Activated protein C generation is greatly decreased in plasma from newborns compared to adults in the presence or absence of endothelium

Activated protein C (APC) generation strongly affects sepsis and thrombosis by inhibition of thrombin generation. However, it is unclear if there are age-related differences in effectiveness of protein C (PC). We studied age effects on plasma APC generation ± endothelium. Defibrinated (Ancrod) plasma (from adults or newborns (umbilical cord)) was recalcified with buffer containing tissue factor ± thrombomodulin (TM) on either plastic or endothelium (HUVEC) at 37oC. Timed subsamples of reaction mixture were taken into either heparin-EDTA or FFRCMK-EDTA solutions and analyzed for APC-PC inhibitor (APC-PCI) or APC-α1antitrypsin (APC-α1AT) by ELISAs. Since heparin converts free APC to APC-PCI, the difference in APCPCI measured in heparin-EDTA and FFRCMK-EDTA samples was equal to free active APC. APC-α2macroglobulin (APC-α2M) was measured as remaining chromogenic activity in heparin-EDTA. Free APC, APC-PCI and APC-α1AT were decreased in newborn compared to adult plasma on plastic. However, APC-α2M made up a larger fraction of inhibitor complexes in newborn plasma. On endothelium, significantly more APC, APC-PCI and APC-α1AT were generated in either plasma compared to that on plastic with excess added TM. APC, APC-PCI and APC-α1AT were also reduced and total APC-α2M increased in newborn plasma on HUVEC. Addition of PC to newborn plasma gave APC generation similar to adult plasma. Thus, free APC, APC-PCI and APC-α1AT generation is reduced in newborn compared to adult plasma with or without endothelium, likely due to reduced plasma PC levels. Endothelium enhances APC generation, regardless of plasma type, possibly because of cell surface factors such as TM, phospholipid and endothelial PC receptor.

Binding of heparin to plasma proteins and endothelial surfaces is inhibited by covalent linkage to antithrombin

Unfractionated heparin (UFH) and low molecular weight heparin (LMWH) are used for prophylaxis and treatment of thrombosis. However, UFH has a short plasma half-life and variable anticoagulant response in vivo due to plasma or vessel wall protein binding and LMWH has a decreased ability to inactivate thrombin, the pivotal enzyme in the coagulation cascade. Covalent linkage of antithrombin to heparin gave a complex (ATH) with superior anticoagulant activity compared to UFH and LMWH, and longer intravenous half-life compared to UFH. We found that plasma proteins bound more to UFH than ATH, and least to LMWH. Also, UFH bound significantly more to endothelial cells than ATH, with 100% of UFH and 94% of ATH binding being on the cell surface and the remainder was endocytosed. Competition studies with UFH confirmed that ATH binding was likely through its heparin moiety. These findings suggest that differences in plasma protein and endothelial cell binding may be due to available heparin chain length. Although ATH is polydisperse, the covalently-linked antithrombin may shield a portion of the heparin chain from association with plasma or endothelial cell surface proteins. This model is consistent with ATH’s better bioavailability and more predictable dose response.

Interactions of heparin and a covalently-linked antithrombin-heparin complex with components of the fibrinolytic system

Unfractionated heparin (UFH) is used as an adjunct during thrombolytic therapy. However, its use is associated with limitations, such as the inability to inhibit surface bound coagulation factors. We have developed a covalent conjugate of antithrombin (AT) and heparin (ATH) with superior anticoagulant properties compared with UFH. Advantages of ATH include enhanced inhibition of surface-bound coagulation enzymes and the ability to reduce the overall size and mass of clots in vivo. The interactions of UFH or ATH with the components of the fibrinolytic pathway are not well understood. Our study utilised discontinuous second order rate constant (k2 ) assays to compare the rates of inhibition of free and fibrin-associated plasmin by AT+UFH vs ATH. Additionally, we evaluated the effects of AT+UFH and ATH on plasmin generation in the presence of fibrin. The k2 values for inhibition of plasmin were 5.74 ± 0.28 x 106 M-1 min-1 and 6.39 ± 0.59 x 106 M-1 min-1 for AT+UFH and ATH, respectively. In the presence of fibrin, the k2 values decreased to 1.45 ± 0.10 x 106 M-1 min1 and 3.07 ± 0.19 x 106 M-1 min-1 for AT+UFH and ATH, respectively. Therefore, protection of plasmin by fibrin was observed for both inhibitors; however, ATH demonstrated superior inhibition of fibrin-associated plasmin. Rates of plasmin generation were also decreased by both inhibitors, with ATH causing the greatest reduction (approx. 38-fold). Nonetheless, rates of plasmin inhibition were 2–3 orders of magnitude lower than for thrombin, and in a plasma-based clot lysis assay ATH significantly inhibited clot formation but had little impact on clot lysis. Cumulatively, these data may indicate that, relative to coagulant enzymes, the fibrinolytic system is spared from inhibition by both AT+UFH and ATH, limiting reduction in fibrinolytic potential during anticoagulant therapy.

Covalently linking heparin to antithrombin enhances prothrombinase inhibition on activated platelets

Factor (F)Xa within the prothrombinase complex is protected from inhibition by unfractionated heparin (UFH), enoxaparin and fondaparinux. We have developed a covalent antithrombin-heparin complex (ATH) with enhanced anticoagulant activity. We have also demonstrated that ATH is superior at inhibiting coagulation factors when assembled on artificial surfaces. The objective of the present study is to determine the ability of ATH vs AT+UFH to inhibit FXa within the prothrombinase complex when the enzyme complex is assembled on the more native platelet system. Discontinuous inhibition assays were performed to determine final k 2-values for inhibition of FXa, FXa within the platelet-prothrombinase, or FXa within prothrombinase devoid of various components. Thrombin generation and plasma clotting was also assayed in the presence of resting/activated platelets ± inhibitors. Protection of FXa was not observed for ATH, whereas a moderate 60% protection was observed for AT+UFH. ATH inhibited platelet-prothrombinase ∼4-fold faster than AT+UFH. Relative to intact prothrombinase, rates for FXa inhibition by AT+UFH in prothrombinase complexes devoid of either prothrombin (II)/activated platelets/FVa were higher. However, inhibition by AT+UFH of prothrombinase devoid of FII yielded slightly lower rates compared to free FXa inhibition. Thrombin generation and plasma clotting was enhanced with activated platelets, while inhibition was better by ATH compared to AT+UFH, thus suggesting an overall enhanced anticoagulant activity of ATH against platelet-bound prothrombinase complexes.

Surface modification of poly(dimethylsiloxane) with a covalent antithrombin-heparin complex for the prevention of thrombosis: use of polydopamine as bonding agent

A modified poly(dimethyl siloxane) (PDMS) material is under development for use in an extracorporeal microfluidic blood oxygenator designed as an artificial placenta to treat newborn infants suffering from severe respiratory insufficiency. To prevent thrombosis triggered by blood-material contact, an antithrombin-heparin (ATH) covalent complex was coated on PDMS surface using polydopamine (PDA) as a "bioglue". Experiments using radiolabelled ATH showed that the ATH coating on PDA-modified PDMS remained substantially intact after incubation in plasma, 2% SDS solution, or whole blood over a three day period. The anticoagulant activity of the ATH-modified surfaces was also demonstrated: in contact with plasma the ATH-coated PDMS was shown to bind antithrombin (AT) selectively from plasma and to inhibit clotting factor Xa. It is concluded that modification of PDMS with polydopamine and ATH shows promise as a means of improving the blood compatibility of PDMS and hence of the oxygenator device.

