14 



up a rough logarithmic plot here on the board of Bennett's estimates. In any 

 event, you can look optimistically at this straight line. With the data we have for 

 pepsin, trypsin, invertase, and so on, this straight line is unquestioned, because 

 we have a true estimate and not just subjective estimates of color. Nevertheless, 

 even these points lie on the line and they show a reasonable biologic relationship. 



We claim that they have a very definite relation. The enzyme is causing 

 the oxidation of the surface of the apple, thus giving the brown color. The per 

 cent survival of the enzyme then follows the relation: 



^" JL = VI 

 n 

 o 



"I" is the number of clusters of ionizations per cubic centimeter volume and "V" 

 is the quantity I have been talking about, the inactivation volume. n is the 



fraction of activity surviving the irradiation, n being the amount left, and n 

 the amount at the start. 



This is really answering Dr. Chargaff's question. When I say it is a 

 mental transformation this illustrates the process. Now I have merely written 

 down the number of roentgens that correspond to this figure of n •,-, I 



multiply tnat by the number of clusters per cubic centimeter in protein, which 

 you can work out from the Bethe formula, and then I come out with a value for 

 "I". 



The inverse of that is then the volume, V. To find the molecular weight, 

 I multiply the volume by the density of the protein, 1.3, and I multiply that by 

 Avogadro's number. Then we conclude that this enzyme has a molecular weight 

 of 760, 000. 



It is assumed that inactivation of the enzyme on that apple surface is due 

 to an effect caused by radiation deposited inside the molecule. I have no proof 

 that this is the case. Actually, a wet surface-migrant energy is perfectly possi- 

 ble. So that I am not claiming that the molecular weight of tyrosinase is 760, 000. 



What I should like to debate this afternoon, or rather, should like to be 

 informed about, is the extent to which this type of reasoning might be true. Is it 

 possible, in point of fact, that an inactivation process of the sort described is 

 close to the truth, or is there something completely different that greatly domi- 

 nates this whole process and that actually renders this whole derivation invalid? 



It would be a nice thing (and I am surprised that more people haven't 

 done it) to study the loss of activity of enzymes in systems such as this and also 

 the same enzymes under equivalent conditions in vitro. We have done a little of 

 this with extremely dry preparations. We have studied the enzymes amylase, 

 invertase, cytochrome oxidase, and succinic dehydrogenase in essentially in vitro 

 systems and in living cells. For example, the amylase was in barley; invertase 

 in yeast cells; and the cytochrome oxidase and succinic dehydrogenase in 

 B. subtilis cells. In all of these studies the effect of irradiation of commercial 

 samples in the dry state and of the organic systems was the same. 



Now I should like to consider the following question. Suppose an ionizing 

 radiation passes near a biological molecule, e.g., a respiratory enzyme unit, a 

 mitochondrion, and produces a primary effect. We know that a part of the action 

 -- Tobias and Zirkle have made this essentially a complete theory (10) -- is that 



