Effect of Oxygen on Inactivation of Trypsin by the Direct Action of Electrons and Alpha-Particles

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Abstract
The radiation dose required to inactivate solid trypsin with 2-Mev electrons and Co60 [alpha]-rays is 30% greater in the absence than in the presence of O2. No oxygen effect is found for [alpha]-irradiations. Possible mechanisms for an oxygen effect when the action is direct are discussed. The existence of an oxygen effect in an in vivo system cannot be used as a test to establish that the radiation acts predominantly through the indirect action of free radicals formed in water. The sensitive volume for trypsin calculated from conventional target theory is much greater than the molecular size, and the value obtained depends on the condition of the irradiation. The limitation of the target theory for determining the size of relatively small molecules are discussed in the light of recent radiochemical experiments, and reasons are advanced why it does not apply to small enzymes.