Surface modification of polydimethylsiloxane with a covalent antithrombin-heparin complex to prevent thrombosis

To prevent coagulation in contact with blood, polydimethylsiloxane (PDMS) was modified with an antithrombin-heparin (ATH) covalent complex using polyethylene glycol (PEG) as a linker/spacer. Using NHS chemistry, ATH was attached covalently to the distal chain end of the immobilized PEG linker. Surfaces were characterized by contact angle and X-ray photoelectron spectroscopy; attachment was confirmed by decrease in contact angles and an increase in nitrogen content as determined by X-ray photoelectron spectroscopy. Protein interactions in plasma were investigated using radiolabeled proteins added to plasma as tracers, and by immunoblotting of eluted proteins. Modification of PDMS with PEG alone was effective in reducing non-specific protein adsorption; attachment of ATH at the distal end of the PEG chains did not significantly affect protein resistance. It was shown that surfaces modified with ATH bound antithrombin selectively from plasma through the pentasaccharide sequence on the heparin moiety of ATH, indicating the ability of the ATH-modified surfaces to inhibit coagulation. Using thromboelastography, the effect of ATH modification on plasma coagulation was evaluated directly. It was found that initiation of coagulation was delayed and the time to clot was prolonged on PDMS modified with ATH/PEG compared to controls. For comparison, surfaces modified in a similar way with heparin were prepared and investigated using the same methods. The data suggest that the ATH-modified surfaces have superior anticoagulant properties compared to those modified with heparin.

Thrombin generation

Generation of thrombin has been established as the critical process leading to coagulation in vivo. Indeed, ex vivo markers of thrombin generation in patients have been useful in detecting thrombosis, while many standard global clot-time tests of haemostasis in blood or plasma samples are simple endpoint measures of the potential to generate thrombin. Thus, there has been a recent surge towards direct measurement of thrombin generation potential in plasma/blood samples as a refined methodology for more precisely assessing procoagulant/anticoagulant/hemorrhagic parameters of the haemostatic status. Presently, however, there is no consensus method for thrombin generation determination. The present treatise gives detailed procedures for available thrombin generation tests, with emphasis on the preferred technology.

Comparison of N-linked glycosylation of protein C in newborns and adults

Protein C (PC) is a major anticoagulant that stems the propagation of thrombin. The activated form of PC (APC), in association with the cofactor protein S, proteolytically converts activated coagulation factors VIIIa and Va into inactive forms. Studies have shown that forms of PC that contain 3N-linked glycans (beta-PC) are functionally distinct from the fully glycosylated 4-glycan type (alpha-PC). Since some findings have also hinted at qualitative differences in PC from newborns and adults, we decided to determine the relative constitution of glycoforms in these age groups. Subtypes of PC in newborn and adult plasmas were distinguished by SDS polyacrylamide electrophoresis and Western blotting, followed by immunological analysis. Newborns were found to have alpha-PC/beta-PC mole ratios of 8.8:1, compared to 2.3:1 in adults. PC was also isolated by immunoaffinity chromatography from newborn and adult plasmas. Glycans were released by protease treatment and studied by mass spectrometry. Results from glycan analysis showed a small range of glycan structures in both age groups. No clear differences were noted between newborn and adult PC microheterogeneity in glycan structures (branching). We conclude that newborns have important differences in PC macroheterogeneity in glycoform content relative to adults. This age-dependent glycosylation variation may have implications in management of PC function in vivo.

Inhibition of the prothrombinase complex on red blood cells by heparin and covalent antithrombin-heparin complex

The role of red blood cells (RBCs) in coagulation is not well understood. Overt exposure of phosphatidylserine on surfaces of RBCs provide docking sites for formation of the prothrombinase complex, which further aids in amplification of coagulation leading to subsequent thrombosis. No studies to date have evaluated heparin inhibition of the RBC-prothrombinase system. Therefore, this study examines the ability of heparin and a covalent antithrombin-heparin complex (ATH) to inhibit the RBC-prothrombinase system. Discontinuous inhibition assays were performed to obtain k₂ values for inhibition of free or prothrombinase-bound Xa by antithrombin and unfractionated heparin (AT + UFH) versus ATH. In addition, components of the complex (prothrombin, RBCs or Va) were excluded prior to reaction with inhibitors to investigate potential mechanisms involved. Inhibition of thrombin generation, fibrinogen conversion and plasma clotting by the RBC-prothrombinase system was also examined. Protection of Xa was observed for AT + UFH and not for ATH reactions. Inhibition rates for ATH were significantly faster when compared with AT + UFH results. The greatest impact on Xa inhibition was observed from factor Va omission for both inhibitors. ATH inhibited thrombin generation, fibrinogen conversion and plasma clotting better compared with AT + UFH. This study determined potential control of coagulation contributed by RBCs. Moreover, greater control of coagulation is achieved by covalently linking heparin to AT.

Modification of polyurethane surface with an antithrombin-heparin complex for blood contact: influence of molecular weight of polyethylene oxide used as a linker/spacer

Polyurethane (PU) was modified using isocyanate chemistry to graft polyethylene oxide (PEO) of various molecular weights (range 300-4600). An antithrombin-heparin (ATH) covalent complex was subsequently attached to the free PEO chain ends, which had been functionalized with N-hydroxysuccinimide (NHS) groups. Surfaces were characterized by water contact angle and X-ray photoelectron spectroscopy (XPS) to confirm the modifications. Adsorption of fibrinogen from buffer was found to decrease by ~80% for the PEO-modified surfaces compared to the unmodified PU. The surfaces with ATH attached to the distal chain end of the grafted PEO were equally protein resistant, and when the data were normalized to the ATH surface density, PEO in the lower MW range showed greater protein resistance. Western blots of proteins eluted from the surfaces after plasma contact confirmed these trends. The uptake of ATH on the PEO-modified surfaces was greatest for the PEO of lower MW (300 and 600), and antithrombin binding from plasma (an indicator of heparin anticoagulant activity) was highest for these same surfaces. The PEO-ATH- and PEO-modified surfaces also showed low platelet adhesion from flowing whole blood. It is concluded that for the PEO-ATH surfaces, PEO in the low MW range, specifically MW 600, may be optimal for achieving an appropriate balance between resistance to nonspecific protein adsorption and the ability to take up ATH and bind antithrombin in subsequent blood contact.

Binding of heparin to metals

Heparin is a major prophylactic and treatment agent for thrombosis. Structurally, this anticoagulant is a polydisperse, highly negatively charged polysaccharide mixture that contains a variable density of sulfate group substituents per molecule. Previous study has shown that heparin molecules have a high affinity for a wide range of metal ions with varying oxidation states. However, reports in literature on binding of heparin to metals have investigated only a small sampling of heparin-metal ion interactions. Since interaction of heparin with fluid phase and cell surface macromolecules in vivo is dependent on the heparin structure when bound in a metal ion complex, a survey of the physical parameters for heparin binding to metals is imperative. Atomic absorption and spectrophotometry experiments were performed for metal quantification, and in this study, the relative values for affinity constants and number of binding sites for heparin binding to several alkaline, alkaline earth, main group, and transition metals in their most common oxidation states are reported. We found an overall trend for heparin-metal affinity to be Mn(2+) > Cu(2+) > Ca(2+) > Zn(2+) > Co(2+) > Na(+) > Mg(2+) > Fe(3+) > Ni(2+) > Al(3+)> Sr(2+), with the trend in N (b) being opposite compared with the K (a).

Immobilization of an antithrombin-heparin complex on gold: anticoagulant properties and platelet interactions

The anticoagulant properties and platelet interactions of gold surfaces modified with an antithrombin-heparin (ATH) complex are reported. ATH was attached to gold through either a short disulfide (linker) or a thiol-terminated polyethylene oxide (PEO) (linker, spacer). Analogous surfaces were prepared with uncomplexed heparin. Antithrombin (AT) uptake was measured before and after selectively destroying the active pentasaccharide sequence of the heparin moiety, and was found to be predominantly through the active sequence on all of the surfaces. AT binding was higher on the ATH surfaces than on the corresponding heparin surfaces. Heparin activity was assessed by an anti-factor Xa assay. The ratio of active heparin density (from the anti-FXa assay) to total heparin density was taken as a measure of heparin bioactivity. The ratio was greater on the ATH- than on the heparin-modified surfaces, i.e. the PEO-ATH surfaces showed the greater proportion of active heparin. Platelet adhesion from flowing whole blood was found to be reduced on PEO- and ATH-modified surfaces compared to bare gold. The PEO-ATH modified surfaces, but not the heparinized surfaces, were shown to prolong the clotting time of recalcified plasma.

