44 I. LESLIE 



in DNA content are always linked to changes in cell number, the amount of 

 PNA in a tissue depends on its physiological function or state, high concen- 

 trations being associated with cell populations which are actively dividing 

 or actively synthesizing protein. Although the proportion of PNA to pro- 

 tein is remarkably similar for a number of different tissues, it is clear that 

 PNA and protein can vary independently around the normal basic level. 

 A higher proportion of PNA to protein than is present in adult or resting 

 cell populations is found in embryonic, regenerating, and neoplastic tissues, 

 and in rapidly dividing bacterial populations. Increased energy intake and 

 treatment with cortisone, thyroxine, estradiol, and insulin will also raise 

 the proportion of PNA to protein. Protein and vitamin C-deficient diets 

 produce a similar effect but act by decreasing the protein to a greater ex- 

 tent than the PNA. Conditions which predominantly reduce the PNA and, 

 hence, the proportion of PNA to protein include a high fat diet and thia- 

 mine and vitamin B12 deficiencies. 



In conclusion, variations in PNA content are produced primarily by 

 changes in energy intake and energy metabolism and, of course, by factors 

 directly affecting nucleic acid synthesis. Secondly, increases in the pro- 

 portion of PNA to protein above the normal adult or resting level invari- 

 ably precede protein synthesis and cell multiplication. 



XIII. Addendum 



An interesting development in the ultramicrotechniques for determining 

 the PNA and the nucleotide content of individual cells has been described 

 by Edstrom.2'^' He has also developed a method of determining the amounts 

 of individual nucleotides in the PNA from a single cell, involving their 

 separation by ionophoresis on a coppei-silk fiber.^^^ While previous iono- 

 phoretic and chromatographic techniques measure quantities of the order 

 of 100 to 1000 /xg. PNA, these methods have been successfully applied to 

 nerve cells containing 200 to 1000 pg. PNA. Motor anterior horn and 

 spinal ganglion cells from the rabbit contained on the average between 

 550 and 560 pg., although the concentrations (mass/cell volume) were 2.4 

 and 1.1%, respectively. 



In their studies on the embryonic development of the sea-urchin, Elson 

 et al}"^^ measured the PNA content by summation of the amounts of in- 

 dividual pentose nucleotides and found that the PNA values by orcinol 

 determinations were on the average 8 % less, while those from phosphorus 

 determinations were 10% more, than the total nucleotide results. The 

 PNA content fluctuated wdthin the range 5 to 6 X lO"* Mg per embryo 



22' J.-E. Edstrom, Biochim. et Biophys. Acta 12, 361 (1953). 



222 J.-E. Edstrom, Nature 172, 809 (1953). 



223 D. Elson, T. Gustafson, and E. Chargaff, J. Biol. Chem. 209, 285 (1954). 



