34 



duced by the oxidation product of irradiation. In the presence of lactate, the 

 free-radical formed is CH3COHCOO, and in the presence of isopropyl alcohol, 

 it is CH3COHCH3. 



The enzymatic activity of this reduced compound was measured with 

 alcohol dehydrogenase from yeast and was found to be 50 percent active. 



KAMEN: Don't you think that 50 percent yield is accounted for by the 

 fact that the enzymatically active DPNH has stereospecificity, whereas, in your 

 experiments, the position of hydrogen in the nicotine-amide portion of the mole- 

 cule is randomized? 



BARRON: I thoroughly agree with you. These experiments suggest 

 that in the enzymatic oxidation of ethanol there is intermediate formation of a 

 free radical. They are also a confirmation of the beautiful experiments of West- 

 heimer and Vennesland (6). 



KAMEN: Can you reoxidize this enzymatically reduced 50 percent? 



BARRON: Yes. 



I want to speak now about the effect of ionizing radiations on proteins. 

 Here my point of view is rather different from that of Dr. Pollard. Proteins are 

 attacked selectively by ionizing radiations and at different points. If we take, for 

 example, proteins with a tyrosine ratio greater than 1, such as serum albu- 



tryptophan 

 min, irradiation produces an increase in the absorption spectrum at Z800 j^ , 

 which is proportional to the X-ray exposure (7). This increase is due to oxida- 

 tion of the tyrosine residue and it is also found after irradiation of tyrosine solu- 

 tions and during the first minutes after addition of tyrosinase to tyrosine. X ir- 

 radiation of a tryptophan solution, on the other hand, produces a decrease in the 

 absorption band at 2800a , which is proportional to the dose. If changes in the 

 absorption spectrum around this wave length are due to a tryptophan attack in 

 the protein molecule, then one would expect a decrease in the absorption spect- 

 rum after irradiation of proteins having a tyrosine ratio lower than 1. Chy- 



tryptophan 

 motrypsin and lysozine were taken as examples of such proteins, since both are 

 rich in tryptophan. X irradiation of these oroteins produced the expected de- 

 crease in the absorption spectrum at 2800 ^ . X irradiation of proteins with 

 small doses leads first to oxidation of the "SH groups and nothing else. Then 

 comes oxidation of the OH groups of serine, and deamination of the free amino 

 groups. When the exposure is increased to 75,000 r, aqueous solutions of serum 

 albumin are precipitated. This phenomenon, which is temperature dependent, 

 seems to be due to rupture of the hydrogen bonds. X irradiated solutions of ser- 

 um albumin can be kept for hours at 3°C without precipitation. As soon as the 

 temperature is raised, precipitation occurs. Polymerization may also take 

 place. When -SH- containing proteins are irradiated, there may be formiation 

 of a dimer, a disulfide protein: 



2 SH-protein ^ protein - S-S-protein + 2H 



This phenomenon takes place with serum albumin that has one -SH 

 group per molecule. Sedimentation studies of normal and X irradiated serum 

 albumin indicated that the S^Qw value of the second peak of the irradiated protein 

 agrees with the values calculated for a dimer. 



CHARGAFF: Does the amount of the second peak depend upon the dose 



