562 RADIATION BIOLOGY 



moting effect of oxygen was found to be slight under the special condition 

 of a near-freezing temperature; yet this result likewise appears ecjually 

 difficult to interpret on either view of the oxygen effect. 



Since much of the effect of ionizing radiation in producing chromosome 

 aberrations in Tradescantia is found to be exerted through the oxygen 

 that is present, it might be possible to increase or decrease the effect by 

 making the oxygen more or less available, respectively, in an active form. 

 In this connection it may be recalled that, according to the scheme sug- 

 gested by Wyss et at. (1948), azide, by inactivating catalase, cytochrome 

 oxidase, and enzymes with related functions, causes more oxygen to be 

 left in the cell and that a part of this oxygen is then converted into 

 hydrogen peroxide by the flavone enzymes present, a process which is 

 increased by the promoting effect of oxygen on the activity of the flavones. 

 In accordance with this, Wyss and co-workers found that azide enhances 

 the spontaneous-mutation frequency. Since carbon monoxide resembles 

 azide in that it too inactivates cytochrome oxidase, it might be thought to 

 have a similar effect on mutagenesis. The effect of carbon monoxide on 

 the production of chromosomal changes in Tradescantia microspores was 

 tested by King, Schneiderman, and Sax (1952). They found that, 

 unlike azide on Staphylococcus and cyanide on Neurospora, it does not act 

 as a mutagen by itself. However, when it was present during exposure 

 to X rays (but not when given as a pre- or posttreatment) it did cause a 

 considerable increase in the aberration frequency. Presumably this was 

 through promotion of breaks by making more oxygen available in some 

 active form, although it is not clear why it had no such effect by itself or 

 in what manner it worked synergistically with the radiation. 



A curious feature of these results was that, when additional oxygen 

 was provided along with the carbon monoxide, besides the amount pres- 

 ent in air, the enhancing effect of the monoxide on aberration production 

 disappeared, and only the relatively small amount of enhancement 

 remained which the additional oxygen alone would have exerted on the 

 radiation effect. This was explained by these investigators on the basis 

 of the fact that oxygen is known to reactivate cytochrome oxidase when 

 the latter is inhibited by carbon monoxide. Thus the oxygen was in this 

 case playing a dual role, that of indirectly (by aiding the cytochrome 

 oxidase) blocking the production of aberrations and of directly promoting 

 them. In view of these complicated counteractions it is not surprising 

 that Haas et al. (1952), in testing the effects of different combinations of 

 carbon monoxide, carbon dioxide, and oxygen on the production of trans- 

 locations in Drosophila, have found that, in some mixtures which include 

 carbon monoxide, the frequency of translocations is actually reduced. 



It has recently been shown by Conger and Fairchild (1951, 1952a, b) 

 that oxygen by itself, applied at 1 atm pressure for 1 hour to dry Trades- 

 cantia pollen, results in a frequency of aberrations equal to that from 



