Properties of a blood-meal-induced midgut lectin from the tsetse fly Glossina morsitans

The properties of a blood-meal-induced lectin (agglutinin) from the midgut ofGlossina morsitans capable of agglutinatingTrypanosoma brucei were studied in vitro. The midgut homogenate from flies that had been fed twice had the highest agglutination activity, followed by that from the once-fed flies and that from the unfed insects. As compared with the bloodstream-form trypanosomes, a much lower concentration of the midgut homogenate was required for agglutination of the procyclic parasites. Furthermore, the agglutination process was specifically inhibited byD-glucosamine. Soybean trypsin inhibitor abrogated agglutination of the bloodstreamform parasites, whereas the procyclics were unaffected. The agglutination process was temperature-sensitive, with little activity being evident between 4° and 15°C. Similarly, heating the midguts to 60°–100°C led to loss of activity. When the midgut homogenate was separated by anion-exchange chromatography, the agglutination activity co-eluted with trypsin activity at approximately 50% NaCl. These results suggest a very close relationship between midgut trypsin-like enzyme and the agglutinin. Since successful agglutination of bloodstream-form trypanosomes requires protease activity, it may be that the enzyme cleaves off some surface molecules on the parasite surface, thus exposing the lectin-binding sites.