120 MICROSOMAL PARTICLES 



microsomal protein may be at least to some extent nonenzymatic, it is quite 

 possible that physiological mechanisms exist for the controlled transfer of the 

 amino acid moieties of aminoacyl adenylates. 



Note Added in Proof 



Zioudrou, Fujii, and Fruton have recently described the synthesis of C 14 - 

 tyrosinyl adenylate and C 14 -glycyltyrosinyl adenylate by a procedure similar to 

 ours. They observed labeling of heated and unheated rat-liver mitochondria 

 by these compounds and by their N-carbobenzoxy derivatives. They have also 

 concluded that the labeling of the mitochondria is due to nonenzymatic acyla- 

 tion (personal communication from Dr. J. S. Fruton; Proc. Natl. Acad. Sci. 

 U. S., in press). 



Further studies in our laboratory indicate that enzymatically synthesized 

 tryptophanyl adenylate can acylate microsomal preparations and also other pro- 

 teins (e.g., bovine serum albumin, ovalbumin) ; these experiments were carried 

 out with systems containing pancreatic tryptophan-activating enzyme, ATP, 

 magnesium ions, and acceptor protein. Labeling of ribonucleic acid prepara- 

 tions was also observed by such systems. The recent findings of Berg and 

 Ofengand [15] and of Schweet, Bovard, Allen, and Glassman [16] are consistent 

 with the possibility that specific binding sites for amino acids exist on soluble 

 ribonucleic acid molecules. Whether such specific binding of amino acids to 

 ribonucleic acid can be obtained with chemically synthesized aminoacyl adenyl- 

 ates remains to be determined. The present studies emphasize the importance 

 of isolating specific proteins in experiments on protein biosynthesis; the recent 

 report of Bates, Craddock, and Simpson [17] on the incorporation of valine into 

 mitochondrial cytochrome c appears to be a significant step in this direction. 



REFERENCES 



1. M. B. Hoagland, E. B. Keller, and G. D. Novelli, Proc. Natl. Acad. Sci. U. S., 

 P. C. Zamecnik, /. Biol. Chem. 218, 345 42, 325 (1956). 



(1956). 10. M. Bergmann and L. Zervas, Ber., 



2. P. Berg, Federation Proc, 16, 152 67, 1192 (1932). 



(1957). 11. F. Lipmann and L. C. Turtle, /. Biol. 



3. M. Karasek, P. Castelfranco, P. R. Chem., 159, 21 (1945). 

 Krishnaswamy, and A. Meister, paper 13 12. P. C. Zamecnik and E. G. Keller, 

 of this volume. /. Biol. Chem., 209, 337 (1954). 



4. M. B. Hoagland, P. C. Zamecnik, and 13. A. Gierer and G. Schramm, Nature, 

 M. L. Stephenson, Biochim. et Biophys. 177, 702 (1956). 



Acta, 24, 215 (1957). 14. R. R. Porter, Biochim. et Biophys. 



5. P. Berg, /. Biol. Chem., 222, 1015 Acta, 2, 105 (1948). 



(1956). 15. P. Berg and E. J. Ofengand, Proc. 



6. W. P. Jencks and F. Lipmann, /. Biol. Natl. Acad. Sci. U. S.. 44, 78 (1958). 

 Chem., 225, 207 (1957). 16. R. S. Schweet, F. C. Bovard, E. 



7. H. S. Moyed and F. Lipmann, /. Bac- Allen, and E. Glassman, Proc. Natl. Acad, 

 teriol, 73, 117 (1957). Sci. U. S., 44, 173 (1958). 



8. K. Moldave and A. Meister, /. Biol. 17. H. M. Bates, V. M. Craddock, and 

 Chem., 229, 463 (1957). M. V. Simpson, /. Am. Chem. Soc, 80, 



9. J. A. DeMoss, S. M. Genuth, and 1000 (1958). 



