Comparison of low-molecular-weight products following reaction of C3-C3b with C3b inactivator and with trypsin.

Substitution of trypsin for Konglutinogen-activating factor (KAF) in the procedure for cleaving C3d from C3-C3b substrate produced a relatively heterogeneous low-molecular-weight fraction (C3d-Tryp) which differed in a number of ways from the KAF-mediated cleavage product (C3d-KAF). The differences were demonstrable by agar and polyacrylamide gel electrophoresis, 125I-labelling, content of immunoreactive 125I-labelled C3d, inhibition of anti-complement antiglobulin reagents and rabbit immunization. By comparison with C3d-KAF, the C3d in C3d-Tryp was more heterogeneous and exhibited a faster electrophoretic mobility in agar at pH 8.6. By contrast to C3d-KAF, C3d-Tryp contained protein carrying C3c antigenic determinants.