The influence of low temperature on the radiation sensitivity of enzymes.

When enzymes are exposed to ionizing radiation at low temperatures there is a progressive decrease in radiation sensitivity: considerably more enzymatic activity remains after the same dose of radiation at low temperature compared to room temperature. Detailed studies of five enzymes reveals the quantitative relationship between radiation sensitivity and temperature during exposure. Although 25 enzymes are shown to display this same relationship, recent reports have denied this effect in three enzymes. In this paper, we investigate two possible artifacts that could cause these discrepancies: 1) inaccurate determination of the temperature of the sample during irradiation, and 2) use of temperature-sensitive dosimeters to measure radiation dose. Procedures are described that carefully control these parameters. Thermoluminescent dosimeters are shown to be independent of temperature effects. These methods are used to investigate one of the enzymes, malate dehydrogenase, that has been reported to have a temperature-insensitive radiation inactivation. The radiation sensitivity of this enzyme is found to show the same temperature dependence as 24 other enzymes.