Effect of hyperthermia and gamma-radiation on Escherichia coli K1060 D-lactate dehydrogenase.

The response of E. coli K1060 D-lactate dehydrogenase (D-LDH), an enzyme located in the cytoplasmic membrane, was studied following 42.5 degrees C hyperthermia and/or gamma-irradiation. The inactivation of D-LDH following the above treatment was used as a tool to probe the role of membrane proteins in the radiation and/or heat sensitivity of cells. No correlation between loss of enzyme activity and cell killing was found, suggesting that D-LDH does not play an important role in hyperthermic cell survival. The results obtained in combined hyperthermia and gamma-irradiation treatments on loss of D-LDH activity and E. coli cell killing suggest that an interaction between heat and radiation occurs at the membrane structure level. Moreover, when cells were heated at 42.5 degrees C in the presence of 10 mM procaine-HCl, both cell killing and loss of D-LDH activity were enhanced. The involvement of membrane structure in the heat sensitivity of cells is strongly indicated by the latter observations. The opposite effect was observed when procaine was present during irradiation in oxic conditions, suggesting that procaine itself can also act as a scavenger towards OH-induced radicals.