Effect of Glucose on Thermal Injury of Yeast that may Define the Maximum Temperature of Growth

SUMMARY: Anaerobically grown yeast, suspended in glucose solution, leaked cell contents non-selectively. The rate of leakage increased with temperature but reached a maximum rate close to the maximum temperature of growth (T max) of the yeast. The total leakage induced by glucose was much greater above T max than below it, because above T max the yeast lost the ability to take up released material. The rate of uptake of 14C-labelled amino acids was also substantially slowed above T max-Yeast heat-stressed in the presence of glucose lost the ability to establish and maintain a concentration gradient of sorbose, and simultaneously ATPase activity could be measured, all characteristics of yeast with a ruptured cytoplasmic membrane. These effects were uniquely caused by utilizable sugars, were essentially independent of sugar concentration and could be partially inhibited by Ca2+ or inhibitors that prevent sugar utilization. Yeast heated above T max in water suspension was essentially undamaged as determined by the tests used, but if glucose was subsequently added below T max an effect of heat damage could be demonstrated. We conclude that one factor that determines the T max of a yeast is the temperature sensitivity of the cytoplasmic membrane in the presence of the utilizable sugar of the growth medium.