein kinase C into the membrane in the absence of added Ca2+. There is no isozyme specificity in this binding. These results suggest that a less-tightly packed headgroup region of the bilayer causes increased insertion of protein kinase C into the membrane. This is a necessary but not sufficient condition for activation of the enzyme in the presence of EGTA.

Animals↗
PubMed1991

Action of insulin in rat adipocytes and membrane properties.

Several small peptides inhibit insulin-promoted glucose uptake in rat adipocytes. At 10 microM peptide concentration, the extent of their inhibition of the insulin effect is related to the ability of these peptides to raise the bilayer- to hexagonal-phase transition temperature in model membranes. Hexane and DL-threo-dihydrosphingosine lower this phase transition temperature in model membranes, and they promote glucose uptake in adipocytes. There is thus an empirical relationship between the action of membrane additives on glucose uptake in adipocytes and their effect on the hexagonal-phase-forming tendency in model membranes. The most potent of the bilayer-stabilizing peptides tested in this work is carbobenzoxy-D-Phe-L-Phe-Gly. This peptide also inhibits insulin-stimulated protein synthesis in adipocytes. In contrast, DL-threo-dihydrosphingosine stimulates protein synthesis. The uptake of [125I]iodoinsulin by adipocytes is inhibited by carbobenzoxy-D-Phe-L-Phe-Gly. The mechanism of action of the bilayer-stabilizing peptides includes inhibition of insulin-dependent protein phosphorylation in adipocytes. The peptides are not specific inhibitors of a single function but are suggested to cause their effects by altering the physical properties of the membrane in a nonspecific manner. These results demonstrate that insulin-dependent functions of rat adipocytes can be modified by membrane additives in a manner predictable from the properties of these additives in model membranes.

Adipose Tissue↗
PubMed1990

Dimerization of the P-glycoprotein in membranes.

Plasma membranes from a CHO cell line, CHRC5, which exhibits multidrug resistance was studied using radiation inactivation analysis. The P-glycoprotein content of the membrane was determined by Western blots. Irradiation resulted in the loss of P-glycoprotein. The dependence of this loss on radiation dose corresponded to a target size of 250 kDa which is the molecular mass of a dimer of the P-glycoprotein. This is strong evidence to indicate that the P-glycoprotein self associates in the membrane.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1989

Studies on the mechanism of action of a bilayer stabilizing inhibitor of protein kinase C: cholesterylphosphoryldimethylethanolamine.

Cholesterylphosphoryldimethylethanolamine is a zwitterionic compound which is a good bilayer stabilizer. As has been found with many other compounds having these properties, cholesterylphosphoryldimethylethanolamine is found to be a potent inhibitor of protein kinase C in both vesicle and micelle assay systems. The kinetics of the inhibition in Triton X-100 micelles was non-competitive with respect to ATP, histone, diolein, phorbol ester and Ca2+. It has a Ki of about 30 microns. The inhibition kinetics as a function of phosphatidylserine concentration is more complex but suggestive of competitive inhibition. Cholesterylphosphoryldimethylethanolamine does not prevent the partitioning of protein kinase C into the membrane. This inhibitor lowers the Ca2+-phosphatidylserine-independent phosphorylation of protamine sulfate by protein kinase C and directly affects the catalytic segment of the enzyme generated by tryptic hydrolysis. Thus, this zwitterionic bilayer stabilizing inhibitor of protein kinase C both competes with the binding of phosphatidylserine as well as affects the active site of protein kinase C.

Animals↗
PubMed1989

Calcitonin inhibits the rise of intracellular calcium induced by thyrotropin-releasing hormone in GH3 cells.

Both human and salmon calcitonins markedly inhibit the TRH-stimulated rise in intracellular [Ca2+] in GH3 cells. Calcitonin also inhibits prolactin release from these cells. Both [Ala] salmon calcitonin and salmon calcitonin (1-23) peptide amide also inhibit this rise in [Ca2+] and also inhibit TRH-stimulated prolactin release from GH3 cells as well as from primary pituitary cell cultures. It is likely that calcitonin inhibits prolactin release in the pituitary by decreasing the extent of the rise of intracellular calcium concentration. Neither an intact disulfide bond at the amino terminus nor residues 24-32 of the carboxyl terminus of salmon calcitonin are required for this inhibition.

