The Occurrence of Amino Acids in Nature 225 



one or two of these twenty amino acids to be absent from a protein preparation. 

 These facts are the more striking when taken together with the fact that in classes 

 {b) and (c) many new species of amino acid have been discovered, as is briefly 

 discussed below. 



Of course there are other amino acids which occur only in protein of a particular 

 type, or only in proteins of a Umited range of organisms or have some other 

 pecuharity of distribution. Of these, the better-authenticated examples are so 

 few that they may conveniently be mentioned here. 



Derivatives resulting from substitution of tyrosine or thyronine with iodine 

 occur in the proteins of the thyroid gland, and have been studied in considerable 

 detail in recent years. Similar compounds, including bromine-substituted deri- 

 vatives, occur in the proteins of a number of marine organisms, including 

 seaweeds, sponges and corals. 



L-Hydroxyproline, in animals, seems characteristically to be present only in 

 connective-tissue proteins of the collagen and elastin groups. Some claims 

 have been made for its occurrence in other proteins but these require verification 

 by isolation, the more so because Radhakrishnan & Giri [2] found by isolation 

 that L-a//ohydroxyproline occurs free in sandal leaves (cf. [3]). These authors 

 have hsted some of the claims that chemically bound hydroxyproline occurs in 

 nature other than in connective-tissue protein. Besides these, may be mentioned 

 claims in respect of seaweeds [4], diatoms [5], sponges [6], fungi and bacteria 

 [7-12], maple-sap peptides [13] and hydrolysates of sou [14-15]. A dehydro- 

 genated form of hydroxyproline, 'oxyminaline', was stated by Minagawa [16] to 

 be a component of fungal pectases. The only proteins in which hydroxy lysine 

 has so far been found are connective-tissue proteins which also contain hydroxy- 

 proline [17]. a-Aminoadipic acid has been reported to occur in the seed proteins 

 of maize [18] and sarcosine in those of the groundnut [19]. Lanthionine was 

 reported as a constituent of locust muscle by Stein [20]. Here there was no 

 ground, as in other cases, for suspecting that it had arisen by degradation of 

 cystine. a-Aminoz5obutyric acid has been claimed as a constituent of casein 

 hydrolysates by Oshima, Yoshihara & Sakamoto [21]. 



There is evidence that Se can take the place of S in the proteins of plants 

 grown on Se-rich soil and of animals feeding on these plants [22-24]. This is 

 the only known natural example of incorporation of foreign amino acids into 

 protein which parallels such observations as the incorporation of artificially 

 administered ethionine. 



ae-Diaminopimelic acid has particular interest since it appears to be confined 

 to bacteria and blue-green algae, among which it has a wide distribution, al- 

 though it is absent from some groups [12]. (Fujiwara & Akabori [25] claim to 

 have detected it in hydrolysates of Chlorella ellipsoidea.) As well as occurring in 

 various non-protein extracts of bacteria, diaminopimelic acid occurs in a com- 

 bined form in the extraction residues. It now seems definitely established that 

 it can be a constituent of the peculiar cell-wall material, which is further dis- 

 cussed below. What does not seem well estabHshed is whether or not diamino- 

 pimelic acid is a constituent of the 'true proteins' of bacteria (cf. [12, 26, 27]). 

 The amino acid described as cystine from alkali-soluble proteins of Corym- 

 bs 



