Serine proteases play an important role in a diverse array of biological processes, including embryogenesis,
metastasis, angiogenesis, thrombolysis and tissue invasion by certain parasites. The latter observation prompted us to explore the possibility that the tissue-invasive ocular parasite Acanthamoeba castellanii elaborates one or more
serine proteases. Acanthamoeba sp. are pathogenic free-living amoebae that can produce an invasive, blinding inflammatory disease of the cornea, termed
Acanthamoeba keratitis. The present study reports the preliminary purification and characterization of a novel
plasminogen activator from an ocular isolate of A. castellanii. The parasite-derived
enzyme has a molecular mass of approx. 40 kDa and produces a single band of lysis on
fibrinogen-
agarose zymographs. Activity of the
enzyme is completely inhibited by treatment with
diisopropylfluorophosphate, indicating that it is a
serine protease. The parasite-derived
serine protease is not inhibited by
amiloride which is a strong inhibitor of
urokinase-type plasminogen activator. Additionally, the
enzyme is not inhibited by
plasminogen activator inhibitor-1 which is the primary physiological inhibitor of both
urokinase and
tissue-type plasminogen activator. It does not cross-react with
antibodies specific for human
urokinase or
tissue-type plasminogen activator. The parasite-derived
enzyme activates
plasminogen from several mammalian species, including human, cow and pig. Thus, it is possible that this parasite-derived
serine protease contributes to the pathogenesis of
Acanthamoeba keratitis.