Animals↗
PubMed1988

The relationship between the bilayer to hexagonal phase transition temperature in membranes and protein kinase C activity.

A number of substances affect the activity of protein kinase C. Among uncharged and zwitterionic compounds, those which activate protein kinase C also lower the bilayer to hexagonal phase transition temperature of dielaidoylphosphatidylethanolamine while substances which inhibit protein kinase C raise this transition temperature. Using this criteria, we have identified 3 beta-chloro-5-cholestene, 5 beta-cholan-24-ol and eicosane as new protein kinase C activators and have shown that Z-Ser-Leu-NH2, Z-Gly-Leu-NH2, Z-Tyr-Leu-NH2, cyclosporin A and cholestan-3 beta, 5 alpha, 6 beta-triol are protein kinase C inhibitors.

Cholesterol↗
PubMed1988

Deletion sequences of salmon calcitonin that retain the essential biological and conformational features of the intact molecule.

Salmon calcitonin has an amino acid sequences that would allow it to form an amphipathic helix from approximately residue 9 to residue 22. We have synthesized a number of analogues of this peptide hormone with deletions in the carboxyl terminus of this putative amphipathic helix. These analogues include deletions of single amino acid residues at positions 19, 20, 21, or 22 as well as deletions of progressively larger segments starting with residue 19 and including deletions of residues 19 and 20; 19, 20, and 21; or 19, 20, 21, and 22. There is a small decrease in the helical content of these analogues compared with the native hormone, both in the presence and absence of amphiphiles. However, the extent of formation of secondary structure, as measured by circular dichroism, is similar for these deletion sequences as it is for the native hormone. In all cases, there is a large increase in the helical content of the peptide in the presence of dimyristoylphosphatidylglycerol, lysolecithin, or sodium dodecyl sulfate. All of the analogues have hypocalcemic activity in vivo in rats, comparable to the native hormone, except for des-Leu19-salmon calcitonin, which is about twice as active as the unmodified hormone. With use of an in vitro assay of adenylate cyclase activation in purified rat kidney membranes, des-Tyr22-salmon calcitonin, des-Leu19,Gln20,Thr21-salmon calcitonin, and des-Leu19Gln20,Thr21,Thr22-salmon calcitonin exhibited about one-tenth the stimulatory activity of the native hormone. Des-Tyr22-sCT and des-Leu19,Gln20,Thr21,Tyr22-sCT were also tested for their activity in inhibiting prolactin release from isolated rat pituitary cells. Both of these analogues exhibited inhibitory activity. Thus, the region of residues 19-22 does not greatly affect either the conformational or the biological properties of salmon calcitonin.

Adenylyl Cyclases↗
PubMed1987

Biologically potent analogues of salmon calcitonin which do not contain an N-terminal disulfide-bridged ring structure.

The disulfide bridge formed between the cysteine residues at positions 1 and 7 of salmon calcitonin (sCT) is not required for biological activity. The analogues [Ala1,7]sCT,[AcmCys1,7]sCT and [AmcCys1,Ala7]sCT (AcmC = S-acetamido-methylcysteine) are linear sequences which retain full hypocalcemic activity in the intact rat and ability to activate adenylate cyclase of rat renal membranes. The secondary structure of these peptides in aqueous solution in the presence or absence of lipid is not greatly perturbed by the opening of the disulfide ring. In contrast with salmon calcitonin, substitution of Cys by AcmCys in human calcitonin results in greatly reduced hypocalcemic activity but no loss in the ability of the peptide to activate renal adenylate cyclase. Thus in vitro activation of adenylate cyclase by human calcitonin analogues is not always correlated with in vivo hypocalcemic potency.

Adenylyl Cyclases↗