Catalytic heterogenity of polyclonal RNA-hydrolyzing IgM from sera of patients with lupus erythematosus.

Various catalytically active IgG antibodies or abzymes have been detected recently 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 first time that IgM from peripheral blood of patients with SLE possesses both DNase and RNase activities: these activities were also present in Fab fragments of the IgM. Both specific enzymic activities of IgM from sera of any single patient are usually 5-10 times higher than those of IgG antibodies. The same preparations of IgM hydrolyze RNA about two order of magnitude faster than DNA. The properties of the RNases of IgM and IgG distinguished them from other known pancreatic and human sera RNases. In addition, the specific activities 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 substrate specificities varied very much for different patients. The findings speak in favor of the generation of a relatively small or an extremely large pool of polyclonal catalytic IgM by the immune system of individual patients.

RNA-hydrolyzing antibodies from peripheral blood of patients with lupus erythematosus.

Experiments and hydrolysis of substrates with known spatial structures (such as yeast tRNAPhe, as well as normal and mutant tRNALys from human mitochondria produced by transcription of the appropriate DNA species, that is, RNA genes) were performed to study the ribonuclease activity of antibodies isolated from blood sera of patients with systemic lupus erythematosus (SLE). The antibody preparations contained two types of ribonuclease activities: the first corresponded to the specificity of ribonuclease A and was found during hydrolysis at low salt concentrations, whereas the second was stimulated by Mg2+ and displayed unique specificity toward double-stranded regions of the substrate. The possible use of the antibody preparations as tools for structural studies of conformational differences between RNA molecules was examined. In experiments with unmodified and mutant tRNALys species differing in one base found in the T-loop, we found that hydrolysis with SLE antibodies can detect small local structural changes in RNA under physiological conditions.