304 



RADIATION BIOLOGY 



free thiol groups of nonprotein nature (glutathione, cysteine) and fixed 

 thiol groups in the side chain of the protein molecule. The free groups 

 act as regulators of cell metabolism and in this manner take part in the 

 processes of cell division and cell growth; the fixed groups have a large 

 number of functions (Barron, 1951). When very small concentrations of 

 thiol reagents are added to cell suspensions, the free thiol groups may be 

 destroyed mthout affecting the fixed thiol groups; an increase in cellular 

 respiration is the consequence. If the concentration of thiol reagent 

 is increased, the fixed thiol groups will also be attacked (sulphydryl 

 enzymes), and an inhibition of respiration ^^dll ensue (Barron, Nelson, 

 and Ardao, 1948). Similar effects can be observed on irradiation of sea 

 urchin sperm and eggs if the irradiation dose and the number of cells in the 

 suspension are carefully controlled. Diminishing the dose of X rays can 

 bring about an increase in respiration of irradiated sea urchin sperm and 

 eggs while there is still delay in the cleavage time (Table 5-8). 



Tablk 5-8. Effect of X Rays on Respiration and Cell Division of Sea Urchin 



Sperm and Eggs 



A striking resistance to the action of X rays has been observed by 

 Barron and Flood (1949) in the respiration of the ameba Chaos. The rate 

 of respiration is not affected by irradiation with 100,000 r, although the 

 irradiated animals have their pseudopodes retracted and have taken a 

 spherical shape. 



A differentiation between cell division and enzyme formation can be 

 shown on X irradiation of strains of yeast (Saccharomyces cerevisiae) that 

 form galactozymase on addition of galactose. Irradiation of these cells 

 with X-ray doses strong enough to inhibit 90 per cent of the cell multi- 

 plication power has no effect on galactozymase formation (Brandt et al., 

 1951). 



EFFECT OF IONIZING RADIATIONS ON TISSUE METABOLISM 



Since the introduction by Warburg (1926) of reliable methods for the 

 measurement of metabolism in tissue slices, numerous investigators 

 studied the effect of X rays on tissue metabolism. The discrepancies and 



