98 THE BIOSYNTHESIS OF PROTEINS 



of some amino acids, like proline, histidine, threonine, tryptophan, leucine 

 is usually strong, whereas that of arginine, glutamic acid or asparagine is 

 often so weak as to be difficult to show (Lipmann, 1958). This may be due, 

 in part at least, to differences in stability of the individual activation 

 enzymes in tissue extracts, as well as to exacting requirements of some of 

 them (Schweet et ah, 1957; Nisman, 1959; Webster, 1959). For instance 

 a purified alanine-activation enzyme from rat liver is very labile, it loses its 

 activity by freezing and thawing, and it is rapidly inactivated by oxygen 

 (Holley and Goldstein, 1959). 



Davie et al. (1956) were able to isolate from beef pancreas an almost pure 

 enzyme which is highly specific for L-tryptophan ; the only other amino 

 acids which react with this enzyme are certain tryptophan analogues 

 (Sharon and Lipmann, 1957). Enzymes which activate other amino acids 

 specifically have now been isolated or purified to various degrees: the 

 enzyme for methionine activation was purified from yeast (Berg, 1956), 

 tryptophan activation enzyme was separated from the enzymes for threo- 

 nine and serine in an extract of pig pancreas (Cole et al., 1957). Tyrosine 

 activation enzymes were also partly purified from the same tissue (Schweet 

 et al., 1957) and from yeast (Van de Ven et al., 1958). Serine activation 

 enzyme was purified from beef pancreas (Webster and Davie, 1959) and a 

 threonine specific enzyme from calf liver (Lipmann et al., 1959). Recently, 

 activation of the dipeptides L-leucyl-L-tyrosine and glycyl-L-leucine has 

 been reported (Tuboi and Huzino, 1960; Brown, 1960). 



Little is known at present about the structure of these activation enzymes 

 and their mode of action. Their active centre probably contains a — SH 

 group, for ^-chloromercuribenzoate or oxygen inhibit the tryptophan 

 enzyme from pancreas (Davie et al., 1956) and the alanine enzyme from 

 rat liver (Holley and Goldstein, 1959). The activity of several activation 

 enzymes is protected by reduced glutathione (Allen et al, 1960). The 

 participation of — SH groups in activation enzymes reminds one of the 

 case of triosephosphate dehydrogenase. This enzyme also contains — SH 

 groups (Rapkine, 1938) to which the activated carboxyl of phosphoglyceric 

 acid is transitorily bound as a thioester (Krimsky and Racker, 1952). 

 Activated carboxyls are also carried by coenzyme A as thioesters. One may 

 therefore suspect the participation of a thioester of amino acids at some 

 stage of the activation process. 



Some activation enzymes cause a rapid formation of hydroxamates 

 although they catalyse the pyrophosphate exchange very poorly (Novelli 

 and De Moss, 1957). Further analysis of this apparent discrepancy 

 might throw light on the reaction mechanism. 



An interesting fact is that the aminoacyl adenylates are very strongly 

 bound to the enzymes. During the pyrophosphate-ATP exchange catalysed 

 by the enzyme in the presence of the corresponding amino acid, no net 



