A dot-blot assay for heparin-binding proteins

Semi-synthetic heparin derivatives: chemical modifications of heparin beyond chain length, sulfate substitution pattern and N-sulfo/N-acetyl groups

The glycosaminoglycan heparin is a polyanionic polysaccharide most recognized for its anticoagulant activity. Heparin binds to cationic regions in hundreds of prokaryotic and eukaryotic proteins, termed heparin-binding proteins. The endogenous ligand for many of these heparin-binding proteins is a structurally similar glycosaminoglycan, heparan sulfate (HS). Chemical and biosynthetic modifications of heparin and HS have been employed to discern specific sequences and charge-substitution patterns required for these polysaccharides to bind specific proteins, with the goal of understanding structural requirements for protein binding well enough to elucidate the function of the saccharide-protein interactions and/or to develop new or improved heparin-based pharmaceuticals. The most common modifications to heparin structure have been alteration of sulfate substitution patterns, carboxyl reduction, replacement N-sulfo groups with N-acetyl groups, and chain fragmentation. However, an accumulation of reports over the past 50 years describe semi-synthetic heparin derivatives obtained by incorporating aliphatic, aryl, and heteroaryl moieties into the heparin structure. A primary goal in many of these reports has been to identify heparin-derived structures as new or improved heparin-based therapeutics. Presented here is a perspective on the introduction of non-anionic structural motifs into heparin structure, with a focus on such modifications as a strategy to generate novel reduced-charge heparin-based bind-and-block antagonists of HS-protein interactions. The chemical methods employed to synthesize such derivatives, as well as other unique heparin conjugates, are reviewed.

Synthesis and characterization of highly sensitive heparin probes for detection of heparin-binding proteins

Three labeled heparin species were synthesized as probes for heparin-binding protein detection. Heparin conjugated with 5([4,6-dichlorotriazin-2-yl]amino)fluorescein can be iodinated to a high specific activity. This probe specifically detected 40 pg histone on a dot blot without affinity purification. Heparin biotinylated on its naturally occurring primary amino groups also detected known heparin-binding proteins in a specific manner. This probe detected lower amounts of collagen I and basic fibroblast growth factor on nitrocellulose membranes than did the iodinated probe, with comparable detection times. To create more attachment sites for biotin, we covalently attached amino groups to the hydroxyl groups of heparin using 3-bromopropylamine hydrobromide. After biotinylation, the amino-rich probe detected heparin-binding proteins at the same or higher sensitivity as the biotinylated native heparin probe, using 100-fold less probe and much shorter detection times. This method of labeling is generally applicable to other polysaccharides, and would be useful when the amount of ligand is limited. We show that these three probes detect essentially the same spectrum of proteins in detergent extract of smooth muscle cell plasma membrane, and expect them to be useful probes for detection of cell-surface heparin receptors.

Molecular design and modeling of protein-heparin interactions

The methods and approaches taken to investigate heparin-apoE peptide interactions have involved a series of steps, including (1) identification of the heparin-binding domains of apoE, (2) determination of the minimal amino acid sequence regions involved in heparin binding, heparin-induced conformational changes, and stability of apoE peptide structures in solution, (3) modeling of these peptide and oligosaccharide structures, and (4) examination of their behavior during molecular dynamics calculations to determine if the modeled complexes simulate the results of the solution study. The heparin-binding regions of apoE were determined by fragmentation of the protein and identification of the heparin-binding fragments by ligand-blotting procedures using 125I-labeled heparin. Studies with synthetic peptide fragments of various lengths and dot-blot procedures with 125I-labeled heparin identified the minimal residues critical for heparin-binding and CD studies established the prominent secondary structures of these domains. These studies also showed that heparin binds to the apoE(211-243) and apoE(129-169) regions to induce and stabilize beta-strand and alpha-helical peptide conformations. Secondary structure algorithms were used to identify the specific residues with the highest probabilities of forming alpha-helix and beta-strand structures. Based on the predictive algorithms, the apoE(211-234) and apoE(129-159) structures were built using the Insight program and their molecular interactions with various heparin oligosaccharide models were investigated by molecular dynamics. In agreement with the solution studies in the presence of salt, the molecular dynamics studies showed that the oligosaccharides stabilized the beta-strand and alpha-helical peptide configurations against simulated thermal denaturations. Further modeling studies are in progress to examine the mechanism of the heparin-induced increase in ordered structure of these peptides.

In-gel ligand blotting with 125I-heparin for detection of heparin-binding growth factors

125I-labeled heparin was used to detect basic fibroblast growth factor (bFGF) in crude tumor cell extracts after separation by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. 125I-labeled heparin bound avidly to native recombinant bFGF immobilized on nitrocellulose and eluted with 1.5-2.0 M NaCl. However, Western transfer of bFGF to nitrocellulose after SDS-polyacrylamide gel electrophoresis destroyed heparin-binding ability. In contrast, bFGF was detected by incubation of the polyacrylamide gels directly with 125I-labeled heparin in a gel overly technique. Heparin affinity and NaCl elution pattern from bFGF in gel were similar to those observed for native bFGF spotted on nitrocellulose. This procedure permitted detection of bFGF in crude extracts of a human astrocytoma cell line. In view of the rapid growth of the heparin-binding fibroblast growth factor gene family, this technique should prove useful for the rapid and sensitive detection of other heparin-binding growth factors.

Isolation and characterization of four heparin-binding cyanogen bromide peptides of human plasma apolipoprotein B

Apolipoprotein B-100 (apoB-100) is the major protein constituent of human plasma low-density lipoproteins (LDL). On the basis of its amino acid sequence [Chen, S.-H., Yang, C.-Y., Chen, P.-F., Setzer, D., Tanimura, M., Li, W.-H., Gotto, A. M., Jr., & Chan, L. (1986) J. Biol. Chem. 261, 12918-12921], apo B-100 is one of the largest monomeric proteins known with a calculated molecular weight of 512937. Heparin binds to the LDL surface by interacting with positively charged amino acid residues of apoB-100, forming soluble complexes in the absence of divalent metals and insoluble complexes in their presence. The purpose of this study was to isolate and characterize the heparin-binding domain(s) of apoB-100. Human plasma LDL were fragmented with cyanogen bromide (CNBr). After delipidation and reduction-carboxymethylation, the CNBr peptides were fractionated by sequential chromatography on DEAE-Sephacel, Mono S, and high reactive heparin (HRH) AffiGel-10; HRH was purified by chromatography of crude bovine lung heparin on LDL AffiGel-10. Heparin-binding peptides were further purified by reverse-phase high-performance liquid chromatography. Heparin-binding activity was monitored by a dot-blot assay with 125I-HRH. The amino-terminal sequences of four CNBr heparin-binding peptides (CNBr-I-IV) were determined. CNBr-I-IV correspond to residues 2016-2151, 3109-3240, 3308-3394, and 3570-3719, respectively, of the amino acid sequence of apoB-100. Each CNBr peptide contains a domain(s) of basic amino acid residues which we suggest accounts for their heparin-binding activity.(ABSTRACT TRUNCATED AT 250 WORDS)