ItiO KARL C. I.ARK 



Although a hugi' (|uaiitity of precursors might not he fouutl in a 

 popuh\tion of randomly Uividing cells, cells undergoing synchronous 

 division and DXA synthesis might well be expected to show such an 

 accumulation. This is apparently not so. In regenerating liver' an 

 increase in deoxynucleoside and deoxynucleotide content is ohseived 

 but the amounts of these are still extremely small and the quantity 

 present at 24 hours (presumably the onset of DXA synthesis) are not 

 strikingly greater (at most o09r) than the amounts found in hepatomas 

 (Schneider and Brownell. 1957; Potter et al., 1957; Rotherham and 

 Schneider, 1958 1 or regenerating liver after 48 hours (following 1)N.\ 

 synthesis). Thus the increase observed appears consistent with the over- 

 all increase in DXA synthetic activity and cell division in this tissue. 



Finally, it has not been demonstrated that all of such material serves 

 as precursoi-s for DXA synthesis (Hecht and Potter. 1956; Potter and 

 Buettner-Janusch, 1958). Especially significant is the fact that no purine 

 deox\'riboside material is observed. 



On the other hand, in regenerating liver and in hepatomas the pro- 

 portion of deoxynucleotides is increased relative to nucleosides and 

 blockage of DXA synthesis following irradiation will result in accumula- 

 tion of deoxj'pyrimidines in the tissue and excretion of such material in 

 the urine (Ord and Stocken, 1958; Parizek et al., 1958; Davidson and 

 Bishop, 1957; ^Main et al, 1957). It seems probable, therefore, that these 

 substances are acting as precursors (see discussion of Fig. 7 below). 



Bacteria have also been found to contain a small pool of dcoxy- 

 pyrimidine compounds (Okazaki and Okazaki, 1958; Okazaki et a/.. 

 1959; Okazaki, 1959; O'Donnell et o/., 1958; Okazaki ct ai. 1960; Lark. 

 1961). To date, almost all of these have been shown to be thymidine 

 derivatives and evidence is accumulating which indicates that they may 

 act as intermediates in the transport or activation of certain sugars 

 (Strominger and Scott. 1959; Okazaki. 1959; Okazaki, 1960; Baddiley 

 and Blumson. 1960). With one exception, no deoxypurine or derivatives 

 thereof have been observed. |In the case of E. coli B. Kanazir. (19541 

 has reported the existence of large amounts of deoxyguanosine within the 

 acid-soluble pool. Other workers, using this and other bacterial strains, 

 have not found any deoxypurines. The quantity of material reported was 

 sufficiently great (0.2 ^.V mg dry weight of cells) that it would seem 

 probable tliat this material would have been observed by others.] 



'In liver, following removal of a portion of the organ, tissue regeneration occurs. 

 When DXA synthesis is followed during this process no SA'nthesis is observed until 

 24-36 hours, at which time a striking synchronous increase in DXA content is 

 observed. For this reason, regenerating liver i^resents an ideal biochemical system for 

 studying certain jirohlems in tlie control of DX.\ l>iosynthesis (Barnum <7 nl.. 1057). 



