Separation of latent, prelatent, and native forms of human antithrombin by heparin affinity high-performance liquid chromatography

When antithrombin substitution strikes back

Commercially available products used for antithrombin supplementation, for example, in extracorporeal life support, may contain latent antithrombin, a hyper-stable strongly procoagulative and anti-angiogenic residue. Latent antithrombin is associated with severe thrombosis in the critically ill. In the manufacturing process of fractionated antithrombin from plasma, heat treatment, citrate, and freeze drying speed up the transformation of native antithrombin to latent antithrombin. Manufacturers are not required to assess and report the latent antithrombin content of their products. When reported, the latent antithrombin fractions in their product range from <1% to 40% of total antithrombin compared with <3% in the healthy adult and less in children. The aims of this work were (1) to convey increased awareness to clinicians who may experience defaulted, expected effect after antithrombin supplementation in, for example, heparin anticoagulation during extracorporeal life support and (2) to urge manufacturers to assess and disclose latent antithrombin content in their products.

Characterization of conformers and dimers of antithrombin by capillary electrophoresis-quadrupole-time-of-flight mass spectrometry

Antithrombin (AT) is a plasma glycoprotein which possesses anticoagulant and anti-inflammatory properties. AT exhibits various forms, among which are native, latent and heterodimeric ones. We studied the potential of capillary electrophoresis-mass spectrometry (CE-MS) using a sheath liquid interface, electrospray ionization (ESI), and a quadrupole-time-of-flight (Q-TOF) mass spectrometer to separate and quantify the different AT forms. For CE separation, a neutral polyvinyl alcohol (PVA) coated capillary was employed. The protein conformation was preserved by using a background electrolyte (BGE) at physiological pH. A sheath liquid of isopropanol-water 50:50 (v/v) with 14 mM ammonium acetate delivered at a flow rate of 120 μL h^-1 resulted in optimal signal intensities. Each AT form exhibited a specific mass spectrum, allowing unambiguous distinction. Several co-injection experiments proved that latent AT had a higher electrophoretic mobility (μ_ep) than native AT, and that these conformers could associate to form a heterodimer during the CE analysis. The developed CE-MS method enabled the detection and quantitation of latent and heterodimeric forms in a commercial AT preparation stored at room temperature for three weeks.

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.

Characterization of the conformational alterations, reduced anticoagulant activity, and enhanced antiangiogenic activity of prelatent antithrombin

A conformationally altered prelatent form of antithrombin that possesses both anticoagulant and antiangiogenic activities is produced during the conversion of native to latent antithrombin (Larsson, H., Akerud, P., Nordling, K., Raub-Segall, E., Claesson-Welsh, L., and Björk, I. (2001) J. Biol. Chem. 276, 11996-12002). Here, we show that the previously characterized prelatent antithrombin is a mixture of native antithrombin and a modified, true prelatent antithrombin that are resolvable by heparin-agarose chromatography. Kinetic analyses revealed that prelatent antithrombin is an intermediate in the conversion of native to latent antithrombin whose formation is favored by stabilizing anions of the Hofmeister series. Purified prelatent antithrombin had reduced anticoagulant function compared with native antithrombin, due to a reduced heparin affinity and consequent impaired ability of heparin to either bridge prelatent antithrombin and coagulation proteases in a ternary complex or to induce full conformational activation of the serpin. Significantly, prelatent antithrombin possessed an antiangiogenic activity more potent than that of latent antithrombin, based on the relative abilities of the two forms to inhibit endothelial cell growth. The prelatent form was conformationally altered from native antithrombin as judged from an attenuation of tryptophan fluorescence changes following heparin activation and a reduced thermal stability. The alterations are consistent with the limited structural changes involving strand 1C observed in a prelatent form of plasminogen activator inhibitor-1 (Dupont, D. M., Blouse, G. E., Hansen, M., Mathiasen, L., Kjelgaard, S., Jensen, J. K., Christensen, A., Gils, A., Declerck, P. J., Andreasen, P. A., and Wind, T. (2006) J. Biol. Chem. 281, 36071-36081), since the (1)H NMR spectrum, electrophoretic mobility, and proteolytic susceptibility of prelatent antithrombin most resemble those of native rather than those of latent antithrombin. Together, these results demonstrate that limited conformational alterations of antithrombin that modestly reduce anticoagulant activity are sufficient to generate antiangiogenic activity.

