The composition, with respect to nucleic acids, of the L.E. bodies resulting from the action of the plasma of patients with systemic lupus erythematosus on substrate leukocyte nuclei in different kinds of preparations was compared microspectrophotometrically with that of control lymphocyte nuclei. Binding of the basic dye methyl green to DNA was uniformly found to be depressed in L.E. bodies as compared with control nuclei. Since Feulgen-revealed DNA, which served as a standard of reference, was relatively unchanged in amount, the Feulgen:methyl green ratios of L.E. bodies were higher than those of lymphocyte nuclei. Acetylation, which covers basic groups of proteins, was found to increase methyl green uptake by DNA of L.E. bodies to values approximating those of control nuclei, with consequent revision, after acetylation of the Feulgen: methyl green ratios of L.E. bodies to values similar to those of lymphocyte nuclei. Ribonuclease was found to reduce methyl green staining; extraction with dilute acid had little effect. These data have been interpreted to indicate (a) the presence in L.E. bodies of DNA-associated proteins whose basic groups compete with the cationic dye for binding sites of DNA and so inhibit methyl green staining, and (b) the DNA itself is not detectibly altered in state or degree of polymerization. Photometric comparison of the mean Feulgen-stainable DNA content per L.E. body with that of control nuclei showed that DNA is not lost in the L.E. transformation of nuclei.

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