DEAN B. COWIE AND RICHARD B. ROBERTS 



31 



Acetate is believed to be used as acetyl-CoA and succinate as succinyl-CoA 

 (15). The phosphorylation of carbohydrates offers another example where the 

 alteration of the intermediates was not immediately apparent from a knowledge 

 of the initial and final compounds. 



Consequently, it seems quite reasonable to assume that most, if not all, 

 of the intermediates of synthesis are in some altered form which can be in- 

 dicated by attachment of an *R' group. 



Protein Protein 



T T 



Homoserine — > Threonine -^ Isoleucine 



t 

 External threonine 



i 

 Glycine 



In the series of reactions above there is no apparent reason why endogenous 

 threonine should not be converted to glycine. However, by attaching an 'R' 

 group there is clearly a difference between the endogenous and exogenous 

 threonine as shown below. 



Protein + R 



T 



Homoserine + R 



+ R 



Protein + R 



t 

 Isoleucine + R 



Threonine 



R^T 

 External threonine 



i 

 Glycine 



To interpret the metabolism of threonine, it is only necessary to assume i) 

 that the equilibrium conditions favor the formation of threonine + R, and 

 2) that the enzyme which splits threonine to glycine cannot attack threonine 

 + R. 



The same concept is equally useful in interpreting the metabolism of other 

 amino acids (that is, lysine, ornithine) in which the externally supplied amino 

 acid is metabolized differently from the endogenous intermediate of protein 

 synthesis. It is also necessary in the interpretation of some of the results 

 obtained with sulfur (24). 



Such an attachment could also provide a mechanism whereby exogenous 

 amino acids can diffuse freely through the cellular membrane but the amino 

 acid intermediates of protein synthesis manufactured by the cell are not lost 



