BIOCHEMICAL MECHANISM OF CELL DIVISION DISTURBANCE 233 



expect a protective action against ionizing radiation. This assumption 

 was checked in an experiment by Strnchkov and (lorkina. They used a 

 highly polymerized DNA (with a molecular weight of 8 to 10 million) 

 extracted from calf thymus by the phenol method. Previous studies 



Table VII. Melanin protective effect on calf thymus DNA in vitro. 



with an 0-004 per cent solutions of such DNA showed that, even at a 

 dose of 500 r there is definitely a fall of the viscosity immediately after 

 irradiation. Experimental results with the DNA protective compounds 

 which interested us are shown in Table VII. We see that neither tyrosine 

 nor melanins, after being dialysed, prevented DNA from depolymerizing, 

 while low molecular weight products of enzymatic oxidation of tyrosine 

 protect DNA completely (under these conditions) from radiation action. 



This protection is rather specific. For example, cysteamine, the 

 universal protecting substance, was not protective under some con- 

 ditions. 



In the light of these investigations we suggest the following course of 

 events in an irradiated cell. The energy absorbed by the cytoplasmic 

 mitochondria effects small physico-chemical shifts in the superstructures 

 of these cell organelles, and the co-ordinated action of enzymes in these 

 organelles is thus disturbed. Polyphenoloxidase activity is released. 

 This enzyme release could be closely related to an inhibition of oxidative 

 phosphorylation. The products of enzymatic oxidation of tyrosine (or 

 substances like tyrosine) of quinonoid or semiquinone-like structure, in 

 abnormally high concentration and having the properties of free 

 radicals, diffuse to a cell nucleus, where they form unstable products 

 with DNA. The blocking of DNA prevents the passage of the cell into 

 mitosis, and that leads to a decrease in the mitotic index. 



Similar processes are possible, for animal organisms where they can 

 explain the observed changes in tyrosine metabolism (Bak, 1955), the 

 role of the adrenals in distant effects (Strelin, 1950; Boyarkin, 1951) and 



