BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 507 



Briefly, three main theories for protein synthesis have been proposed: 

 (a) protein synthesis results from a reversal of proteolysis; (b) protein 

 synthesis involves the intervention of energy-rich (presumably phosphate) 

 bonds; (c) protein synthesis occurs through a "template" mechanism. 

 These three hypotheses will now be considered in turn. 



a. Protein Synthesis by Reversal of Proteolysis 



This is obviously the simplest possible hypothesis: proteases and pepti- 

 dases, acting on amino acids, are the agents of protein synthesis. 



It is a well-known fact that, when proteolytic enzymes act on mixtures 

 of polypeptides, they can produce ''plasteins," which are substances 

 resembling proteins. However, the average molecular weight of the plas- 

 teins does not exceed 1,200 according to Ecker,^^^ who concludes that they 

 are little more than a complex mixture of peptides. Virtanen^^^ believes 

 that small peptides are synthesized by a phosphorylation process and 

 large ones by reversal of proteolysis. 



As has been pointed out by Linderstr0m-Lang,i*^ there is no satisfactory 

 evidence as yet that protein synthesis ever occurs when proteolytic enzymes 

 are made to act on polypeptides; there have, however, recently been 

 claims by Bresler and his co-workers'^^'^^ that, if a hydrolysate of a protein 

 by a protease is submitted to very high pressures (e.g., 6,000 atm. for 18 

 hr. at 38°), resynthesis of the specific protein occurs to a certain extent. 

 But it should be added that such a mechanism can hardly be physiological 

 and, furthermore, that Bresler's claims for amylase activity have not been 

 confirmed by Talwar et al.,^^'^ who found that high pressures inactivated 

 the enzyme and who obtained no evidence of synthesis. 



While it is doubtful whether proteolytic enzymes play an important 

 part in protein synthesis, it is a well-established fact that they can catalyze 

 peptide bond synthesis (Bergmann and Fruton^*^), as well as transpeptida- 

 tion reactions (see Fruton'" and Chantrenne'*^ for a full discussion of 

 these questions). But even though the transpeptidase activity of proteo- 

 lytic enzymes is a factor which aids in the understanding of how amino acids 



18* P. G. Ecker, /. Gen. Physiol. 30, 399 (1947). 

 18* A. I. Virtanen, Makromol. Chevi. 6, 94 (1951). 

 >8« K. Linderstr0m-Lang, Bull. soc. chim. biol. 22, 339 (1940). 



1" S. E. Bresler, M. V. Glikina, A. P. Konikov, N. A. Selezneva, and P. A. Finogenov, 

 Izvest. Akad. Nauk S. S. S. R., Ser. Fiz. 13, 396 (1949). 



188 S. E. Bresler, M. V. Glikina, and A. M. Tongur, Doklady Akad. Nauk S. S. S. R. 

 78, 543 (1951). 



189 A. G. Pasynskil and D. L. Talmud, Doklady Akad. Nauk S. S. S. R. 85, 1361 (1952). 



190 G. P. Talwar, E. Barbu, J. Basset, and M. Macheboeuf, Bull. soc. chim. biol. 33, 

 1793 (1951). 



1" M. Bergmann and J. S. Fruton, Ann. N. Y. Acad. Set. 45, 409 (1944). 



