BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 489 



correlation between the basophilia or ultraviolet absorption of different 

 tissues and their PNA content. This parallelism was already made apparent 

 by Brachet's'^ earlier estimations of the PNA content of various tissues; 

 subsequently, different and better methods have been devised for quanti- 

 tative estimations of PNA, without altering the initial conclusions. Exten- 

 sive reviews of the whole question have been given by Davidson,^^ ''^ who 

 has made important experimental contributionstto the subject: Davidson^^ 

 comes to the conclusion that the nucleic acid content of different tissues, 

 as determined by chemical methods, is generally in accordance with the 

 values which might be expected on histological grounds. 



Thus, it will be no surprise to learn that glandular organs, synthesizing 

 large amounts of proteins, such as pancreas, salivary glands, and gastric 

 and intestinal mucosae, are rich in PNA; that the same is true, to a some- 

 what lesser extent, for organs where mitoses are frequent (spleen, thymus, 

 lymph nodes, testis, various tumors) ; and that kidney, brain, heart, and 

 lung have a much lower PNA content. This question has already been 

 discussed in Chapter 16. 



Biochemical work on embryonic material from Davidson's laboratory^" '^^ 

 has quantitatively demonstrated yet another cytochemical finding (Thor- 

 elP^) : synthesis of PNA always precedes protein synthesis, so that a rising 

 protein content is characteristic of embryonic differentiation. This conclu- 

 sion is perfectly in keeping with the cytochemical work done on amphibian 

 embryos, which has been discussed earlier in this review. It has also been 

 found by Mandel et alP that during development of the brain in chick 

 embryos, synthesis of PNA is linked to protein synthesis, while DNA syn- 

 thesis is related to nuclear multiplication. 



Another interesting case is to be found in liver, where fasting or admin- 

 istration of a protein-poor diet is followed by a decrease in basophilia and 

 a parallel drop in the actual PNA content (Davidson^*). In contrast to this, 

 the DNA content of the liver is not affected when the protein content of the 

 diet is altered (Mandel et al.,^^ Campbell and Kosterlitz^^) . Campbell and 

 Kosterlitz*^ finally draw the conclusion that "the PNA content of a unit 

 of liver cells is determined mainly by the protein content of the diet," an 

 opinion which is also shared by Munro et al. f^ these workers recently dis- 



'« J. N. Davidson, Cold Spring Harbor Symposia Quant. Biol. 12, 50 (1947). 

 " J. N. Davidson, "The Biochemistry of Nucleic Acids." Methuen, London, 1950. 

 8" J. N. Davidson, I. Leslie, and C. Waymouth, Biochern. J. 44, 5 (1949). 

 81 I. Leslie and J. N. Davidson, Biochim. et Biophys. Acta 7, 413 (1951). 

 " P. Mandel, R. Bieth, and R. Stoll, Bull. soc. chim. biol. 31, 1335 (1949). 

 »3 P. Mandel, M. Jacob, and L. Mandel, Bull. soc. chim. biol. 32, 80 (1950). 

 84 R. M. Campbell and H. W. Kosterlitz, Endocrinology 6, 308 (1950). 

 «5 R. M. Campbell and H. W. Kosterlitz, Biochim. et Biophys. Acta 8, 664 (1952). 

 *' H. N. Munro, D. J. Naismith, and T. W. Wakramanayake, Biochern. J. 54, 198 

 (1953). 



