BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 505 



more rapidly than in any other fraction, including mitochondria and 

 nuclei. A high PNA concentration, characteristic of the microsomes, is 

 therefore more important for protein synthesis than energy-producing 

 systems present in the mitochondria according to Hultin.^** 



Later work by Tyner et aZ.^^^'^" using glycine-C'^ by Keller^^* with 

 labeled leucine, and by Lee et al}^'^ using cystine-S^^ all in the rat, has 

 entirely confirmed these early results; the same conclusion, i.e., that 

 incorporation is greater in microsomal protein than in all other cellular 

 fractions, is also reached by Smellie et al.,^-'^ who worked with formate-C^^, 

 glycine-N'*, and methionine-S^*. 



Work on homogenates in vitro also confirms the exceptional activity of 

 the microsomal fraction in the incorporation of amino acids: such work 

 has been done by Borsook et aZ."" and by Siekevitz,^^^ who both emphasize, 

 however, the importance of energy-yielding reactions for successful incor- 

 poration. For instance, Siekevitz"- finds that incorporation of labeled 

 alanine by liver homogenates requires the presence of both mitochondria 

 and microsomes, the activity being greatest in the latter. The uptake is 

 greatly increased by the addition of a-ketoglutarate to the system, while 

 dinitrophenol and the hexokinase shunt are greatly inhibitory. 



Experiments by Crampton and Haurowitz,^^- with antigens labeled with 

 radio-iodine, have also shown that the antigen is very rapidly deposited in 

 the microsomes : about 50 % of the antigen deposited in the liver or spleen 

 is found in this fraction a few minutes after injection. One hour later, most 

 of the antigen is present in the mitochondria. This means, according to 

 Haurowitz and Crampton,'" that "since antibody synthesis is nothing but 

 modified synthesis of normal plasma proteins, the synthesis of these pro- 

 teins seems to take place in the cytoplasmic granules of liver, spleen and 

 other tissues." 



Another fact, which again is in agreement with the view that microsomes 

 are especially concerned with protein synthesis, is the observation of 

 Munro et al."* that, when rats are submitted to a protein-free diet, the 

 PNA content of the liver decreases only for the microsomes: neither cell 



>«« E. P. Tyner, C. H. Heidelberger, and G. A. LePage, Federation Proc. 11, 300 (1952). 

 >«' E. P. Tyner, C. H. Heidelberger, and G. A. LePage, Cancer Research 12, 158 (1952). 

 i«« E. B. Keller, Federation Proc. 10, 106 (1951). 

 169 N. D. Lee, J. T. Anderson, R. Miller, and R. H. Williams, J. Biol. Chem. 192, 733 



(1951). 

 "" H. Borsook, C. L. Deasy, A. J. Haagen-Smit, G. Keighley, and P. H. Lowy, /. 



Biol. Chem. 184, 529 (1950). 

 "1 P. Siekevitz, /. Biol. Chem. 195, 549 (1952). 



"2 C. F. Crampton and F. Haurowitz, Federation Proc. 11, 464 (1952). 

 1^' F. Haurowitz and C. F. Crampton, Exptl. Cell Research Suppl. 2, 45 (1952). 

 174 "P. W. Wikramanayake, F. C. Heagy, and H. N. Munro, Biochim. et Biophys. Acta 



11, 566 (1953). 



