296 RADIATION BIOLOGY 



serum albumin become inhomogeneous by the formation of a continuous 

 series of molecules of both lower and higher molecular weight when irra- 

 diated at room temperature, whereas no change is observed on irradiation 

 at the temperature of liquid air (Svedberg and Brohult, 1939). The 

 number of molecules remaining unsplit is an exponential function of the 

 dose. However, as the data are given for only one concentration, it is 

 not possible to apply the usual criteria for discrimination between the 

 direct and the indirect action. Undoubtedly both effects are found on 

 irradiation of proteins with large doses of X rays. 



EFFECT OF IONIZING RADIATIONS ON AMINO ACIDS 



The decomposition of amino acids by ionizing radiations has been known 

 for a long time. The decomposition of tyrosine by X rays proceeds with 

 an ionic yield of 0.166 (Stenstrom and Lohmann, 1928), whereas on irra- 

 diation with a rays the yield drops to 0.003 (Nurnberger, 1937). Irradia- 

 tion with a. particles also produces deamination (Loiseleur, 1933). This 

 problem has been recently studied quantitatively and under varied con- 

 ditions by Dale (Dale, Davies, and Gilbert, 1949a, b; Dale and Davies, 

 1949) and by Weiss (Stein and Weiss, 1949c). When amino acids are 

 irradiated with large doses of X rays (166,000 r), the formation of ammo- 

 nia increases with the concentration of irradiated amino acid. For 

 example, the ionic yield of glycine, which is 0.3 on irradiation of a 0.001 M 

 solution, increases to 1 on irradiation of a 0.1 M solution and reaches 2.9 

 on irradiation of dry glycine. Since the absence of oxygen and the addi- 

 tion of H2O2 has no measurable effect on the ionic yield, it seems that, of 

 the irradiation products of water, only OH radicals and H atoms are 

 effective (Dale, Davies, and Gilbert, 1949a). In Table 5-2 are given the 

 ionic yields of amino acids on X irradiation of 0.13 M solutions. The 

 efficiency of irradiation with a particles is 15 to 20 per cent of that of X 



Table 5-2. Deamination of Amino Acids by Ionizing Radiations (X Rays) 

 (Calculated from the data of Dale, Gray, and Meredith, 1949) 

 Amino Acid* Ionic yield f 



Glycine 1.08 



Alanine . 96 



/3- Alanine . 57 



Histidine 1 . 57 



Lysine 4.92 



Arginine 1 . 00 



Glycylglycine 1 . 66 



Glycylglycine and HCl . 50 



Leucylglycine . 88 



Diglycylglycine . 93 



Proline 



* 0.13 M, X-ray dose 166,000 r. 



t Number of NH3 molecules produced per ion pair. 



