Angiostatin is an anti- angiogenic 38- 45 kDa proteolytic fragment of Plasminogen, a 92- 100 kDa glycosylated blood zymogen that serves as the precursor for Plasmin. Plasminogen is produced primarily in the liver, but also in other tissues. Angiostatin circulates in the plasma, binds endothelial cells and myeloid cells, is present in platelet granules, and is excreted in the urine. Human Plasminogen contains an N-terminal activation peptide between amino acids (aa) 1- 98, five characteristically folded kringle domains (aa 103- 561), and a peptidase S1 domain (aa 581- 808). Cleavage of the activation peptide produces mature Plasminogen, while further cleavage between Arg580 and Val581 by tPA (tissue plasminogen activator) produces the disulfide-linked two-subunit enzyme plasmin that dissolves fibrin clots. Angiostatin was first identified as consisting of kringles 1- 4, a form called K1- 4. Human Angiostatin (K1- 4, aa 99- 459) shares 79- 80% aa sequence identity with mouse, rat, canine, feline, porcine and bovine K1- 4. Other anti- angiogenic forms include kringles 1- 3 (K1- 3), or 1- 4 plus most of kringle 5 (K4.5). K4.5, which is reported to be the most active form, occurs in vivo by autoproteolysis of mature Plasminogen in the presence of either a sulfhydryl donor or cell surface actin, while matrix metalloproteins such as MMP3, 7, 9 and 19 can create multiple forms.
