Trypanosoma cruzi: characterization and isolation of a 57/51,000 m.w. surface glycoprotein (GP57/51) expressed by epimastigotes and bloodstream trypomastigotes.

We have recently described the application of a purified glycoprotein of 25,000 Mr (GP25) of Trypanosoma cruzi in serodiagnosis of Chagas' disease. Purified GP25 lacks appreciable immunogenicity in some animal species, in spite of being generally antigenic to parasitized hosts. The underlying cause for these contrasting observations has not been determined, but it may relate to the drastic extraction conditions used in the original isolation procedure, and possible damage inflicted to the native form of this antigen. This report describes the molecular properties of a GP25-related primary antigen, and a fast performance liquid chromatographic (FPLC) procedure to attain its isolation under gentle conditions. The expression of GP25-related antigen by epimastigotes or bloodstream forms was investigated with a high-affinity monoclonal antibody to GP25, SC11G10, as well as with monospecific antisera to GP25. Immunochemical analysis of Nonidet P-40 lysates supplemented with protease inhibitors indicated that GP25 is not synthesized as such; instead, a 57,000 Mr component (GP57) was identified as the primary antigen product. Partial enzymatic conversion to GP25 was observed when inhibitors were deliberately omitted from cell extracts. Most significantly, GP57 was established as the primary biosynthetic product in [35S]-labeled bloodstream trypomastigotes after immunoprecipitation with SC11G10 antibody. This analysis when applied to metabolically labeled epimastigotes has consistently revealed a minor antigen component of 51,000 Mr (GP51), in addition to GP57. The former was identified as the antigenically related product exposed at the parasite cell surface after external radioiodination of viable trypanosomes. Access to the native form of this widely distributed surface glycoprotein should stimulate the investigation of functional and structural aspects of its immunologic activity.