BASIC RADIATION BIOCHEMISTRY 



261 



relation between chemical change of an organic substance (e.g., car- 

 boxypeptidase, Fig. 4-2) and X-ray dose, which was explained by the 

 protection effect, is only a special case of two solutes in solution, although 

 before the irradiation starts only one type, active molecules, is present. 

 By the action of radiation, originally active molecules lose their activity 

 and become a second kind of solute, inactive molecules. This second kind 

 of solute is still competing for radicals, but the reaction is not measured 

 since the criterion of the radiation effect is enzymatic activity. 



In order to illustrate the protection effect we shall describe a particular 

 experiment (Dale, 1942). In this experiment the activity of the enzyme 

 D-amino acid oxidase is measured in the absence and in the presence of a 

 protective substance (Table 4-1). The active enzyme consists of a pros- 

 thetic group, alloxazinadeninedinucleotide (D), and a specific protein 

 (P), neither of these alone being enzymatically active. When this 

 enzyme (D + P) acts on its substrate alanine (A), the oxidation of 

 alanine which ensues is measured manometrically by the uptake of oxygen 

 over a convenient period, usually of the order of 10 min. Thus the oxy- 

 gen uptake of the total system P + D + A represents the initial activity 

 of the enzyme. If we now irradiate a solution of D alone and also solu- 

 tions of D of the same concentration to which different concentrations of 

 a protective organic substance have been added^in this case the dipep- 

 tide leucylglycine (L) which cannot be oxidized by the enzyme — it will be 

 seen that the oxygen uptake decreases with decreasing concentrations of 

 L to the value of D irradiated alone. This experiment also demonstrates 

 that the prosthetic group which is not an enzyme can be affected by 

 radiation. 



Table 4-1. Inactivation of Alloxazinadeninedinucleotide by 4000 r of X 



Radiation in Presence and Absence of Leucylglycine as Protector 



{Reproduced by perynission of the editors of the Biochemical Journal.) 



* The asterisk denotes irradiation of contents included in the parentheses. 



With the dilution and protection effect as the characteristic features of 

 the indirect action of radiation, it is easily seen why irradiation experi- 

 ments on biologically active substances carried out up to ten years ago 



