Volume 40, Issue 2
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



Phospholipase A (PLA) plays an important pathogenic role in infections caused by several microorganisms and has been implicated in host cell invasion. The mechanism of host cell penetration by the intracellular protozoan involves several steps; we have investigated the role of PLA in cellular invasion by the tachyzoite stage of this parasite. We assayed invasion of human fibroblast monolayers by measurement of the selective incorporation of H-uracil into growing intracellular parasites. Exogenous PLA from snake venom () increased the penetration of fibroblasts by , while horse antiserum to venom inhibited penetration. An irreversible PLA inhibitor, p-bromophenacyl bromide, blocked penetration without metabolically disabling the parasite. When host fibroblasts were preincubated with this drug, penetration was not affected, supporting a role for parasite rather than host cell PLA in the penetration process. Another PLA inhibitor, nordihydroguaiaretic acid, also inhibited penetration. We assayed extracellular tachyzoites, purified from host cell debris, for PLA activity by radiometric detection of fatty acid release from labeled membranes. Sonically disrupted parasites contained a low level of calciumdependent PLA with maximum activity at pH 8.5–9.0. These experiments suggest that a phospholipase is implicated in host cell invasion.


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