Accurate determination of DNA content in single cell nuclei stained with Hoechst 33258 fluorochrome at high salt concentration.

In an attempt to achieve accurate quantification of DNA levels in cell nuclei, we studied the influence of salt concentration on the fluorescence of cell nuclei complexed with Hoechst-33258 (Hoe) fluorochrome. The fluorescence of cell nuclei was compared with that of extracted DNA as well as that of nucleosome core. Conformational changes in these complexes were examined by measuring both fluorescence anisotropy and fluorescence lifetime in the nanosecond region. The results showed that the fluorescence of DNA-Hoe was quenched by the nucleosomal structure, there being an associated increase in anisotropy and a decrease in the fluorescence lifetime; however, the fluorescence was restored to the original level by the addition of a high concentration of NaCl, CsCl, or LiCl. The reduction in fluorescence may have been due to loss of fluorescence energy caused by collision of the fluorophore with histones in the nucleosome. The addition of 1 M NaCl to the medium used for staining with Hoe greatly stabilized the fluorescence of DNA in cell nuclei. The DNA content of individual cell nuclei was determined by comparing the fluorescence of these nuclei with that of a standard DNA solution. For lymphocytes and liver ploidy cells, reasonably accurate values were obtained by applying the present method.