Ortho-phenanthroline inhibition of DNA synthesis in mammalian cells

Ortho-phenanthroline (OP) inhibits DNA synthesis in cultures of eukaryotic cells. After 6 h of exposure of L1210 leukemic cells to 5 μM OP, DNA synthesis was virtually stopped while RNA and protein synthesis remained at 82 and 70% of control levels. When normal human lymphocytes were stimulated by phytohemagglutinin (PHA), the presence of 5 μM OP caused complete inhibition of the expected increase in DNA synthesis but caused only partial inhibition of the expected increase in RNA and protein synthesis. OP inhibition of DNA synthesis can be prevented or reversed by removing the OP or by adding the metal ions Ni(II), Co(II), Cu(II), Zn(II) or Fe(II). The relative effectiveness of these metal ions in reversing the OP inhibition correlates with the order of the stability constants for their respective OP-metal complexes. This correlation is altered, however, by at least two factors: the toxic effects of some metals on the cells and the presence of serum proteins. This correlation suggests that the added metal ions function by binding OP and displacing it from inhibitory complexes rather than by replacing a metal ion required in DNA synthesis. Addition of OP to cell cultures followed by addition of metal ions appears to be an effective technique to inhibit and reinitiate DNA synthesis.