Surface modification with an antithrombin-heparin complex for anticoagulation: studies on a model surface with gold as substrate

Gold was used as a substrate for immobilization of an antithrombin-heparin (ATH) covalent complex to investigate ATH as a surface modifier to prevent blood coagulation. Three different surface modification methods were used to attach ATH to gold: (i) direct chemisorption; (ii) using dithiobis(succinimidyl propionate) (DSP) as a linker molecule and (iii) using polyethylene oxide (PEO) as a linker/spacer. The ATH-modified surfaces were compared to analogous heparinized surfaces. Water contact angles and X-ray photoelectron spectroscopy confirmed the modifications and provided data on surface properties and possible orientation. Ellipsometry measurements showed that surface coverage of DSP and PEO was high. ATH and heparin densities were quantified using radioiodination and quartz crystal microbalance, respectively. The surface density of ATH was greatest on the DSP surface (0.17 microg cm(-2)) and lowest on the PEO (0.05 microg cm(-2)). The low uptake on the PEO surface was likely due to the protein resistance of the PEO component. Using radioiodinated antithrombin (AT), it was shown that ATH-immobilized surfaces bound significantly greater amounts from both buffer and plasma than the analogous heparinized surfaces. Immunoblot analysis of proteins adsorbed from plasma demonstrated that surfaces chemisorbed with PEO, whether or not subsequently modified with ATH, inhibited non-specific adsorption. The immunoblot response for AT was stronger on the DSP-ATH than on the heparin surfaces, thus confirming the results from radiolabelling. The ATH surfaces again showed higher selectivity for AT binding than analogous heparin-modified surfaces, indicating the enhanced anticoagulant potential of ATH for biomaterial surface modification.

Anticoagulant mechanisms of covalent antithrombin-heparin investigated by thrombelastography. Comparison with unfractionated heparin and low-molecular-weight heparin

We have developed an antithrombin-heparin covalent complex (ATH) which inhibits coagulation enzymes by two mechanisms: directly, or by catalytic activation of plasma antithrombin (AT). Anticoagulation by ATH was compared to unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) using a blood-based, tissue factor (TF)-activated thrombelastography (TEG) assay. Simplified TEG assays with plasma or purified plasma components were used to determine the contribution of the direct and catalytic mechanisms to ATH efficacy. Low anti-Xa concentrations of UFH inhibited clot formation significantly more than equivalent concentrations of ATH or LMWH in blood and plasma. ATH had reduced ability to catalyse AT-mediated thrombin (IIa) inhibition compared to UFH. However, at high anti-Xa concentrations, ATH had similar anticoagulant activity to UFH. ATH and non-covalent AT+UFH directly inhibited clotting to a similar degree in AT-deficient plasma. IIa-ATH complexes, which are limited to catalytic inhibition, displayed impaired anticoagulation compared to free ATH, and the magnitude of this effect increased significantly as anticoagulant concentration increased. Kinetic experiments indicated that the rate of reaction of AT with IIa is lower when catalysed by ATH versus UFH. In conclusion, at low anti-Xa doses catalytic inhibition is the primary mechanism of ATH anticoagulation, and the catalytic potential of ATH is reduced relative to UFH. However, the direct inhibitory activity of ATH is comparable to non-covalent AT+UFH, and at high anti-Xa doses the direct inhibitory activity of ATH may play a larger role in anticoagulation.

Intravascular targeting of a new anticoagulant heparin compound

Since most thrombotic reactions occur on the vessel wall, interaction of anticoagulants with vascular components is critical. Heparin (H) is the primary drug for treatment and prevention of thrombosis. To improve H's efficacy and bioavailability, a covalent complex of H and its biological target, antithrombin (AT), was developed. While H has a short, variable intravenous half-life leading to unpredictable anticoagulation, clearance of covalent ATH complex is slower. H's variable anticoagulant effect arises from interactions with plasma and vessel wall proteins. ATH has increased bioavailability due to lower plasma protein and endothelial binding relative to H. Pharmacodynamic studies demonstrate that the AT moiety can regulate ATH binding to target tissues. For example, blood vessel binding is enhanced using ATH containing recombinant AT with oligomannose structures that can interact with endothelial mannose receptor lectins. Furthermore, recent work has shown that inhibition of factor VIIa/tissue factor complex by ATH is significantly faster than AT + H. Similarly, thrombin bound to endothelial thrombomodulin is inhibited more efficiently by ATH than by AT + H, which might improve regulation of thrombin generation. Overall, linking H to AT may prevent unwanted protein interactions and allow vessel wall sites to be targeted. This review examines ATH biodistribution.

Effect of covalent antithrombin-heparin complex on developmental mechanisms in the lung

We have developed a potent antithrombin (AT)-heparin conjugate (ATH) that is retained in the lung to prevent pulmonary thrombosis associated with respiratory distress in premature newborns. During continuing maturation, pulmonary angiogenesis in premature infants would be a crucial process in lung development. A naturally occurring latent form of antithrombin (L-AT) has antiangiogenic effects on lung vascularization. However, impact of latent ATH (L-ATH) on developing lung vascularization is unknown. Thus, effects of L-AT and L-ATH on fetal murine lung development were compared. Lung buds from embryonic day 11.5 (E11.5) Tie2-LacZ mouse embryos were incubated in DMEM plus FBS supplemented with PBS, AT, L-AT, heparin, ATH, or L-ATH. Vasculature of cultured explants was quantified by X-galactosidase staining. RNA was analyzed with murine gene probes for angiopoietin (Ang)-1, Ang-2, fibroblast growth factor 2 (FGF2), platelet endothelial cell adhesion molecule (PECAM), and vascular endothelial growth factor (VEGF). FGF2-supplemented medium was used to test contribution to effects of L-AT and L-ATH on angiogenesis. Epithelial branching morphogenesis was inhibited by L-AT (P = 0.003) and heparin (P < 0.001). L-AT and heparin decreased relative vascular area compared with PBS, ATH, and L-ATH. Expressions of all genes studied were downregulated by L-AT. However, L-AT and L-ATH inhibited branching morphogenesis and vasculature with added FGF2. These findings indicate that covalent linkage of AT to heparin negates disruptive effects of these moieties on lung morphology, vascularization, and growth factor gene expression. ATH may have enhanced safety as an anticoagulant during vascular development.

Treatment of endothelium with the chemotherapy agent vincristine affects activated protein C generation to a greater degree in newborn plasma than in adult plasma

INTRODUCTION

Activated protein C (APC) is well-established as a physiologically important anticoagulant. During development, plasma concentrations of protein C and alpha(2)macroglobulin, factors involved in APC generation, differ from adult levels. Chemotherapy drugs can perturb endothelial expression of PC-activating receptors. This study examines the effect of chemotherapy treatment of endothelium on APC generation in newborn and adult plasma.

MATERIALS AND METHODS

APC generations were initiated on endothelial cells treated with vincristine or media by recalcifying defibrinated plasma with buffer containing thromboplastin. APC generation was terminated by mixing timed subsamples into FFRCMK-EDTA or heparin, followed by EDTA. APC-PCI and APC-alpha(1)AT were assayed by ELISA. APC-alpha(2)M was measured chromogenically. Since heparin converts free APC to APC-PCI, the difference between APC-PCI detected in heparin subsamples and APC-PCI detected in FFRCMK-EDTA subsamples gave the free APC. Cellular expression of EPCR and TM were measured by flow cytometry and Western blot.

RESULTS

Vincristine-treated endothelium decreased free APC generation in newborn plasma to a greater degree than in adult plasma. APC-PCI levels in both adult and newborn plasma were unaffected by chemotherapy. Vincristine treatment reduced levels of APC-alpha(1) AT and APC-alpha(2) M to a greater degree in newborn plasma versus adult plasma. Expression of EPCR was reduced in cells treated with vincristine. Conversely, TM was reduced on the cell surface, but increased in whole cell lysates.

CONCLUSIONS

The differential response of newborn and adult plasma PC components to chemotherapy-mediated changes in cell surface components may be a factor in the increased risk of thrombosis in children receiving chemotherapy.

