132 BARBARA E. HOLMES 



of organic phosphates. This could be a very interesting system for 

 studying an initial reaction and its final visible effect. 



The manifest effect of irradiation on the cell nucleus and the con- 

 sequences to cell life have made us regard this as the main target and 

 think of initial effects as being within the nucleus; occasionally, how- 

 ever, events caused by simple enzyme inhibitions, probably uncon- 

 nected with the nucleus, obtrude themselves upon our notice. 



A striking example is given by the work of Gordon (1956) who found 

 that the growth of young mung bean plants coukl ])e stimted for one or 

 more days by low doses of X-rays. He traced this effect to an immediate 

 fall in the auxin content of the irradiated plant. This in turn was found 

 to be due to a considerable decrease in the usual formation of auxin 

 from trytophan, probaljly because of an inhibition of the enzyme re- 

 sponsible for the last stage of the transformation. Gordon considered 

 that the enzyme responsible was that causing the oxidation of indole- 

 acetaldehyde to auxin and could show that extracts made from bean 

 plants immediately after irradiation showed a lower enzyme activity 

 than extracts from normal plants. The initial action of the X-rays. l)io- 

 logically speaking, was apparently the inactivation of this enzyme, 

 but it is not yet possible to say what, in its structure or surroundings, 

 made it particularly sensitive. 



Another interesting case of the obvious manifestation of the result 

 of irradiating an enzyme system is given by Ungar et al. (1955) who 

 studied the irradiation of an adrenal gland during its perfusion with 

 blood containing A.C.T.H. At the end of the experiment, when the 

 total dose had reached 2,000 r, it was found that the usual production 

 of adrenal steroids had been much reduced. It was possible to show 

 that definite steps in the formation of these steroids had been inhibited 

 and the effect must have been on specific enzymes or on the formation 

 of these enzymes. 



To come at last to events in the nucleus, it has, of course, struck all 

 workers that the huge nucleoprotein molecides offer a target that 

 cannot be missed and that some of the initial effects of irradiation must 

 be within them. This idea will be discussed later, hut it is worth ])ointing 

 out that it has proved easier to produce changes in other mechanisms 

 connected with the mitotic process than to show any such effect. 



For instance, it was deduced by Pelc and Howard (1953) from auto- 

 radiographs and later demonstrated directly by ourselves on the re- 

 generating rat liver (Holmes and Mee, 1955) that fairly small doses of 

 X-rays given before the synthesis of deoxyribonucleic acid had started, 

 were effective in sto])ping this synthesis. Im])ortant work by Bollum 

 and Potter (1960), supported by the indejDendent evidence of Ord and 



