Proteins 39 



the highly complex protein molecules are constructed. These are 

 drawn upon mainly from the free amino acid "pool" of the cell and 

 by means of enzyme activity are united with ATP through their 

 carboxyl group (Hoagland, 1955) . The ATP provides energy for 

 the synthesis and comes largely from the mitochondria. In this way 

 an activated amino acid adenylate is produced. The enzymes con- 

 cerned are distributed in the soluble fraction (cell sap) of many, if 

 not all, cells though some activity also may be associated with par- 

 ticulate cell structures such as the microsomes. There may be other 

 ways of "activation" of amino acids but these are little more than 

 conjecture at the present time (Wieland and Pfleiderer, 1957— re- 

 view) . Uncertainty exists, too, about the role of vitamin B i2 in 

 protein biosynthesis; it seems to be true enough that amino acids 

 are incorporated into protein more slowly when there is a shortage 

 of B 12 . 



Activated amino acids are next bound to the RNA present in a 

 fraction of the cell sap. The latter, known as soluble RNA (SRNA) 

 may well serve as an intermediate stage between activated amino 

 acid and ribonucleoprotein situated in the microsomes (Hoagland 

 et al. 1957) . Perhaps the reaction goes on in the following manner: 



Amino acid -f ATP ^ Amino acid AMP + PP 



Amino acid - AMP + SRNA ?± Amino acid - SRNA + AMP 

 There seem to be specific binding sites for each amino acid. At any 

 rate SRNA must have a certain terminal nucleotide sequence be- 

 fore it can act as an amino acid receptor and the amino acids so 

 attached are not present in peptide linkage (Simkin, 1959— review). 



Much evidence now exists for the view that the microsomes 

 which are storehouses of ribonucleoprotein are important sites of 

 protein synthesis (Chantrenne, 1958; Loftfield, 1957) . They may 

 well be the end of an assembly line which begins in the nuclear 

 DNA and actually manufactures the structural protein of the cell. 

 Here, too, are produced the proteins destined for secretion by the 

 cell, both normal and abnormal, e.g. serum albumin, antibody, 

 Bence-Jones protein, myeloma protein and pancreatic juice pro- 

 tein. The microsomes are not, however, the only site of protein 

 synthesis though they certainly account for a large part in some 

 cells. Mitochondria can build some protein, including cytochrome 