Protein adsorption on polyurethane catheters modified with a novel antithrombin-heparin covalent complex

Highly anticoagulant covalent antithrombin-heparin complex (ATH) was covalently grafted onto polyurethane catheters to suppress adsorption/activation of procoagulant proteins and enhance adsorption/activation of anticoagulant proteins for blood compatibility. Consistency of catheter coating was demonstrated using immunohistochemical visualization of ATH. The ability of the resulting immobilized ATH heparin chains to bind antithrombin (AT) from plasma, as measured by binding of (125)I-radiolabeled AT, was greater than that for commercially-available heparin-coated catheters, and much greater than for uncoated catheters. Complementary measurements of antifactor Xa (FXa) activity and plasma protein binding were also performed. Both ATH-coated and heparin-coated catheters demonstrated functional binding of exogenous AT. However, the ATH-coated catheters gave a trend towards elevated anti- FXa activities/AT binding ratios, consistent with the higher active pentasaccharide content in starting ATH. Western blot analysis of proteins adsorbed to catheters after incubation with rabbit plasma established protein binding profiles that showed AT and albumin as major plasma proteins adsorbed to ATH-coated catheters, while AT and altered forms of fibrinogen were major plasma protein species adsorbed to heparinized catheters.

Analysis of Inhibition Rate Enhancement by Covalent Linkage of Antithrombin to Heparin as a Potential Predictor of Reaction Mechanism

Antithrombin (AT) inhibition of coagulation enzymes is catalyzed by unfractionated heparin (UFH) and other heparinoids. Reaction proceeds either via conformational activation of the inhibitor or template-mediated binding of both inhibitor and protease. We investigated if the relative inhibition rates of AT + UFH and covalent AT-heparin conjugate (ATH) with coagulation factors might be indicative of the mechanism involved. Rates were determined by discontinuous assay and mechanisms were probed by a variety of binding studies with UFH or ATH heparin chains. Rates were increased more than 2-fold with ATH over AT + UFH in reactions with thrombin, factor (F) VIIa + tissue factor + Ca2+ + lipid, FIXa and FXIa, but not with FXa or FXIIa. In comparison, UFH or ATH heparin binding (evidence of a template mechanism) was only observed with thrombin, tissue factor, FIXa and FXIa. Thus, inhibition rate enhancement by conjugation of AT with heparin were predictive of inhibitor.enzyme template bridging by heparin. Rationales behind this novel concept are discussed.

Chronic performance of polyurethane catheters covalently coated with ATH complex: A rabbit jugular vein model

Covalent complexes of antithrombin (AT) and heparin (ATH) have superb anticoagulant activity towards thrombin and factor Xa. Stability of polyurethane central venous catheters covalently modified with radiolabeled ATH was studied using a roller pump with saline or protease P-5147. Saline wash removed loosely bound ATH molecules to decrease graft density from 26 to 12 pmol/cm2. However, only slightly more ATH was removed by strong protease (from 12 to 7 pmol/cm2). To evaluate ATH-coated, heparin-coated, and uncoated catheters, a chronic rabbit jugular vein model was developed with catheters maintained for up to 30-106 days. Lumen occlusion was tested by drawing blood twice daily. Although unmodified or heparin-coated catheters occluded within 5-7 days after insertion, all ATH catheters remained patent throughout the experiment. Scanning electron microscopy (SEM) analysis of heparin and uncoated catheters revealed extensive thrombosis (lumen+mural) while ATH catheters were unaffected. Visual observation showed significant deposition of protein and cells on control and heparin-modified catheters and, to a lesser degree, on ATH-coated surfaces. SEM showed no fibrin inside or outside of ATH catheters, which remained patent in extended studies out to 106 days. Although atomic force microscopy showed ATH coatings to be rough, 6-fold higher anti-factor Xa activity likely contributed to increased patency. Our data confirm that ATH-modified catheters are stable and have superior potency compared to heparin or control catheters.

Covalent antithrombin-heparin complexes

Unfractionated heparin (UFH) and low molecular weight heparin (LMWH) have been utilized as primary anticoagulants for thrombosis prophylaxis and treatment. However, a number of biophysical and safety limitations have led to development of new anticoagulants. Covalent antithrombin-heparin (ATH) complexes may address many of these issues. Early ATH products were prepared that had increased intravenous half-lives relative to UFH but lacked any improvement in anti-factor Xa activity or had no catalytic activity or reactivity against thrombin. However, a recent conjugate developed by Chan et al. has displayed a number of superior properties. Chan et al. ATH has an increased direct thrombin inhibition rate and can catalyze coagulant enzyme inhibition by exogenous antithrombin with very high specific activity. Unlike UFH, clot-bound thrombin is readily inhibited by ATH and, at similar antithrombotic efficacy, the ATH has improved bleeding profiles compared to heparins. Given the preclinical findings, Chan et al. ATH may warrant clinical trial testing for control of clot propagation.

Selective cleavage of heparin using aqueous 2-hydroxypyridine: Production of an aldose-terminating fragment with high anticoagulant activity

Unfractionated heparin (UFH) was partially depolymerized by heating at 115 degrees C with aqueous 2-hydroxypyridine. Compared to starting UFH, no significant loss of anticoagulant (anti-Xa) activity was observed. Products consisted of polysaccharide fragments and small quantities of ammonia, sulfate, and hexuronic acid. Fragments with aldose termini that reacted with [3H]NaBH4 (fragment A) were of relatively uniform size (6000 D) and increased as depolymerization time increased. Fragment A contained the anticoagulant activity, with 90-94% and 24-31% binding to Sepharose-thrombin and Sepharose-antithrombin, respectively. In contrast, a non-reducing fragment B that did not react with [3H]NaBH4 was more heterogeneous (6000-10,000 D) and did not have anticoagulant activity or Sepharose-antithrombin affinity. Given the polysaccharide 3H-incorporation, small release of monosaccharide products, and fragment A end-group analysis, thermolysis of UFH is likely limited to one site per molecule when protected by 2-hydroxypyridine. Thus, an anticoagulant fragment A is hydrolytically released from UFH leaving a variable-length fragment B complete with linkage region.

Structural Effects of a Covalent Linkage Between Antithrombin and Heparin: Covalent N-Terminus Attachment of Heparin Enhances the Maintenance of Antithrombin's Activated State

We have produced a molecule comprising of permanently-activated covalently linked antithrombin and heparin (ATH). This study was designed to elucidate the covalent linkage point(s) for heparin on antithrombin and conformational properties of the ATH molecule. ATH was produced using Schiff base/Amadori rearrangement by incubating antithrombin with unfractionated heparin for 14 d at 40 degrees C. ATH was then digested using Proteinase K, and the heparin-peptide was reacted with NaIO4/NaBH4/mild acid to degrade the heparin moiety. Sequencing of the remaining peptide was performed by Edman degradation with linkage point confirmation by LC-MS. The degree of insertion of the reactive center loop (RCL) of antithrombin into the A-sheet of ATH was examined using synthesized antithrombin RCL peptides. Binding between the peptides and ATH, and the formation of ATH in the presence of the peptides were tested. CD was used to further examine the secondary and tertiary structures of ATH. The results suggest that heparin is conjugated to the amino terminal of antithrombin in the majority of ATH molecules, proximal to the previously determined heparin binding domain of antithrombin. From the linkage data, a model is proposed for the structure of ATH. Studies using the RCL peptides and CD analysis of ATH support this model.

Effect of substrate and fibrin polymerization inhibitor on determination of plasma thrombin generation in vitro

BACKGROUND

Thrombin generation potential, a critical haemostatic measure, can be determined by continuous detection of total thrombin or direct subsampling. However, differences between methods exist in area under the curve or peak thrombin calculated. Also, impact of anticoagulants on thrombin generation may vary depending on mode of analysis.

OBJECTIVE

We studied the effect of components on thrombin generation in the presence or absence of anticoagulants.

