Purification and properties of plasminogen activators from epithelial cells

Two epithelial plasminogen activators were purified from the serum-free conditioned medium of guinea pig keratocytes (GPK) and human breast epithelial (BEB) cells in culture. The cells were cultured on Cytodex 3 microcarrier beads in Eagles' minimum essential medium. The purification procedure was essentially as described by Rijken and Collen (1981) [J. Biol. Chem. 265, 7035-7041]. The specific activities of the purified GPK and BEB activators were 12500 and 6000 IU/mg. Unlike other tissue activators, both the epithelial activators had an isoelectric point of approximately 4.7 +/- 0.2. Pure enzymes were shown to be homogeneous by dodecyl sulphate/polyacrylamide gel electrophoresis with an apparent molecular mass of 62 +/- 2 kDa under reducing conditions. Immunological experiments have shown that both the activators are different from urokinase and do not cross react with anti-urokinase antibodies. Both GPK and BEB activators bound tightly to fibrin clots in vitro. Preliminary N-terminal sequence results indicate that both the epithelial activators appear to be similar to one another but different from melanoma and other tissue activators. These findings indicate that the plasminogen activators secreted by epithelial cells represent a unique and different class of tissue plasminogen activator.