RADIOACTIVE IXDK'ATORS IN BIOCHEMISTRY 979 



Incorporation of ^^P into deoxyribonucleic acid of a growing organ, 

 e.g. a tumour, is of a very different type. Even if some rephosphorylation 

 in such molecules cannot be excluded, the incorporation of -"^-P into 

 deoxyribonucleic acid of a growing organ takes place mainly in the 

 course of mitotic processes in which necessarily the whole molecule is 

 involved. This appears from the parallelism often found between the 

 mitotic figure and the rate of ^^p incorporation into deoxyribonucleic 

 acid of the organ. In organs in which appreciable cell division takes 

 place, such as in the bone marrow, the thymus gland, the intestinal 

 mucosa, or the spleen, administration of labelled phosphate to the rat 

 is promptly followed by a remarkable formation of labelled deoxyribo- 

 nucleic acid molecules. This is not the case in the liver or kidney of the 

 fully-grown animal in contrast to the corresponding organs of the newly- 

 born rat. We shall see later, when discussing the life-cycle determination 

 of blood corpuscles, that the deoxyribonucleic acid phosphorus of the 

 circulating avian red corpuscle and of the mammalian white corpuscle 

 in which mitotic processes do not occur is entirely stable during the 

 life-time of such particles, and that incorporation of ^^P into the deoxy- 

 ribonucleic acid of such particles takes place only during their formation. 

 The Jensen-sarcoma of the rat grows by about 1% per hour, and its 

 deoxyribonucleic acid content increases correspondingly by about 1%. 



In the Jensen-sarcoma the probability of the incorporation of a ^^P 

 atom into a ribonucleic acid molecule was found to be 2 — 3 times higher 

 than that of its incorporation into a deoxyribonucleic acid molecule^^^^ 

 As during the 2-hour experiment the percentage additional formation 

 of ribonucleic acid in the growing tumour cannot differ much from that 

 of deoxyribonucleic acid, the above figures indicate that out of three 

 32P atoms at least 1 — 2 are incorporated into ribonucleic acid by a process 

 which does not involve synthesis of ribonucleic acid molecules from early 

 precursors. 



Table 8. — Effect of Irradiation with an X-ray 

 Dose of 880 r on the Incorporation of ^■'C into 

 Deoxybibonucleic Acid Ptjrines and into Tissue 

 Proteins Prepared from the Livers of 55 New- 

 born Fed Rats (Mean Values) 



(") EuLEB, Hevesy and Solodkowska, Arkiv Kemi 26, A Nr. 4. (1948). 

 62* 