METHODS

The continuous method was conducted with plasma +/- fibrin(ogen) +/- fibrin polymerization inhibitor. Plasma contained slow-reacting TG5134 substrate at 37 degrees C and reaction was started with dilute thromboplastin in CaCl(2)/Tris buffer. Absorbance (405 nm) was recorded over time and free thrombin calculated from total thrombin activity. For the subsampling method, similar plasma mixtures +/- TG5134 were reacted and free thrombin measured directly as the difference in activity against S2238 substrate of timed subsamples taken into EDTA or EDTA + antithrombin + heparin.

RESULTS

Slow-reacting substrate in the continuous method acted as a competitor for thrombin, giving delayed but greater free thrombin than direct subsampling. These differences persisted to varying extents with all anticoagulants tested. In either method, presence of polymerization inhibitor increased the amount of free thrombin. Continuous method detection of alpha(2)macroglobulin complexes was hampered by sensitivity limits leading to inordinate free thrombin calculations. Especially with hirudin, although free thrombin remained at the end of the subsampling method, continuous method calculations assumed no residual free thrombin.

CONCLUSION

In vitro plasma thrombin generation is delayed and increased by slow-acting substrate and fibrin polymerization inhibitor.

Biodistribution of covalent antithrombin-heparin complexes

We have developed a covalent antithrombin-heparin (ATH) complex with advantages compared to non-covalent antithrombin:heparin (AT:H) mixtures. In addition to increased activity, ATH has a longer intravenous half-life that is partly due to reduced plasma protein binding. Given ATH's altered clearance, we investigated biodistribution of ATH in vivo. ATH made from either human plasma-derived AT (pATH) or recombinant human (produced in goats) AT (rhATH) was studied. 125I-ATH + unlabeled carrier was injected into rabbits at different doses. 131I-labeled albumin was administered just before sacrifice as a marker for trapped blood in tissues. Immediately after sacrifice, animal components were removed, weighed, and subsamples were counted for gamma-radioactivity. Percent recoveries of ATH in various organs/compartments at different time points were calculated, and kinetic distribution plots generated. At saturating doses, early disappearance of rhATH from the circulation was much faster than pATH. Co-incident with clearance, 26 +/- 3% of dose for rhATH was liver-associated compared to only 3.7 +/- 0.5% for pATH by 20 min. Also, at early time periods, >60% of all extravascular ATH was liver-associated. Analysis of the vena cava and aorta suggested that vessel wall binding might also account for initial plasma loss of rhATH. By 24 h, most of pATH and rhATH were present as urinary degradation products (51 +/- 3% and 63 +/- 8%, respectively). In summary, systemic elimination of ATH is greatly influenced by the form of AT in the complex, with liver uptake and degradation playing a major role.

The influence of age on in vitro plasmin generation in the presence of fibrin monomer

BACKGROUND AND OBJECTIVES

The components of the fibrinolytic system interact to generate plasmin from its zymogen form, plasminogen. At birth, all the components of the fibrinolytic system are present but with differing plasma concentrations. The present study was undertaken to explore the effect of physiological, age-dependent factors of the fibrinolytic system during childhood on the capacity to generate plasmin.

DESIGN AND METHODS

Total plasmin generation was measured in venous plasma from umbilical cords and adults, on plastic and cell surfaces, in the presence of fibrin monomer, Desafib. Plasminogen, its inhibitors alpha2-antiplasmin and plasminogen activator inhibitor type 1, and plasmin-alpha2-antiplasmin complex in the time samples were assayed by enzyme-linked immunosorbent assay. The effect of addition of plasminogen on the plasmin generation in cord plasma and the effect of lipoprotein on adult and cord plasmin generation were measured.

RESULTS

On the surface of human umbilical vein endothelial cells, onset of plasmin generation was earlier (40 min) compared to plastic (60 min) but total plasmin generation was similar on both surfaces. The addition of plasminogen to cord plasma increased plasmin generation. Supplementation of lipoprotein in adult plasma had an inhibitory effect, but there was no significant effect in cord plasma.

INTERPRETATIONS AND CONCLUSIONS

Plasmin generation is reduced in newborn compared to adult plasma. Decreased plasmin generation in cord plasma is likely due to decreased plasminogen concentration. The antifibrinolytic effect of lipoprotein is more pronounced in adults as compared to newborns due to the presence of higher plasminogen concentration.

The fibrinolytic system in newborns and children

The fibrinolytic system comprises a series of serine proteases that interact to cleave fibrin into fibrin degradation products. Although all key components of the fibrinolytic system are present at birth, important age-dependent, quantitative and qualitative differences are present during childhood as compared to adults. These differences include decreased plasma concentrations of plasminogen, tissue-type plasminogen activator and alpha2-antiplasmin, increased plasma concentrations of plasminogen activator inhibitor-1, as well as a decrease in both plasmin generation and overall fibrinolytic activity. Increasing evidence suggests that these age-dependent differences may contribute to the development of specific childhood diseases and influence the course of fibrinolytic therapy, particularly in newborns. This review aims to summarize the available information on the age-dependent features of the fibrinolytic system in newborns and children in healthy and disease states and the impact of these features on fibrinolytic therapy.

Effect of heparins on thrombin generation in hemophilic plasma supplemented with FVIII, FVIIa, or FEIBA

Central venous catheters assist infusion of coagulation factors in hemophiliacs but can be problematic due to mechanical dysfunction, infection, and thrombosis. The effect of low molecular weight heparin, unfractionated heparin, or covalent antithrombin-heparin complex on thrombin generation in factor concentrate-supplemented hemophilic plasma were studied. Thrombin levels were similar to normal plasma after the addition of factor eight inhibitor bypassing agent to hemophilic plasma. At 0.2 U/ml of heparin, covalent antithrombin-heparin inhibited free thrombin generation to a greater degree than heparin and low molecular weight heparin. Covalent anti-thrombin-heparin may give a greater anticoagulant response in hemophilic plasma supplemented with factor VIII or factor VIIa than with factor eight inhibitor bypassing agent. Requirements for heparin in hemophilic patients with thrombosis may depend on the procoagulant treatment used.

In vivo rabbit acute model tests of polyurethane catheters coated with a novel antithrombin-heparin covalent complex

Catheter use has been associated with an increased risk of thrombotic complications. The objective was to make catheters less thrombogenic with the use of antithrombin-heparin covalent complex (ATH). The antithrombotic activity of ATH-coated catheters was compared to uncoated (control) and heparincoated catheters in an acute rabbit model of accelerated occluding clot formation. Anaesthetized rabbits were pre-injected with rabbit (125)I-fibrinogen, followed by insertion of test catheters into the jugular vein. Blood was drawn and held in a syringe, reinjected, then flushed with saline. The experiment was terminated when blood could no longer be withdrawn (occluding clot). Efficacy was defined as the ability of catheters to remain unoccluded. Clot formation, determined by measuring deposition of radiolabeled fibrin, was a secondary endpoint. ATH-coated catheters were resistant to clotting for the full 240-minute duration, while uncoated and heparin-coated catheters had an average clotting time of 78 and 56 minutes, respectively. The patency of ATH coating was dependant on intact heparin pentasaccharide sequences, rather than the chemistries of the basecoat, the PEO spacer arm, or the antithrombin (AT) protein. The ATH coating was stable to ethylene oxide sterilization, modest abrasion, protease attack, and the coating did not appear to leach off the catheter. Surface tension measurements showed that the ATH modified surface was more hydrophilic than uncoated control catheters or heparin-coated catheters. Thus, ATH-coated catheters are better at preventing clots than uncoated or heparin-coated catheters and show promise as an alternative to the currently available catheters in reducing thrombotic complications associated with its use.

Isoform composition of antithrombin in a covalent antithrombin-heparin complex

Antithrombin (AT) circulates in two isoforms, alpha- (90-95%) and beta-AT (5-10%). AT inhibits clotting factors such as thrombin and factor Xa, a reaction catalyzed by heparin. Heparin has been used in many clinical situations but suffers from limitations such as a short intravenous half-life, bleeding risk, and the inability to inhibit thrombin bound to fibrin clots. In order to overcome some of heparin's limitations, we prepared a covalent AT-heparin complex (ATH) that has increased intravenous half-life, reduced bleeding risk, and can directly inhibit clot-bound thrombin. However, structural analysis is required to further develop this promising antithrombotic agent. It was found that the proportion of isoforms in ATH (55% alpha-AT, and 45% beta-AT) was significantly different than that in the commercial AT starting material (80% alpha-AT and 20% beta-AT). Further analysis of the rate of heparin-catalyzed inhibition of thrombin by AT isoforms prepared from ATH revealed that the beta-variant reacted approximately 2-fold faster.

