48 A. G. PASSYNSKY 



of ionizations arising in the cell at even comparatively Ioav irradiation 

 doses certainly do not pass without trace but remain in the cell in the 

 form of hundreds of changed or damaged molecules of various sub- 

 stances, including such important ones as nucleic acids and nucleopro- 

 teins, proteins, enzymes, etc. 



One of the main types of chemical change in protein molecules on 

 irradiation seems to be the oxidation of SH-groups. In the work with 

 Pavlovskaya (1956) we showed that the action of X-rays on 2 per cent 

 aqueous solutions of crystalline egg albumin, gives an oxidation of 

 SH-groups with an ionic yield of 1:1. The same result was obtained 

 when dry preparations of crystalline egg albumin were irradiated ; the 

 total amount of oxidized SH-groups was 6 x lO^^ per g protein, while 

 the ionization number for the irradiation dose of 5 x lO^r was about 

 8-5x1018 (Passynsky and Pavlovskaya, 1960). With a 20 per cent 

 protein solution (assumed conditionally in the form of egg albumin) 

 in the protoplasm the lethal dose of 600 r may lead to the oxidation of 

 only 1 SH-group per 3,000 molecules. When taking into consideration 

 the distribution of ionizations within the cell it may be assumed that 

 the j3roportion of chemically changed protein molecules in the cell can 

 hardly be greater than 0-01 per cent of the total number of j^rotein 

 molecules. Similar results are obtained, according to available data, 

 for other chemical changes in protein molecules, such as the breakage 

 of peptide bonds, deamination, etc. when doses of the order of 600 to 

 1,000 r are used. It seems that the transition of a great number of 

 protein molecules to the excited or activated state at the expense of the 

 absorbed radiation energy, which is accompanied by a re-grouping of 

 some of the links in polypeptide chains and by corresponding structural 

 changes, is of a considerable importance. These changes result in the 

 increase of the aggregation of particles, decrease of solubility, activity 

 augmentation of functional groups of protein, revealed, in particular, 

 in the experiments described above by the increase of ^sg -methionine 

 binding. 



It should be noted that the effects of the oxidation of SH-groups in 

 protein molecules (in which these groups are the most radiosensitive 

 areas) can hardly be explained by the formation of organic radicals. 

 On irradiation of dry trypsin with fast electrons or y-rays in oxygen, 

 Alexander (1957) observed an increase of inactivation which he inter- 

 preted by the formation of -RO2 radicals from the primary -R ones, 

 without, however, quantitatively analysing this suggestion. In our 

 work (Passynsky and Pavlovskaya, 1960) the number of oxidized SH- 

 groups and that of radicals measured by the ESR method was com- 

 pared for one and the same preparation of dry egg albumin under the 



