Heparin And Pf4

Heparin and heparan sulfate constitute a distinct class of glycosaminoglycans (GAGs). GAGs are long, linear polymers composed of repeating disaccharide subunits. Heparin and heparan sulfate belong to a family of polysaccharide species, the chains of which are made up of alternating 1-4-linked and variously sulfated residues of hexuronic (D-glucuronic or L-iduronic) acid and D-glucosa-mine. The two substances differ in their hexuronic acid composition and pattern of substitution, with heparin having a higher content of sulfates and, consequently, a greater linear charge density. Commercially available heparin preparations are heterogeneous and polydisperse, consisting of polysaccharide fragments ranging in length from 3000 to 30,000 Da (10-100 saccharide residues) (see Chapter 7).

Heparan sulfate, together with chondroitin sulfate and dermatan sulfate, is widely distributed in the body, whereas heparin is found only in lung, ileum, skin, lymph nodes, thymus, and appendix, sites where mast cells are concentrated (Gomes and Dietrich, 1982). Metachromatic granules of mast cells are the major reservoir of heparin (Metcalfe et al., 1979). Heparan sulfate and other GAGs are also found in mast cell granules but are expressed mainly on the surface of nearly all adherent mammalian cells in the form of proteoglycans, consisting of oligosaccharides covalently linked to a core protein (syndecan) (Hook et al., 1984).

PF4, a heparin-binding protein normally found in platelet a-granules, is secreted when platelets are activated by various stimuli. Human PF4 is a member of a large family of homologous proteins, encoded by genes located on chromosomes 4 and 17, which have been designated "chemokines," and they are involved in chemotaxis, coagulation, inflammation, and cell growth (Zlotnik et al., 2000; Rollins, 1997; Luster, 1998). This family has been separated into four branches, designated CX3C, CXC, CC, and C, based on the relative position of the first two conserved cysteines. PF4 belongs to the CXC family, which includes, among others, interleukin-8 (IL-8), interferon-g-inducible protein (IP-10), platelet basic protein (PBP), and two proteins derived from PBP by proteolytic cleavage: b-thromboglobulin (b-TG) and neutrophil-activating protein-2 (NAP-2). Human PF4 is a symmetrical, tetrameric molecule made up of identical subunits, each containing 70 amino acid residues of known sequence (Poncz et al., 1987), including two disulfide bonds, a single tyrosine, but no tryptophan. The molecule is positively charged at physiological pH (Handin and Cohen, 1976). The crystal structure of human PF4 has been resolved (Zhang et al., 1994). Lysine residues on the exterior faces of a-helices at the COOH-terminus of each monomer are critical for heparin binding (Loscalzo et al., 1985). However, residues located elsewhere on the tetramer are probably also important for this interaction (Maccarana and Lindahl, 1993; Mayo et al., 1995a).

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