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A B ST R AC T Human retinoblastoma cells were proven to possess some very unusual DNApolβ species. Being 23.5 kDa monomers, which itself is not common for the DNApolβ superfamily members, these chromatin associated proteins manifests most of the DNApolβ-specifc functional peculiarities making them legitimate targets for DNA repair cytostatic inhibitors. Particularly, these tumor specific enzymes were found to be very sensitive to -promoted magnetic isotope effects (MIE) caused a marked DNA sequence growth limitation as well as a formation of the size-invalid, i.e. too short in length, DNA fragments, totally inappropriate for the DNA repair purpose. This MIE-DNApolβ match may serve a starting point for further move towards the paramagnetic path in current developments of anti-cancer strategies.K E Y WO R D S retinoblastoma; magnetic isotope effects (MIE); DNA repair

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.