Conformational changes of complement components C3 and B induced at higher temperature

When purified C3 or B of human complement was incubated at various temperatures for 30 min, B lost most of its antibody combining capability at 46 degrees C, and C3 lost it at higher than 50 degrees C. When C3, heated B, D and Mg++ ions were incubated, there was a precipitous decrease in C3 conversion in the presence of heated B between 44 and 46 degrees C. No C3 conversion was observed in the presence of B heated at 50 degrees C. When C3 heated higher than 50 degrees C was incubated with B, D and Mg++ ions, C3 conversion decreased dramatically, but B was converted almost normally, suggesting that B could be complexed with heated and conformationally altered C3 and cleaved by D. The fluorescence intensity of heated C3 excited at 288 nm gradually decreased between 44 and 46 degrees C. The fluorescence 288 nm gradually decreased between 44 and 46 degrees C. The fluorescence intensity of C3 was slightly increased by 1-anilino-8-naphthalene sulfonate (ANS) at 50 degrees C and significantly increased at 56 degrees C, while ANS enhancement of fluorescence of B began at 46 degrees C and was significant at 50 degrees C, indicating that the surface of B and C3 became hydrophobic between 44 and 46 degrees C, and 46 and 50 degrees C, respectively. These results suggest that conformations of C3 and B have low melting points at which they change confirmations drastically.