102 NITROGEN METABOLISM 



substances which are beHeved to be peptides. Before at- 

 tempting to decide whether such requirements are absolute 

 it should be noted that in many instances the nature of the 

 response is determined by the composition of the medium 

 and the period for which the cultures are incubated [22]. 

 Strepogenin is the name given to acid-labile material pre- 

 sent in enzymic digests of proteins and required for the 

 growth of certain streptococci and Lh. casei in synthetic 

 media. The strepogenin activity of protein digests cannot 

 be explained solely in terms of their content of glutamine 

 or asparagine since neither of these substances replaced the 

 digest factor for a strain of Strep, faecalis [48] and the res- 

 ponse of Lb. casei to glutamine was different from that to 

 strepogenin [47]. Moreover, the activity of both glutamine 

 and GSH, unlike strepogenin, was destroyed by autoclaving. 

 From their experiments with digests of crystalline insulin, 

 Sprince and Woolley concluded that glutamic acid and 

 glycine are two components of strepogenin, and of a large 

 number of synthetic peptides, only tripeptides exhibited any 

 activity, serylglycylglutamic acid being the most effective, 

 though none was as active as the protein digest factor(s). 

 Strepogenin was antagonized by peptides containing as- 

 partic acid and also by lycomarasmin, a peptide secreted 

 by Fusarium ly coper sici and responsible for the wilting of 

 tomato plants. Lycomarasmin is composed of asparagine, 

 glycine and a-hydroxyalanine, with the two latter sharing 

 a common nitrogen atom, but the detailed structure is not 

 known. From these various experiments, it seems likely that 

 strepogenin contains a y-glutamyl residue and perhaps func- 

 tions as a stable source of such groups for the synthesis of 

 various co-factors. It is worthy of note that organisms res- 

 ponding to strepogenin also readily decompose glutamine, 

 and this may be another example of an essential nutrient 

 being utilized more efficiently when it is supplied in the 

 form of a peptide. The connection between strepogenin and 

 the dicarboxylic amino-acids and their amides is, however, 

 far from clear [cf. 42], and it is not yet possible to account 

 for all the experimental results. Gravis and intermedius 

 strains of Corynehacterium diphtheriae also require peptide 



