Materials and Methods Cell cultureThe HL-60 human myeloid leukemia cell line has been purchased from the Hungarian Cell Bank, Pasteur Institute of Hungary, Szeged, NCBI Code C427. Cells were maintained in suspension culture at +37°C under 5% CO 2 /air in RPMI 1640 (Gibco, UK) supplemented with 10% FCS and antibiotics: 100 U/mL Penicillin and 100 µg/mL Streptomycin. The cells were subcultured three times weekly, ATRA (Sigma, USA). This procedure has been originally adopted by Olins et al. [25] and then modified by Roy et al. [26]. Chromatin fractionationThe cells were precipitated by centrifugation at 12,000 rpm for 20 min (+4°C). The pellets were then suspended and homogenized in 5 volumes of 10 mM Tris-HCl (pH 7.80)/200 mM sucrose/1.5 mM EDTA/20 mM MgCl 2 /0.5% (v/v) Triton X100 using the glass-teflon Potter-Elvehjem homogenizer with a snug-fitting pestle (1,800 rpm).Homogenates were filtered through the 5-layer cotton graze and

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DNA Polymerase Beta (EC 2.7.7.7) is found to be operated by magnetic isotope effect (MIE) of Calcium once the Mg2+ ions replaced with the stable 43Ca2+ isotopes inside the enzyme catalytic sites. The isotope mentioned is the only paramagnetic species of the Calcium isotopic set with a 0.135 natural abundance value and the negative 7/2 nuclear spin providing a nuclear magnetic moment equal to 1.317 Bohr magnetons. As compared to the Mg/40Ca substitution, a 2.25-fold enzyme inhibition has been shown to provethe43Ca-MIE dependent mode of the catalysis turning down.An ion-radical mechanism based on the singlet – triplet conversion of the enzyme generated intermediates (ion-radical pairs) is found to be engaged once the paramagnetic metal isotope involved into the catalysis studied.The MIE promotes a primary reaction in DNA synthesis constituting in electron transfer between the ion – radical forming partners, [Ca(H2O)n2+] and [Ca2+(dNTP)]. Once the metal isotope substitution takes place inside just one of two DNA Polymerase Beta catalytic sites, a consequent43Ca – promoted inhibition leads to a residual synthesis of shorted DNA fragments that counts 25 – 35 nucleotides in length contrasting with the 180n – 210n DNA produced by either intact or40Ca – loaded polymerase. Being occurred simultaneously with a marked MIE – promoted enzyme inhibition, this fact itself makes possible to consider these short (“size-invalid”) DNA segments hardly efficient in the DNA base – excision repair. The latter is a survival factor in leukemic cells where the DNApolβ was found overexpressed. That supports a standpoint considering theDNApolβ a legitimate target for antitumor agents since its inhibition deprives the malignant cell from a DNA base – excision repair in neoplasma. A possible trend making role of these data in the current developments on a novel concept - establishing chemical background for cancer therapies is in a focus.

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