Effect of Hyperthermia on CHO DNA Polymerases α and β

The heat sensitivity of polymerase β (pol-β) has been studied in an attempt to assess the molecular lesions for heat killing and heat radiosensitization. Viable cells were heated, and then polymerase activity was determined in crude cell extracts by measuring the incorporation of $[{}^{3}{\rm H}]{\rm TMP}$ into an exogenous, activated calf thymus DNA. The thermal inactivation of pol-β activity was shown to correlate with hyperthermic cell killing and radiosensitization. Pol-β activity decreased exponentially when cells were heated at temperatures of 43°C and above, and a 10-min 45.5°C heat shock reduced pol-β activity by 80%. Also, pol-β activity showed a 3-fold greater heat sensitivity than polymerase α (pol-α) activity at 43.5 and 45.5°C. When cells were heated at temperatures of 42.2-42.5°C, pol-β activity decreased, but as for cell killing, tolerance to any further treatment was observed after 2 hr of heating. Thermal tolerance also was observed as a decrease in the rate of pol-β inactivation at 45.5°...