| Domain Structure of b-Thromboglobulin: The domain structure of the b-thromboglobulin monomer is represented. The b-thromboglobulin monomer is an 8,800 molecular weight peptide which is derived via NH2-terminal proteolysis of a precursor molecule LAPF-4 (low affinity platelet factor-4). Although the COOH-terminal domain of b-thromboglobulin is characterized by a clustering of basic lysine residues, the affinity of b-thromboglobulin for heparin is significantly weaker than that of platelet factor-4. In its native state, b-thromboglobulin is a homotetramer consisting of four, identical, noncovalently-associated peptide chains. b-Thromboglobulin (b-TG), is a low molecular weight, heparin-binding, platelet-derived protein (1). It is similar to platelet factor-4 (PF-4) in that it is localized within the platelet a-granule at levels reported to range from 8.1-24.2ug per 109 platelets (2,3). The relative concentration of b-TG in platelets exceeds that of plasma by 260,000-fold (4) making b-TG a convenient marker of platelet activation. Structurally, b-TG is analogous to PF-4 in that, in its native state, b-TG is a tetramer (1) consisting of four identical 8800 molecular weight peptide chains (5). In contrast to PF-4, b-TG exhibits a lower affinity for heparin and also exists as a larger molecular weight species known as low affinity PF-4 (LAPF-4) (2). b-TG is derived from the proteolytic removal of four NH2-terminal amino acid residues from a LAPF-4 (6,7). Immunological screening of partially fractionated supernatant from activated platelets revealed a highly basic form of b-TG distinct from LAPF-4 (7). This basic b-TG species, termed platelet basic protein (PBP), was subsequently isolated (8) and later concluded from immunological, peptide sequencing, and proteolytic processing studies to be a higher molecular weight precursor form of both LAPF-4 and b-TG (9,10). The physiological function of b-TG is not known. While early studies suggested that the precursor forms of b-TG were mitogenic for mouse fibroblasts (8,11), it was later concluded that this activity was due to growth factor contamination (10). b-TG has also been reported to inhibit prostacyclin-I2 production by endothelial cells (12), however, the relevance of this effect has been called into question (13,14). The chemotactic activity of platelet a-granule proteins for human fibroblasts has been attributed to both PF-4 and b-TG (15). Source: Human plasma Purity: Human b-TG is prepared from the supernatant of activated platelets by heparin-agarose affinity chromatography and gel filtration (1,2). Purity is assessed by SDS-PAGE analysis. Concentration: 0.89mg/ml. Form: Supplied as a liquid in 20mM Tris, 150mM NaCl, 2mM CaCl2, pH7.4. Localization: Platelet a-granule (3) Mode of Action: Heparin-binding protein Extinction coefficient: E1%1cm, 280nm=2.6, Calculated based upon amino acid sequence and molecular weight Structure: Homotetramer (monomer, Mr~8800) (5). Applications: Suitable as a marker of platelet activation. Other applications not tested. Recommended Dilution: Optimal dilutions to be determined by the researcher. Storage and Stability: May be stored at 4C for short-term only. For long-term storage, aliquot and store at -20C. Aliquots are stable for at least 6 months at -20C. For maximum recovery of product, centrifuge the original vial after thawing and prior to removing the cap. Further dilutions can be made in assay buffer. |
