In vitro cadmium-DNA interactions: cooperativity of cadmium binding and competitive antagonism by calcium, magnesium, and zinc.

The carcinogenic effects of cadmium (Cd) can be inhibited by the administration of the physiological essential metals, zinc and magnesium, while calcium injection is ineffective. Interactions of these metals with DNA could possibly account for such observations. Therefore, the binding of Cd to DNA in vitro and the effect of Ca, Mg, and Zn on Cd binding were investigated. Various concentrations of CdCl2 (0.1 to 250 microM) containing radioisotopic Cd (109Cd) were incubated at 24 degrees C for 1 hr with 100 micrograms purified double-stranded calf thymus DNA (0.1 mg/ml in 2 mM Tris-HCl, pH 7.4) both with and without Ca (80 microM), Mg (70 microM), and Zn (50 microM). Free and DNA-bound Cd were separated by gel filtration (Sephadex G-25) and quantitated by gamma spectrometry. Scatchard analysis revealed two Cd-binding sites and a positive slope at bound-Cd concentrations less than or equal to 1.4 microM, indicative of positive cooperative binding. Cooperativity of Cd binding was abolished when heat-denatured DNA was used. Analysis of high-affinity (HA) sites showed 0.093 mumol of HA sites/mg DNA (0.0305 sites per DNA base). Double reciprocal plots indicated an apparent dissociation constant (KD) for the Cd-DNA complex of 24.8 microM, and showed Ca, Mg, and Zn to be competitive antagonists of Cd binding to HA sites. The KD of Ca, Mg, and Zn were 110.7, 78.6, and 39.3 microM, respectively. Results indicated the cooperativity of Cd binding to double-stranded DNA and competition by Ca, Mg, and Zn for Cd-binding sites in DNA. The relative capability of these physiological essential metals to antagonize HA Cd-DNA binding (i.e., Zn greater than Mg greater than Ca) parallels their in vivo capacity to antagonize Cd-induced carcinogenesis.