Temperature Dependence of Ionizing Radiation Effect on Dry Lysozyme and Ribonuclease

Abstract

The effect of ionizing radiation on subsequent activity of dry enzyme preparations in a vacuum has been extensively interpreted in relation to the hypothesis of D. E. Lea, that a single ionization event occurring within a biologically active molecular structure destroys its activity (1, 2). Most such irradiations of enzymes for direct action' on catalytic activity have been carried out near room temperature. The Lea hypothesis implies no dependence on temperature, but both for invertase (3) and for catalase (4) the direct action of radiation on catalytic activity was found some time ago to be temperature-dependent. Irradiation of these enzymes at liquid air temperature showed sensitivities, respectively, about one-half and two-thirds the values near room temperature. Brustad's work on trypsin (5) has now considerably extended the precision and the low-temperature range of such work. The temperature dependence for dry irradiation is confirmed by the trypsin work, but without indications of plateaulike increments in target mass with increasing temperature, a suggestion made in the reports of the earlier data. Brustad analyzed the trypsin data in analogy with the Arrhenius dependence of reaction rate on temperature, noting three characteristic regions. At temperatures below 100?K trypsin showed no effect of temperature on radiation sensitivity. From 100?K to about 170?K there was a slight dependence, and at temperatures above 170?K the temperature activation was more