MICROSOMES AND RNA PARTICLES 21 



microsomes after intravenous injection of labeled amino acids 

 (glycine, lysine and leucine). The same result was also obtained by 

 Hultin (1950), who used glycine-^^N and worked with the chick. The 

 uptake in vivo of the amino acid by the microsomal protein took 

 place more rapidly than in any other fraction, including mitochon- 

 dria and nuclei. He concluded that a high RNA concentration, 

 which is characteristic of the microsomes, is more important for 

 protein synthesis than the energy-producing systems present in the 

 mitochondria. 



Later work by Tyner et al. (1952) with glycine-^'*C, by Keller 

 (1951) with labeled leucine, and by Lee et a/. (1951) with cystine-^^S, 

 all in the rat, quickly confirmed these early results. The same con- 

 clusion, i.e. that incorporation is greater in microsomal protein than 

 in all other cellular fractions, has also been reached by Smellie et al. 

 (1953), who worked with formate-^*C, glycine-^^N and methionine- 

 ^^S, by Hendler (1959), who studied the oviduct of the hen, and by 

 Rabinovitz and Olson (1956) for reticulocytes. 



Work on the 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 al. (1950) and by 

 Siekevitz (1952), who both emphasized, however, the importance 

 of energy-yielding reactions for successful incorporation in vitro. 

 For instance, Siekevitz (1952) found that incorporation of labeled 

 alanine by liver homogenates requires the presence of both mito- 

 chondria 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 is greatly inhibitory. The incorpo- 

 ration is thus linked to energy production through oxidative phos- 

 phorylation. 



In a return to the in vivo experiments, Allfrey et al. (1953, 1955a, b) 

 found that there is a close correlation between the RNA content of 

 the microsome fraction pellet and the rate of protein synthesis in a 

 tissue, and that in pancreas the protein pellet serves as a precursor 

 material in the synthesis of the secretory proteins. Similar conclu- 

 sions have been reached by Siekevitz and Palade (1958) who also 

 worked on pancreas, and by Oota and Osawa (1954) and Martin 



References p. 50/54 



y 



