Plasmin is processed in the
conditioned medium of HT1080
fibrosarcoma cells producing fragments with the domain structures of the
angiogenesis inhibitor,
angiostatin, and
microplasmin.
Angiostatin consists of kringle domains 1-4 and part of kringle 5, while
microplasmin consists of the remainder of kringle 5 and the
serine proteinase domain. Our findings indicate that formation of
angiostatin/
microplasmin involves reduction of
plasmin by a
plasmin reductase followed by proteolysis of the reduced
enzyme. We present evidence that the Cys461-Cys540 and Cys511-Cys535
disulfide bonds in kringle 5 of
plasmin were reduced by
plasmin reductase.
Plasmin reductase activity was secreted by HT1080 and Chinese hamster ovary cells and the
human mammary carcinoma cell lines MCF-7, MDA231, and BT20 but not by the monocyte/macrophage cell line THP-1. Neither primary foreskin fibroblasts, blood monocyte/macrophages, nor macrovascular or microvascular endothelial cells secreted detectable
plasmin reductase. In contrast, cultured bovine and rat vascular smooth muscle cells secreted small but reproducible levels of
plasmin reductase. Reduction of the kringle 5
disulfide bonds triggered cleavage at either Arg529-Lys530 or two other positions C-terminal of Cys461 in kringle 5 by a
serine proteinase.
Plasmin autoproteolysis could account for the cleavage, although another
proteinase was mostly responsible in HT1080
conditioned medium. Three
serine proteinases with apparent Mr of 70, 50, and 39 were purified from HT1080
conditioned medium, one or more of which could contribute to proteolysis of reduced
plasmin.