Thiol Proteinase Inhibitor in the Ascitic Fluid of Sarcoma 180 Tumor-bearing Mice

Abstract A thiol proteinase inhibitor (TPI) has been purified from the ascitic fluid of Sarcoma 180 tumor-bearing mice. The molecular weight of the inhibitor was estimated to be 67,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the substance inhibited papain, cathepsins B and L, but not cathepsins H and D and trypsin. The inhibitor also liberated kinin upon treatment with trypsin or mouse glandular kallikrein, indicating that the inhibitor is a kininogen, and the kinin liberated upon trypsinization was identified as bradykinin. An immunoreactive TPI with a molecular weight indistinguishable from that of the ascites TPI was found in plasma of non-tumor-bearing mice as well as that of tumor bearers. Plasma levels of immunoreactivity were increased up to twice the normal levels in tumor bearers inoculated with Sarcoma 180 or 3LL tumor cells. Supplementation of the purified ascites TPI into Sarcoma 180 culture medium caused a significant suppression of cell growth as well as [ 3 H]thymidine incorporation at a concentration below that normally present in plasma. In contrast, addition of ascites-TPI to cultured mouse embryonic cells caused enhancement of cell growth as well as [ 3 H]thymidine incorporation. These results indicate that in mice responding to tumor growth, a TPI corresponding to a kininogen is induced which may regulate tumor growth by countering tumor-related proteolytic activity.