2 16 AMINO ACIDS, PEPTIDES AND PROTEINS 



PEPTIDES 



Most likely the number of pure peptides that have been isolated from higher plants 

 represents only a small portion of the total. Thus, the presence of a large number of 

 unidentified, acid labile, ninhydrin-positive substances have been detected by paper chro- 

 matography in higher plants (23). Many of these may turn out to be new peptides. Also, 

 a chromatographic examination of rye grass extracts showed the presence of a number of 

 peptides (25). The discussion below is limited to peptides which have been isolated in 

 what is believed to be a pure state. 



Glutathione, gamma-L-glutamyl-L-cysteinyl-glycine, was first isolated from yeast 

 but has now been shown to be very widely distributed and can be detected by paper chroma- 

 tography in the extracts from many higher plants (23). A cyclic peptide, named evolidine, 

 has been isolated from the leaves of Evodia xanthoxyloides . The complete structure of 

 this cyclic heptapeptide has been reported (26). A dipeptide isolated from beans has been 

 shown to be gamma-L-glutamyl-S-methyl-L-cysteine (27). 



Peptides which contain non-amino acid moieties are also known. Pteroyl-L-glutamic 

 acid, also called folic acid is an example of this group and can be detected in the leaves 

 of a large number of green plants, (cf. Chap. 14) Another representative of this group 

 of peptides is the "lathyrus factor", isolated from Lathyrus odoratus seeds (28). (cf. Chap. 

 14) Pantothenic acid, an integral part of the coenzyme A molecule (Chap. 11) is a peptide 

 of pantoic acid and b6ta-alanine. As its name indicates, it is universally distributed in 

 nature although at very low concentrations. It is a growth factor for many organisms. 



CH, O 



I ^ II 

 HOCHoC - CHCNHCHoCHoCOOH 



1 I '' 



CH3 OH 



pantothenic acid 



Mistletoe contains a cardiotoxic, necrotizing substance named viscotoxin which ap- 

 pears to be a peptide. Its structure is still subject to controversy. According to Winter- 

 feld and Rink the toxic material contains a hydrogenated naphthalene ring, a glucuronic 

 acid radical and a tetrapeptide chain (29). However, later work by Samuelson (30) indi- 

 cates that the material does not contain any sugars but is a peptide composed of eleven 

 different amino acids. Among other peptides with uncertain structures one can list a 

 basic substance with weak antibiotic activity isolated from wheat (31) and the so-called 

 "allergens" present in plant pollens (32). 



PROTEINS 



All living systems contain a large number of different proteins. These may differ 

 in the amino acid composition, in the sequence of the amino acids, in the non-amino acid 

 constituents, in molecular weight and in those factors that determine the conformation of 

 the protein. In order to elucidate the structure of a given protein it is necessary that the 

 substance be separated from non-proteinaceous material as well as from other proteins. 

 This is sometimes a most formidable task and a number of different criteria have to be 

 used in order to establish the homogeneity of a given sample. 



The complexity and diversity of the proteins has prompted a number of different 

 classification schemes. However, these have only been partially successful. Plant pro- 

 teins have been classified according to their source, thus one speaks of seed or leaf 



