518 J. BRACKET 



cently by Pavlova^^^ that protease-free ribonuclease exerts no influence on 

 oxidative phosphorylations in homogenates. 



The obvious next step, under such circumstances, was to study the 

 possible effects of ribonuclease on the incorporation of amino acids into the 

 proteins of homogenates. As we already know from Siekevitz's work, such 

 an incorporation occurs mostly in the microsomes and requires the presence 

 of an ATP generating system. Recent experiments of Allfrey et al.,^^'' al- 

 though still of a preliminary nature, have shown that the integrity of PNA 

 is necessary for the incorporation of amino acids in the proteins of the 

 microsomes: adding ribonuclease to the system again strongly inhibits the 

 incorporation process. Similar observations have, still more recently, been 

 reported by Zacmenik and Keller,-^* who found that the incorporation of 

 amino acids into the proteins proceeds very well in anaerobiosis, provided 

 an adequate ATP generating system is present. All that is needed for the 

 reaction to proceed are the microsomes, a nondialyzable soluble fraction, 

 and the ATP generating system. The incorporation mainly occurs in the 

 proteins of the microsomes and, again, ribonuclease acts as an inhibitor. 

 This simplified biochemical system of Zacmenik and Keller^^* will certainly 

 be helpful in the understanding of the biochemical mechanisms of protein 

 synthesis. Two factors of the machinery are already identified, ATP and 

 PNA. 



As repeatedly emphasized earlier, the microsomes are especially rich in 

 PNA. This is the reason why the present writer suggested, some 10 years 

 ago, that they must be very important in protein synthesis. Impressive 

 experimental support for this opinion has been recently brought forward 

 by Allfrey et al}^"^ who found that there is a correlation between the PNA 

 content of the microsome fraction pellet and the rate of protein synthesis 

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

 material in the synthesis of the secretory proteins. Similar results have been 

 obtained and the same conclusions have been drawn by Oota and Osawa,^^^ 

 working on plant material. It thus seems to be a general property of all 

 living organisms that microsomes, because of their high PNA content, play 

 an exceedingly important role in protein synthesis; very similar conclusions, 

 based mainly on recent cytochemical evidence, have also been reached by 

 Pollister"" in an excellent review of the problem. 



The importance of PNA in protein synthesis in disrupted bacteria and in 

 microsomes seems thus to have been established beyond doubt; but does 



"6 M. V. Pavlova, Doklady Akad. Nauk S. S. S. R. 92, 641 (1953) . 

 2" V. G. Allfrey, M. M. Daly, and A. E. Mirsky, /. Gen. Physiol. 37, 157 (1953). 

 "«» P. C. Zamecnik and E. B. Keller, J. Biol. Chem. 209, 337 (1954). 

 2«9 Y. Oota and S. Osawa, Biochim. et Biophys. Acta 15, 162 (1954). 

 "" Pollister, A. W., "Dynamics of Growth Processes," pp. 33-169. Princeton Univ. 

 Press, Princeton, 1954. 



