Human Beta Thromboglobulinb-thromboglobulin (b-TG), is a low molecular weight, heparin-binding, platelet-derived protein. It is similar to platelet factor-4 (PF-4) in that it is localized within the platelet alpha-granule at levels reported to range from 8.1-24.2 g per 109 platelets. The relative concentration of b-TG in platelets exceeds that of plasma by 260,000-fold 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 consisting of four identical 8800 molecular weight peptide chains. 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). b-TG is derived from the proteolytic removal of four NH2-terminal amino acid residues from a LAPF-4. Immunological screening of partially fractionated supernatant from activated platelets revealed a highly basic form of b-TG distinct from LAPF-4. This basic b-TG species, termed platelet basic protein (PBP), was subsequently isolated 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.
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, it was later concluded that this activity was due to growth factor contamination. b-TG has also been reported to inhibit prostacyclin-I2 production by endothelial cells, however, the relevance of this effect has been called into question. The chemotactic activity of platelet alpha-granule proteins for human fibroblasts has been attributed to both PF-4 and b-TG.
Human b-TG is prepared from the supernatant of activated platelets by heparin-agarose affinity chromatography and gel filtration. The purified protein is supplied in 25 mM Hepes, 150 mM NaCl pH 7.4 and should be stored at -80 C. Purity is assessed by SDS-PAGE analysis.