Mechanisms responsible for catalysis of the inhibition of factor Xa or thrombin by antithrombin using a covalent antithrombin-heparin complex

Covalent antithrombin-heparin (ATH) complexes, formed spontaneously between antithrombin (AT) and unfractionated standard heparin (H), have a potent ability to catalyze the inhibition of factor Xa (or thrombin) by added AT. Although approximately 30% of ATH molecules contain two AT-binding sites on their heparin chains, the secondary site does not solely account for the increased activity of ATH. We studied the possibility that all pentasaccharide AT-binding sequences in ATH may catalyze factor Xa inhibition. Chromatography of ATH on Sepharose-AT resulted in >80% binding of the load. Similar chromatographies of non-covalent AT + H mixtures lead to a lack of binding for AT and fractionation of H into unbound (separate from AT) or bound material. Gradient elution of ATH from Sepharose-AT gave 2 peaks, a peak containing higher affinity material that had greater anti-factor Xa catalytic activity (708 units/mg heparin) compared with the peak containing lower affinity material (112 units/mg). Sepharose-AT chromatography of the ATH component with short heparin chains (<or=12 monosaccharides) resulted in active unbound (40%) and bound fractions (190 and 560 units/mg, respectively). Factor Xa-ATH or thrombin-ATH inhibitor complexes gave chromatograms on Sepharose-AT with more unbound material compared with that of free ATH. Also, ATH did not bind to Sepharose-heparin, and the intrinsic fluorescence due to activation of AT in ATH by its heparin chain was reversed at higher [NaCl] than that required to dissociate non-covalent AT.H complexes. Thus, exogenous AT can compete with the AT moiety of ATH for binding to the covalently linked heparin chain, leading to catalytic inhibition of factor Xa or thrombin. These data may suggest that access to pentasaccharide units in non-covalent AT.H complexes by free AT may be facile.

Overexpression of the 78-kDa glucose-regulated protein/immunoglobulin-binding protein (GRP78/BiP) inhibits tissue factor procoagulant activity

Previous studies have demonstrated that overexpression of GRP78/BiP, an endoplasmic reticulum (ER)-resident molecular chaperone, in mammalian cells inhibits the secretion of specific coagulation factors. However, the effects of GRP78/BiP on activation of the coagulation cascade leading to thrombin generation are not known. In this study, we examined whether GRP78/BiP overexpression mediates cell surface thrombin generation in a human bladder cancer cell line T24/83 having prothrombotic characteristics. We report here that cells overexpressing GRP78/BiP exhibited significant decreases in cell surface-mediated thrombin generation, prothrombin consumption and the formation of thrombin-inhibitor complexes, compared with wild-type or vector-transfected cells. This effect was attributed to the ability of GRP78/BiP to inhibit cell surface tissue factor (TF) procoagulant activity (PCA) because conversion of factor X to Xa and factor VII to VIIa were significantly lower on the surface of GRP78/BiP-overexpressing cells. The additional findings that (i) cell surface factor Xa generation was inhibited in the absence of factor VIIa and (ii) TF PCA was inhibited by a neutralizing antibody to human TF suggests that thrombin generation is mediated exclusively by TF. GRP78/BiP overexpression did not decrease cell surface levels of TF, suggesting that the inhibition in TF PCA does not result from retention of TF in the ER by GRP78/BiP. The additional observations that both adenovirus-mediated and stable GRP78/BiP overexpression attenuated TF PCA stimulated by ionomycin or hydrogen peroxide suggest that GRP78/BiP indirectly alters TF PCA through a mechanism involving cellular Ca(2+) and/or oxidative stress. Similar results were also observed in human aortic smooth muscle cells transfected with the GRP78/BiP adenovirus. Taken together, these findings demonstrate that overexpression of GRP78/BiP decreases thrombin generation by inhibiting cell surface TF PCA, thereby suppressing the prothrombotic potential of cells.

Effect of covalent serpin-heparinoid complexes on plasma thrombin generation on fetal distal lung epithelium

Extravascular coagulation within the lung airspace is a hallmark of respiratory distress syndrome (RDS) in premature infants. We previously showed that covalent antithrombin-heparin complex (ATH) is superior to noncovalent antithrombin (AT) + heparin (H) mixtures at inhibiting plasma thrombin generation on rat fetal distal lung epithelium (FDLE) in vitro. However, heparin cofactor II (HC) has been shown to selectively inhibit thrombin, which may be advantageous if other enzyme activities are present in the airspace. We compared the abilities of ATH, covalent HC-heparin complex (HCH), and covalent HC-dermatan sulfate (HCD) to inhibit thrombin generation on FDLE in plasmas from either adults or newborns. In the presence of ATH, peak free thrombin generation in adult plasma on the cell surface was reduced by 92% compared with controls (no anticoagulant). However, whereas HCH reduced peak free thrombin generation in adult plasma by 81%, HCD was only able to reduce activity by 33%. All covalent complexes caused a greater decrease in thrombin activity compared with that with the corresponding noncovalent serpin + heparinoid mixtures. Experiments in plasma from newborns resulted in peak free thrombin that was less than or equal to that in adult plasma when covalent conjugates were added. Relative peak free thrombin was proportional to rate of prothrombin consumption and amount of thrombin-inhibitor complexes formed. In vivo, experiments in newborn rats showed that a greater percentage of intratracheally instilled ATH and HCH could be recovered in lung lavage fluid compared withwith that for HCD. In summary, ATH, HCH, and HCD are inhibitors of thrombin generation on FDLE superior to the corresponding noncovalent mixtures, with ATH and HCH being more potent than HCD. Covalent conjugates of AT or HC with H may be preferred in treatment of extravascular coagulation.

The effects of chemotherapeutic agents on the regulation of thrombin on cell surfaces

Thromboembolic disorders are common in cancer patients. Two major contributing factors are central venous catheters for drug delivery and the use of l-aparaginase, which decreases the plasma antithrombin level, but the causes of the hypercoagulable state in these patients are not fully understood. In this study, the T24/83 cell line was used as a model to investigate the effects of chemotherapeutic agents on cell surface thrombin regulation. Plasma thrombin generation and prothrombin consumption was increased in most of the treated cells, particularly vincristine- and adriamycin-treated cells (P < 0.05), compared with controls. However, no free thrombin generation or prothrombin consumption was observed in factor VII (FVII)-depleted plasma. No significant differences in the levels of thrombin-alpha2-macroglobulin (IIa-alpha2M) and thrombin-anti-thrombin (TAT) were observed between controls and any of the treatments, except for vincristine- and adriamycin-treated cells, which showed a significant difference in TAT production (P < 0.05). Also, there was an upregulation in tissue factor (TF) mRNA expression in etoposide-, methotrexate- and vincristine-treated monolayers compared with controls, as well as an upregulation in TF protein production in vincristine-treated cells. The data suggests that thrombin generation occurs via the extrinsic (TF-dependent) coagulation pathway on cell surfaces and that some chemotherapeutic agents are able to upregulate TF mRNA and protein expression in T24/83 cells.

