The binding of anti-DNA antibodies as measured fluorometrically by ethidium bromide.

The phenanthridine dye ethidium bromide (EB) intercalates with double-stranded DNA (dsDNA) resulting in an enhancement of fluorescence. Single-stranded DNA (ssDNA) does not show this fluorescent enhancement. Purified IgG from patients with Systemic Lupus Erythematosus (SLE) containing anti-dsDNA antibodies competes with EB for binding to DNA resulting in a decrease in fluorescence. This study has shown that antibodies which bind ssDNA in the Millipore filter radioimmunoassay displace EB from dsDNA showing that antigenic determinants are available for binding in the double-stranded molecule. This study introduces the EB assay and presents a comparison with the Millipore filter assay.