Arrhenius Analysis of Heat Survival Curves from Normal and Thermotolerant CHO Cells

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Abstract
The temperature compatible with biphasic hyperthermia-survival curves in Chinese hamster ovary (CHO) cells was increased from 42.5-44.degree. C by acute heat conditioning (10 min, 45.degree. C). An Arrhenius analysis of heat-survival curves of both continuously heated and acutely heat-conditioned cells suggests 3 thermodynamically distinct categories of thermotolerance. Thermotolerance developed during continuous heating (below 43.degree. C) and expressed in terms of biphasic survival curves was characterized by .DELTA.H [enthalpy] = 418 kcal/mol and .DELTA.S [entropy] = 1257 cal/mol K (controls .DELTA.H = 302 kcal/mol, .DELTA.S = 893 cal/mol K), both approximately 1.4-fold greater than for nonthermotolerant cells (thermotolerance I). After acute conditioning, the resistant subpopulation indicated by the biphasic survival curves of cells heated between 42 and 44.degree. C resulted in .DELTA.H and .DELTA.S not significantly different from those of cells continuously heated at temperatures below 43.degree. C (thermotolerance II). Here, thermotolerance appeared to be directly associated with the shift from 43-45.degree. C in the temperature limit for biphasic survival curves. After acute conditioning, thermotolerance to 45-48.degree. C (monophasic survival curves) and to 42-44.degree. C in terms of the initial phase of biphasic survival curves was characterized by a .DELTA.H of 123 kcal/mol and .DELTA.S of 325 cal/mol K, neither one significantly different from that of normal cells inactivated above 43.degree. C (thermotolerance III). The change in Gibbs free energy, .DELTA.G, for heat killing of normal and thermotolerant cells was similar and ranged from 19.7-21.5 kcal/mol, values in the range commonly reported for protein denaturation (22.5 .+-. 5 kcal/mol). The thermodynamic pathways of heat killing or the intermediate denaturation states apparently differ for the 3 types of thermotolerant and normal cells.