212 



AMINO ACIDS, PEPTIDES AND PROTEINS 



Another interesting class contains new heterocyclic and alicyclic amino acids. 

 Thus in addition to the long familiar histidine, proline and tryptophan, the compounds 

 listed in Table 3 must now be added. It will be noted that with the finding of azetidine-2- 

 carboxylic acid and pipecolic acid a homologous series of 4, 5 and 6-membered nitrogen- 

 containing rings is now established. A new cyclopropyl derivative, 1-aminocyclopropane- 

 1-carboxylic acid, has been isolated from pears (11), and hypoglycine A from the fruits 

 ol Blighia sapida (12). This compound, whose structure is shown below, has the ability 

 to decrease the blood glucose level of experimental animals. 



/ \ 



H2C=C CH CH^ CH(NH2)C02H 



hypoglycine A 



The number of naturally occurring dicarboxylic acids and amides has also been ex- 

 tended. Representative compounds are listed in Table 4. There is particular interest in 

 gamma-methyleneglutamine since evidence exists that in the peanut this compound plays a 

 major role in nitrogen transport (13). In most cases, however, the biological function of 

 these compounds, if any, is still obscure. 



New sulfur containing amino acids have also been discovered. Table 5. As already 

 mentioned, great caution is required in those cases where the only difference between the 

 previously known compounds and the newer ones is in the oxidation state of the sulfur. 

 There is always the possibility that these substances are artifacts which are formed during 

 isolation. S-methylcysteine suKoxide which has been isolated from cabbage (14) and from 

 turnip roots (15) appears to be a true plant constituent since S-methylcysteine cannot be 

 oxidized to the sulfoxide at room temperature (15). This is contrary to the behavior of 

 methionine which is oxidized under those conditions. Alliin, which is also a sulfoxide, 

 has been isolated from garlic (16). (cf. Chap. 14) 



Possibly, alliin is also closely related biochemically to cyclo alliin, isolated from 

 onion bulbs (17), since the latter is the cyclized form of alliin. Djenkolic acid was first 

 isolated from the urine of natives of Java who had eaten the djenkol bean but was later 

 shown to occur in the bean (18). Inspection of the structural formula (Table 5) shows that 

 this compound can be considered to be a thio acetal between cysteine and formaldehyde. 



A number of amino acids have been discovered in higher plants in which the presence 

 of a hydroxyl group is the major distinction from the more common compounds. Table 6. 

 However, this is not meant to imply that the biosynthesis of these compounds proceeds by 

 oxidation of the parent compounds. Several other hydroxy amino acids have been isolated 

 but their structure assignments are not yet definite (10a). Another interesting compound 

 which may be considered in this group is canavanine, which, as the structure below indi- 

 cates, is a hydroxyguanidine derivative. The similarity of this compound to arginine is 



NH 

 II 

 H2N-C-NOCH2CH2CHCO2H 



H NH2 



sufficient to permit hydrolysis of canavanine by arginase to urea and canaline, 

 H2NOCH2CH2CH(NH2)C02H. The latter compound is frequently found together with canava- 

 nine in jack bean meal and other leguminous species (19). 



