Nonspecific ("pseudo-plasmal") dye-binding in the Feulgen nuclear stain and its blocking by azocarmin G.

In Feulgen nuclear staining nonspecific dye-binding due to the "pseudo-plasmal reaction" is intensified in isolated cells with intact cytoplasm, and cannot be eliminated by the post-irradiation method. Fluorescence intensity in the cytoplasm sometimes exceeds that of specific nuclear fluorescence, especially in brain and heart muscle cells, and it was almost impossible to perform cytofluorometric DNA quantification on such specimens. Various kinds of aldehyde-blocking agents such as sodium borohydride, 2,4-dinitrophenylhydrazine, aniline, and sodium pyrosulfite were effective in reducing the "pseudo-plasmal reaction". But the blocking effects were not complete because of additional release of reactive aldehyde groups during subsequent Feulgen hydrolysis. Acidic azocarmin G produced a complete block of all "pseudo-plasmal reaction" in acriflavine-Feulgen nuclear staining, allowing accurate DNA-cytofluorometry to be carried out.