Trans-sialidase: a unique enzyme activity discovered in the protozoan Trypanosoma cruzi.

Trypanosoma cruzi, the agent of Chagas' disease, an ailment characterized by a progressive chronic fibrotic myocarditis and degeneration of tissues that are innervated by the autonomic nervous system, is a voracious sialic acid eater from glycoconjugates of the surrounding medium. This is accomplished through an active trans-sialidase residing on the surface membrane of the trypomastigote stage, which is the parasite form that invades vertebrate cells. The existence of the enzyme was proposed and established only 7 years ago and yet a flood of information on the subject is already available. Trans-sialidase is able to reversibly transfer sialic acid alpha(2-->3)-linked to an external Gal beta from the host cell surface sialoglycoconjugates to a terminal Gal beta of an appropriate acceptor on the parasite surface. In the absence of an acceptor, the enzyme acts as a hydrolase transferring sialic acid to water. Trans-sialidase belongs to a highly heterogeneous gene family of surface molecules sharing with each other and with bacterial neuraminidases variable degrees of nucleotide sequence homology and common motifs. It has been proposed that sialylation of the parasite surface catalyzed by trans-sialidase is necessary for successful invasion of the host cell, but the evidence available is still indirect. Another function could be a protection from lysis by the alternative pathway of complement while the parasite is circulating in the acute phase of the disease.