Various catalytically active IgG antibodies or abzymes have been detectedrecently in the sera of patients with several autoimmune pathologies including systemic lupus erythematosus(SLE), where their presence is most probably associated with autoimmunization. Here we show for the firsttime that IgM from peripheral blood of patients with SLE possesses both DNase and RNase activities: theseactivities were also present in Fab fragments of the IgM. Both specific enzymic activities of IgM fromsera of any single patient are usually 5-10 times higher than those of IgG antibodies. The same preparationsof IgM hydrolyze RNA about two order of magnitude faster than DNA. The properties of the RNases of IgMand IgG distinguished them from other known pancreatic and human sera RNases. In addition, the specificactivities of the RNase activity of polyclonal IgM with the polymer substrates [RNA > poly(U) > or =poly(A) >> poly(C)], the observed range of optimal pHs, of apparent Km values for substrates and of substratespecificities varied very much for different patients. The findings speak in favor of the generationof a relatively small or an extremely large pool of polyclonal catalytic IgM by the immune system ofindividual patients.

Keywords: Antibodies, Catalytic, Catalysis, Immunoglobulin G, Immunoglobulin M, Immunoglobulins, Fab, Kinetics, Lupus Erythematosus, Systemic, RNA, Reference Values, Research Support, Non-U.S. Gov, Ribonucleases, Substrate Specificity