Inhibition of fibrin-bound thrombin by a covalent antithrombin-heparin complex

Numerous studies have shown that fibrin-bound thrombin (IIa) is protected from inhibition by antithrombin (AT) + heparin (H) due to the formation of a ternary fibrin.IIa.H complex. We investigated factors affecting the inhibition of fibrin.IIa by a covalent complex of AT and H (ATH). The rate of IIa reaction with ATH was decreased 2-3-fold by fibrin monomer as compared to 57-fold for AT + heparin with high AT affinity. Furthermore, although the reaction of AT + H with a IIa mutant with decreased H binding (RA-IIa) was inhibited 2-3-fold in the presence of fibrin, reaction rates of ATH + RA-IIa were not reduced by fibrin. The relative difference in the effect of fibrin on the ATH reaction with RA-IIa compared to that for reactions of AT + H with RA-IIa is consistent with the fact that, in the absence of fibrin, the rate of the ATH reaction with RA-IIa relative to IIa was much less reduced (8-fold) compared to the corresponding reactions of AT + H (decreased 306 fold). Similarly, the addition of excess H in the absence of fibrin gave only a small decrease in rate of ATH + IIa reaction. However, in the presence of fibrin, the addition of 40-fold excess H decreased the rate of ATH inhibition of IIa by 1 order of magnitude. Experiments with ATH containing low molecular weight heparin chains with low AT affinity showed that IIa inhibition requires ATH with long chains that activate the AT moiety. Finally, electrophoresis of fibrin +/- ((125)I-)IIa +/- ((125)I-)ATH on native and denaturing gels showed that ATH forms ATH-IIa complexes that remain bound to fibrin through the ATH component. Thus, ATH is a potent inhibitor of fibrin-bound IIa, likely due to the formation of fibrin.ATH-IIa as opposed to fibrin.IIa.H ternary complexes.

Decreased concentrations of heparinoids are required to inhibit thrombin generation in plasma from newborns and children compared to plasma from adults due to reduced thrombin potential

Thrombin generation is decreased and delayed in plasma from newborns and children compared to adults. We hypothesized that lower doses of heparinoid anticoagulants are required to give similar thrombin generation in newborn (umbilical cord) and child plasmas compared to that of adults. Thrombin generation was performed in either the absence or presence of unfractionated heparin (UFH), low molecular weight heparin (LMWH) or a covalent antithrombin-heparin complex (ATH). After contact activation and recalcification of each plasma, thrombin activity was measured by periodic sub-sampling into chromogenic substrate. UFH inhibited thrombin generation to a greater extent compared to LMWH in all plasmas. Cord plasma was more sensitive to inhibition and displayed a greater difference in the effectiveness of UFH compared to LMWH than other plasmas. Lower concentrations of UFH and LMWH were required to inhibit thrombin generation in cord and child plasmas compared to adult plasma. In comparison, ATH strongly inhibited thrombin generation in all 3 plasmas. Similar peak thrombin concentrations were observed at lower ATH concentrations (0.1 U/mL) compared to either UFH (0.25 U/mL) or LMWH (0.25 U/mL). As with UFH and LMWH, cord plasma was more sensitive to inhibition by ATH than the other plasmas and lower ATH concentrations inhibited thrombin generation in cord and child plasmas compared to adult plasma. Decreased thrombin generation with heparinoids in cord and child plasmas compared to adult plasma coincided with decreased rates of prothrombin consumption and increased proportion of thrombin-alpha2-macroglobulin inhibitor complexes. In summary, lower doses of UFH, LMWH or ATH result in similar peak thrombin generation in newborn and child plasmas compared to adult plasma. Cord plasma was the most sensitive to inhibition, with ATH being more effective than UFH or LMWH.

Mechanisms responsible for the failure of protamine to inactivate low-molecular-weight heparin

Protamine is unable to completely reverse the anticoagulant effect of the low-molecular-weight heparins (LMWH), a fact of clinical importance given the rapid increase in use of LMWH in clinical practice. This investigation sought to determine the mechanism by which LMWH were able to resist protamine-mediated inactivation. Affinity fractionation of LMWH by passage through a protamine column, with subsequent determination of molecular mass and sulphate charge density, demonstrated that the protamine-resistant fraction in LMWH is an ultra-low-molecular-weight fraction with low sulphate charge density. This group of molecules was not found in unfractionated heparin, even when species of similar molecular mass were compared. We then determined that different commercially available LMWH varied in their ability to be neutralized by protamine, and that this variability correlated with the total sulphate content of the LMWH. We conclude that reduced sulphate charge, not molecular mass, is the principle reason that protamine is unable to fully inactivate LMWH. Furthermore, different LMWH vary in their ability to be neutralized by protamine, suggesting that product-specific recommendations for neutralization might be developed.

Fibrinogen has a rapid turnover in the healthy newborn lamb

The half-lives for coagulation factors in the healthy newborn infant are not known and may be different than for the adult. We measured the half-life for fetal sheep fibrinogen and compared it to the half-life of adult sheep fibrinogen. Fibrinogen was purified from adult and fetal sheep plasma and radiolabeled with either 125I or 131I. The half-lives for these fibrinogens were determined in the adult sheep and newborn lamb. In addition, the fetal and adult sheep fibrinogens were compared by reptilase time, thrombin clotting time, sialic acid content, and the behavior of the N-glycans derived from these fibrinogens on the immobilized lectin, Sepharose-concanavalin A. Finally, the in vivo response of coinjected radiolabeled fibrinogens to increasing doses of infused thrombin was determined. The fetal sheep fibrinogen differed from the adult as indicated by a prolonged reptilase time and an increased sialic acid content (fetal: 10-11 residues/340 Kd versus adult: 8-9 residues/340 Kd). The latter was also reflected in differing chromatographic profiles for the N-glycans on Sepharose-concanavalin A. The half-lives for both the adult and fetal fibrinogen were significantly more rapid in the newborn lamb (fetal: 47 +/- 2.0 h; adult: 46 +/- 2.4 h, mean +/- SEM) than in the adult (fetal: 116 +/- 6.5 h; adult: 121 +/- 6.9 h). Finally, the adult and fetal sheep fibrinogen responded to thrombin in an identical fashion in the intact animal. In summary, both adult and fetal fibrinogen have faster half-lives in the lamb compared tot he adult, despite a higher sialic acid content for the fetal fibrinogen.(ABSTRACT TRUNCATED AT 250 WORDS)

Maternal low-carbohydrate high-protein diet affects mandibular growth in diabetic newborn rats

Dams with 7 pups each were randomly assigned to two different diets. Twelve dams were fed a normal (20%) protein diet and were divided into two groups of 4 and 8 animals. Pups from group 1 (n = 28) were injected with citrate buffer as a control. Pups from group 2 (n = 56) were injected with streptozotocin. Twelve additional dams were fed a 40% protein diet. They were also divided into two groups of 4 and 8 animals. Pups from group 3 (n = 28) were injected with citrate buffer as a control. Pups from group 4 (n = 56) were injected with streptozotocin. Forty-eight hours later, diabetic status was determined using Dextrostix. On Day 15, pups were injected with [14C]proline to determine collagen synthesis and 45Ca to study mineralization. After the pups were killed, blood glucose levels were determined. Then mandibles were removed. Milk from each dam was also collected after injection of oxytocin. At the time of killing, blood glucose levels in diabetic pups were less than earlier levels, though still higher than those of controls on either diet. The weights of body and mandible, collagen contents, and the total calcium contents in the diabetic group were in general less than those of the nondiabetic group on the 20 and 40% protein diets. 45Ca uptake in the diabetic group was significantly increased compared with those of the nondiabetic rats on both diets. The percentage reduction in the mandibles of diabetic rats from those of nondiabetic rats on the 40% protein diets was consistently less than that of animals on the 20% protein diets. The higher protein contents of the maternal milk in the 40% protein group may partly be responsible for the smaller impairment of mandibular development in the diabetic over nondiabetic animals. It is concluded that maternal low-carbohydrate high-protein diets will play indirectly a beneficial role in the development of the mandibles of diabetic newborns.

