The capacity to activate the complement system and to fix isolated C1 of different monoclonal IgM proteins and various fragments was studied. All the 8 different IgM proteins tested fixed isolated C1, but the dose of the IgM preparations necessary for the fixation of the 50% of the available C1 activity strongly differed from each other. The complement activating (anti-complementary) effect of the IgM preparations was weak. (Fcμ)5 fragments prepared by tryptic digestion from 4 different IgM proteins and separated by gel filtration at pH 8.0 could fix C1 and their anti-complementary effect significantly increased with respect to the original IgM preparations. It was demonstrated that the C1-fixing ability of the (Fcμ)5 fragment was lost after gel filtration at pH 3.0 and a C1-fixing low mol. wt peptide fraction was released from the (Fcμ)5. This peptide recombined with the non-C1-fixing (Fcμ)5 and restored its ability for the C1-fixation. It was shown, furthermore, that the (Fcμ)5 preparation which could not fix isolated C1, activated the complement system via the alternative pathway.
ASJC Scopus subject areas