BIOSYNTHESIS OF PROTEINS 271 



The essential energy-exchange processes of the cell are 

 associated with the enzymes which have been found in the 

 mitochondria. It is clear that the main biological function 

 of the mitochondria consists in the formation of energy-rich 

 compounds. 



Rather unexpectedly, it was found that the bulk of the 

 enzymes associated with the metabolism of the nucleic acids 

 (ribonucleic acid, RNA, and desoxyribonucleic acid, DNA) 

 was localised in the mitochondria."^ This led to the idea 

 that nucleic acid synthesis might be localised in the mito- 

 chondria. However, direct experiments did not verify this 

 suggestion."^ In the same way all contemporary investi- 

 gators agree that the actual synthesis of proteins does not 

 occur in the mitochondria. 



At one time T. Caspersson"^ suggested that the nucleus 

 is the main centre for the synthesis of proteins, but direct 

 studies of biosynthesis in fragments of algae (Acetabularia) 

 containing no nuclei, carried out by J. Hammerling"' and 

 later by J. Brachet and H. Chantrenne,"® disproved this 

 hypothesis. On the contrary, the numerous results obtained 

 by J. Brachet and R. Jeener,"^ T. Hultin,"^ E. B. Keller,"' 

 P. Siekevitz and P. C. Zamecnik^'^" have shown quite definitely 

 that the proteins of the cytoplasm are synthesised directly 

 in the microsomes. The nucleus only takes part indirectly in 

 the synthesis of proteins, perhaps by controlling the forma- 

 tion of the microsomes themselves. 



According to Siekevitz, ^'^^ however, the incorporation of 

 amino acids into the proteins of the microsomes is only 

 observed ^vhen microsomes and mitochondria are incubated 

 together. Under these circumstances the reaction proceeds 

 much faster when the conditions are suitable for oxidative 

 phosphorylation. The mitochondria may also be incubated 

 without the microsomes in a solution containing an oxidis- 

 able substrate and co-factors. In this case the mitochondria 

 form a high-energy factor, Avhich enters the solution, and in 

 the presence of this the isolated, incubated microsomes retain 

 their ability to incorporate labelled alanine. 



Thus we have here Tvhat might be called a division of 

 labour between the two sorts of protoplasmic structures. One 

 system of enzymes is localised in the mitochondria in which, 