A comparison of two procedures used for complexing Fe(III) with human apotransferrin. II. Uptake of Fe(III) by K-562 cells from 55Fe . transferrins and Fe . [3H]transferrins

Samples of human apotransferrin (apo . HTr) were saturated with Fe(III) by two different techniques, a method employing excess trisodium citrate to chelate Fe(III) and a nonchelating approach which involves the ferroxidase activity of ceruloplasmin to convert Fe(II)----Fe(III). The samples were radiolabelled with either 55Fe or 3H. Using an initial molar Fe/apo . HTr ratio of 2.0-2.1, preparations of human transferrin with bound Fe (Fe . HTr) using the citrate method invariably contained 2.2-2.4 atoms Fe/molecule, whereas Fe . HTr (ceruloplasmin method) contained 2.0 atoms/molecule as shown by spectrophotometric and radioactivity measurements. Uptake of Fe from these Fe . HTr preparations by K-562 cells grown in a serum-free medium was marginally, but consistently, more rapid from 55Fe . HTr (citrate) than from 55Fe . HTr (ceruloplasmin). Taking account of the different Fe contents of the Fe . HTr preparations, the rate measured over a 2-h period amounted to approximately 12,700 and 16,100 Fe atoms/(cell . min) for Fe . HTr (ceruloplasmin) and Fe . HTr (citrate), respectively. However, cell binding by the two Fe . [3H]HTr preparations did not differ significantly over the 8-h incubation period. Furthermore, from the 3H distribution, the quantities of Fe . HTr bound reversibly at the cell surface and contained within the cell were similar for the two Fe . HTr preparations. The results indicate that apo . HTr may bind Fe in different ways depending on the method of Fe presentation and that the Fe . HTr product can donate Fe to K-562 cells at a rate which may reflect the method used for Fe-complex formation.

A comparison of two procedures used for complexing Fe(III) with human apotransferrin: I. Physicochemical properties of the Fe(III) . transferrin products

Samples of human Fe.transferrin (Fe.HTr) were prepared from a single batch of apotransferrin (apo.HTr) by either the Fe(III)-citrate or the Fe(II)-ceruloplasmin (ferroxidase) method. By using 55Fe, 55Fe.HTr prepared by the citrate method and 55Fe.HTr prepared by the ceruloplasmin method contained 2.2-2.3 and 2.0 Fe/mol, respectively. For both 55Fe.HTr preparations, the isotope was shown to be associated with the protein from the measurement of absorbance at 465 nm and dialysis studies. However, passage of the 55Fe.HTr (ceruloplasmin) reaction mixture through DEAE-cellulose caused 55-60% of 55Fe to be lost from the protein, although no decrease in absorbance at 465 nm was observed. Ion-exchange chromatography of 55Fe.HTr (citrate) did not induce loss of 55Fe. Absorbance measurements showed significant differences between the two Fe.HTr preparations with respect to the ratios A212/A278 and A463/A278. Using an excitation wavelength of 275 nm, the fluorescence intensity ratios relative to apo.HTr were 0.275 and 0.309 for Fe.HTr (citrate) and Fe.HTr (ceruloplasmin), respectively. Electron spin resonance (ESR) measurements confirmed that Fe.HTr (citrate) and Fe.HTr (ceruloplasmin) were saturated with Fe. Hyperfine coupling constants and other features of the resonance profile revealed distinct differences between the two Fe.HTr preparations. Dialysis against H2O caused Fe.HTr (citrate), but not Fe.HTr (ceruloplasmin), to lose absorbance at 465 nm. The ESR profile of Fe.HTr (citrate), after dialysis against H2O, was reduced to multiple splittings and a lack of resolution of the central hyperfine structure. Addition of Na2CO3 restored the absorbance (465 nm) and the ESR pattern of Fe.HTr (citrate). In contrast, these properties of Fe.HTr (ceruloplasmin) were little affected by dialysis against H2O. However, the addition of trisodium citrate to Fe.HTr (ceruloplasmin) caused a reduction in absorbance at 465 nm and a change in ESR profile to resemble that of Fe.HTr (citrate) after dialysis in H2O; these changes, caused by citrate binding to Fe.HTr (ceruloplasmin), were restored to normal by the addition of Na2CO3. The data indicate that different protein conformations result from complexing Fe(III) with apo.HTr by these two different procedures. The two Fe.HTr products may differ, conceivably, in their abilities to transfer Fe to cells.

On the interaction between 5-hydroxytryptamine and N-acetylneuraminic acid under aqueous conditions

A complex designated 5-HT-NeuAc was formed between 5-hydroxytryptamine (5-HT) and N-acetylneuraminic acid (NeuAc) under aqueous conditions. Complex formation was encouraged by exposure to light (3000-3800 A; 1 A = 0.1 nm) and freeze-drying and the freeze-dried complex was isolated by gel filtration chromatography. Although stable to rechromatography on Bio-Gel P-2 if H2O was the eluent, 5-HT-NeuAc dissociated into the free components when placed in 0.1 M NaCl. Chemical analyses of the isolated complex showed that an equimolar amount of 5-HT and NeuAc was present and that all group functions were intact; these data suggested that the association between 5-HT and NeuAc was noncovalent. Spectrophotometric measurements demonstrated a small increase (approximately 12%) in extinction coefficient (275 nm) and a large increase (340- to 440-fold) in fluorescence emission (340 nm) compared with 5-HT alone. Data obtained from 1H nuclear magnetic resonance spectroscopy (250 MHz) of 5-HT and NeuAc standards compared closely to published reports. In comparison, measurements made with 5-HT-NeuAc showed that all 5-HT protons were slightly deshielded; of the NeuAc protons, slight deshielding of H8 and significant shielding of H3eq, H3ax, and H6 was observed. From these observations, a model describing the association between 5-HT and NeuAc is proposed.

The role of proteolytic enzymes derived from crude bacterial collagenase in the liberation of hepatocytes from rat liver. Identification of two cell-liberating mechanisms

Crude bacterial collagenase was chromatographed on DEAE-cellulose to yield three peaks with proteolytic activity: an arginine esterase (DEAE-1), gelatinase (DEAE-2) and a caseinolytic activity (DEAE-3). The arginine esterase and gelatinase activity fractions were slightly contaminated with each other but neither possessed caseinolytic activity; the caseinolytic fraction was devoid of arginine esterase and gelatinase activities. In addition, crude collagenase was fractionated by ZnII-affinity chromatography to produce a gelatinase peak (ZnII peak 1), which was free from arginine esterase and caseinolytic activities. The four fractions were compared to crude collagenase in their ability to liberate rat hepatocytes by using either liver slices or a standard perfusion technique. Compared to crude collagenase (0.05-0.1% w/v), which produced 70-80% liver digestion with approximately 80% cell viability, digestion with equivalent quantities of the isolated enzymic activities was relatively poor. Gelatinase activity (ZnII peak 1) was wholly ineffective and DEAE-1 and DEAE-2 each possessed only slight digestive properties. Hepatocyte liberation by the caseinolytic activity, DEAE-3, was partially successful (30-40% digestion, 25-30% viability) but only a portion of liver tissue was digested regardless of the quantity of DEAE-3 used. However, by mixing certain fractions before perfusion two gelatinase-dependent, cell-releasing mechanisms were identified: (a) DEAE-3 with ZnII peak 1 and (b) DEAE-1 mixed with either DEAE-2 or ZnII peak 1. Each system compared creditably with the digestive properties of an equivalent activity of crude collagenase. At present we are attempting to determine any differences between hepatocytes produced by the two enzymic mechanisms.

The structural heterogeneity of the carbohydrate moiety of desialylated human transferrin

Human transferrin consists of a single chain polypeptide which supports two N-glycosidically linked glycans at sequons a and b. Glycopeptides were released from human transferrin by proteolytic digestion, desialylated by mild acid hydrolysis, and then isolated by chromatographic methods. The structures of the glycans located on each sequon were determined by a combination of analytical techniques including Smith degradation, permethylation, and enzymic degradation. Approximately 79% of the total glycan from sequon a was of the biantennary type as previously described by Dorland and his colleagues (FEBS Lett. 77, 15-20 (1977)). The remaining 21% consisted of a mixture of triantennary and tetraantennary glycans, each amounting to approximately 10% of the total glycan for this sequon. The triantennary structure resembled that described for the N-glycosidic triantennary glycans of bovine fetuin by Nilsson and his colleagues (J. Biol. Chem. 254, 4545-4553 (1979)). Of the tetraantennary glycan, approximately half of the structures were incomplete, i.e., one antenna terminated by N-acetylglucosamine. On sequon b, 81% of the glycan was biantennary, identical to those biantennary glycans of sequon a, and the reminder was triantennary, also of the fetuin type. The glycan structures and their locations on the polypeptide are related to the known subpopulations of human transferrin.