Binding Specificity and Affinity of Acriflavine for Nucleic Acids

Abstract

A set of conditions was developed for the specific binding of acriflavine to the DNA of intact squamous cells. This was achieved through a series of studies into the relative affinities for dye between DNA and various biopolymers by an agar gel diffusion technique. Specificity was ascertained by DNase and RNase treatment of the cells. The final conditions, based on an estimated DNA-to-dye ratio of 4:1, required a constant cell count of 100,000 and dye at a concentration of 0.0025 μg per ml in 10 ml of phosphate buffer, pH 6.0–7.4. These quantities were dictated by the sensitivity limitations of the analytical apparatus. To make use of standard fluorometric instrumentation, the whole cell population method for determining average values was followed. Free dye was analyzed after cell samples were stained, and the difference between this value and that of an aliquot of working dye was taken as the amount of bound dye. To ensure cell-free residues of dye, cells were removed by centrifugation through Teflon membrane filters. The average amount of dye which was bound to the DNA of normal squamous cells was 1.6 × 10⁻⁷ μg (0.7 × 10⁻¹⁵ mole) per cell, and the amount bound to HeLa cells was 2.3 × 10⁻⁷ μg (1 × 10⁻¹⁵ mole). These values were highly replicable, making it possible to use them as an expression of the DNA content of the cells. The mildness of the staining conditions, and the preservation of cellular integrity in this technique may permit quantitative measurements of the DNA content of living cells.