Fractionation of proteins by heparin chromatography

Heparins are negatively charged polydispersed linear polysaccharides which have the ability to bind a wide range of biomolecules including enzymes, serine protease inhibitors, growth factors, extracellular matrix proteins, DNA modification enzymes and hormone receptors. In this chromatography, heparin is not only an affinity ligand but also an ion exchanger with high charge density and distribution. Heparin chromatography is an adsorption chromatography in which biomolecules can be specifically and reversibly adsorbed by heparins immobilized on an insoluble support. An advantage of this chromatography is that heparin-binding proteins can be conveniently enriched using its concentration effect. This is especially important for separating low abundance proteins for the analysis in two-dimensional electrophoresis (2DE) or other proteomics approaches. Heparin chromatography is a powerful sample-pretreatment technology that has been widely used to fractionate proteins from extracts of prokaryotic organism or eukaryotic cells. As an example, the fractionation of fibroblast growth factors (FGFs) from the extract of mouse brain microvascular endothelial cells (MVEC) is now introduced to demonstrate the procedure of heparin chromatography.

Heparin chromatography to deplete high-abundance proteins for serum proteomics

BACKGROUND

Serum is a very informative sample for disease diagnosis. However, a few of the high-abundance proteins existing in serum make the identification of disease-specific serum biomarkers extremely challenging using currently available technologies. A highly promising first step for most analytical approaches of serum is to deplete as many of the high-abundance proteins as possible.

METHODS

We introduced the traditional method of heparin chromatography coupled with protein G sepharose to deplete the high-abundance proteins for serum proteomics.

RESULTS

Compared with the multiple affinity removal system (MARS) column (a commercial version to deplete 6 major proteins in serum), heparin chromatography can deplete more high-abundance proteins in a single step, especially many high molecular-weight proteins. Using this simple and inexpensive method to pretreat serum for 2-DE analysis, more protein spots can be visualized. IgGs depletion by protein G sepharose can further enhance the resolution of the resulting serum proteome.

CONCLUSIONS

Heparin chromatography coupled with protein G appears to be an efficient and economical strategy to pretreat serum for serum proteomics.

Purification and characterization of recombinant human antithrombin containing the prelatent form in Chinese hamster ovary cells

Antithrombin (AT) is a serine proteinase inhibitor and a major regulator of the blood coagulation cascade. AT in human plasma has two isoforms, a predominant alpha-isoform and a minor beta-isoform; the latter lacks N-glycosylation at Asn 135 and has a higher heparin affinity. From the difference in its folding states, the AT molecule can be separated into three forms: a native form, a denatured and inactive form known as the latent form, and a partially denatured form called the prelatent form. In this study, we purified and characterized recombinant human AT (rAT) containing the prelatent form produced by Chinese hamster ovary (CHO) cells. When rAT was purified at physiological pH, its specific activity was lower than that of plasma-derived human AT (pAT). The latent and prelatent forms were detected in rAT by using hydrophobic interaction chromatography analysis. However, when rAT was purified at alkaline pH, the prelatent form was reversibly folded to the native form and the inhibitory activity of rAT increased to a value similar to that of pAT. Highly purified rAT was analyzed and compared with pAT by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, circular dichroism spectroscopy, amino acid composition, N-terminal sequence, monosaccharide composition, peptide mapping, and heparin-binding affinity. From these analyses, rAT was found to be structurally identical to pAT, except for carbohydrate side-chains. rAT in CHO cells had a high beta-isoform content and it caused a higher heparin affinity than by pAT and also pH-dependent reversible inhibitory activity.

Pasteurization of antithrombin without generation of the prelatent form of antithrombin

Human antithrombin (AT) is the major inhibitor of blood coagulation and has also been shown to exert anti-inflammatory and anti-angiogenic effects. Pasteurization of pharmaceutical AT products is usually performed at 60 degrees C for 10h in the presence of sodium citrate as stabilizer, sometimes in combination with sucrose. These stabilizers significantly decrease the aggregation and denaturation of AT, but during the pasteurization, a small amount of latent AT (LAT), a partially denatured form, is usually generated, as is an equal amount of another latent form of AT, the so-called prelatent AT (PLAT). The LAT formed during pasteurization has a rather low affinity to heparin and is easily removed by using a second heparin affinity chromatography step in the production process. This is in contrast to the PLAT, which has a slightly lower affinity to heparin than does native AT, which makes it hard to remove. Hence, four commercial products of pasteurized AT were previously shown to contain about 4% of PLAT. In the present work, an alternative pasteurization method is presented, where 2M ammonium sulfate and 50% sucrose are used as stabilizers. During this pasteurization, no, or trace amounts ( < 0.5%), of PLAT may be generated with no formation of aggregates. Moreover, the pasteurized AT has the same specific thrombin-inhibiting activity when compared to incubation in the presence of citrate and sucrose. Heparin affinity high-performance liquid chromatography was used for the determination of PLAT, LAT, and AT.