Sialic acids and sialidases play important roles in cellular interactions and modulate the recognition of pathogenic microbes by mammalian host cells. Protozoan parasites of the genus Trypanosoma express a unique sialic acid-metabolizing enzyme. This enzyme, named trans-sialidase (TS), catalyzes the transfer of sialic acids from host glycoconjugates to acceptor molecules of the parasite plasma membrane. In African trypanosomes, the agents of sleeping sickness, TS is found only in forms developing within the insect vector, and the enzyme sialylates the major surface protein. In Trypanosoma cruzi, the causative agent of Chagas' disease in Central and South America, TS is expressed both in the insect and mammalian forms of the parasite. The T. cruzi enzyme has been biochemically characterized, and the gene encoding the enzyme has been cloned. The enzyme sialylates abundant mucin-like molecules present on the surface of the parasite. Several lines of evidence suggest that TS and sialic acid acceptors on the surface of T. cruzi participate in host-parasite interactions and mediate the initial stages of the trypanosomes' invasion of host cells.

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The trypanosomatid flavoprotein disulfide reductase, trypanothione reductase, is shown to catalyze oneelectron reduction of suitably substituted naphthoquinone and nitrofuran derivatives. A number of such compounds have been chemically synthesized, and a structure-activity relationship has been established; the enzyme is most active with compounds that contain basic functional groups in side-chain residues. The reduced products are readily reoxidized by molecular oxygen and thus undergo classical enzyme-catalyzed redox cycling. In addition to their ability to act as substrates for trypanothione reductase, the compounds are also shown to effectively inhibit enzymatic reduction of the enzyme's physiological substrate, trypanothione disulfide. Under aerobic conditions, trypanothione reductase is not inactivated by these redox-cycling substrates, whereas under anaerobic conditions the nitrofuran compounds cause irreversible inactivation of the enzyme. When tested for biological activity against Trypanosoma cruzi trypomastigotes, many of the test compounds were trypanocidal, and this activity correlated with their relative ability to act as substrates for trypanothione reductase. The activity of the enzyme with these redox-cycling derivatives constitutes a subversion of its normal antioxidant role within the cell. For this reason these compounds may be termed "subversive" substrates for trypanothione reductase.

The protozoan parasite Cryptosporidium parvum is a significant cause of diarrheal disease worldwide. Attachment to and invasion of host intestinal epithelial cells by C. parvum sporozoites are crucial steps in the pathogenesis of cryptosporidiosis. The molecular basis of these initial interactions is unknown. In order to identify putative C. parvum adhesion-and invasion-specific proteins, we raised monoclonal antibodies (MAbs) to sporozoites and evaluated them for inhibition of attachment and invasion in vitro. Using this approach, we identified two glycoproteins recognized by 4E9, a MAb which neutralized C. parvum infection and inhibited sporozoite attachment to intestinal epithelial cells in vitro. 4E9 recognized a 40-kDa glycoprotein named gp40 and a second, >220-kDa protein which was identified as GP900, a previously described mucin-like glycoprotein. Glycoproteins recognized by 4E9 are localized to the surface and apical region of invasive stages and are shed in trails from the parasite during gliding motility. The epitope recognized by 4E9 contains ␣-N-acetylgalactosamine residues, which are present in a mucin-type O-glycosidic linkage. Lectins specific for these glycans bind to the surface and apical region of sporozoites and block attachment to host cells. The surface and apical localization of these glycoproteins and the neutralizing effect of the MAb and ␣-N-acetylgalactosaminespecific lectins strongly implicate these proteins and their glycotopes as playing a role in C. parvum-host cell interactions.Cryptosporidium parvum, an intestinal Apicomplexan parasite, is a significant cause of diarrheal disease worldwide (15,17). In immunocompetent individuals, the disease is usually self-limiting, but it may be chronic and life threatening in immunocompromised patients such as those with AIDS. Recently, the parasite has gained notoriety as the causative agent of numerous outbreaks of waterborne diarrheal disease. There is currently no effective specific therapy approved for disease caused by this parasite.Infection is initiated by ingestion of oocysts, which undergo excystation to release sporozoites. Attachment of sporozoites to epithelial cells and subsequent invasion of the host cell membrane are crucial primary steps in the pathogenesis of cryptosporidiosis. The ultrastructural aspects of attachment and invasion have been characterized in detail (10,24,33,34). Sporozoites attach to host cells by their anterior pole. Attachment is followed by invagination of the host cell plasma membrane, which extends along the surface of the sporozoite and eventually completely surrounds it, leading to formation of a parasitophorus vacuole where the parasite undergoes further development in a unique intracellular but extracytoplasmic location.Using in vitro models of sporozoite attachment to epithelial cells, we previously showed that attachment was dose and time dependent and was influenced by pH, divalent cations, and the degree of differentiation of host cells (20,22). Further, attachment could be inhibited by polyclo...