24 NITROGEN METABOLISM 



with a reducing agent such as cysteine, lactate or for- 

 mate [28, 25]. Similarly, a loss of serine deaminase activity 

 was prevented by glutathione, formate or AMP. During 

 investigations of the nutrition of lactobacilli and Strep, 

 faecalisy organisms exacting towards aspartic acid, Stokes, 

 Larsen and Gunness observed that they grew well in the 

 absence of this amino-acid provided the medium contained 

 adequate amounts of biotin. They therefore suggested that 

 biotin plays an importarit role in aspartic acid metabolism, 

 and perhaps especially in the aspartase system, although 

 they were unable to demonstrate the presence of the latter 

 in these organisms [57]. 



Meanwhile Lichstein and his colleagues were studying 

 the catabolism of aspartic acid and serine by Esch. coli, Pr. 

 vulgaris and Bacterium cadaveris. They found that of the 

 known B vitamins only biotin was capable of reactivating 

 washed cell suspensions whose deaminase activity had been 

 reduced by standing for 30 minutes in molar phosphate 

 buffer, pH 4 [42]. This loss of deaminase activity and subse- 

 quent reactivation by biotin or by much larger concentra- 

 tions of AMP was only exhibited by cells which had been 

 grown in the presence of a yeast extract. A concentrate of 

 compounds containing biotin was prepared from yeast and 

 was found to be a hundred times more effective in enhancing 

 the aspartase activity of 'aged' suspensions than was to be 

 expected on the basis of its biotin content. Moreover, when 

 fresh, a partially resolved cell-free preparation of aspartase 

 was activated either by yeast extract or by biotin plus 

 adenylic acid, but after standing at 0° C, only the former 

 was effective [41]. A bound form of biotin, termed biocytin, 

 has been isolated recently from yeast, crystallized and shown 

 to be £-N-biotinyl-L-lysine [69]. Biocytin itself is probably 

 not the natural coenzyme, since, unlike the impure concen- 

 trates, its activity is only comparable to that produced 

 by an equivalent amount of free biotin [68]. Wood and 

 Gunsalus have prepared from Esch. coli a purified cell-free 

 system possessing serine and threonine deaminase activity, 

 and although both adenylic acid and glutathione were 

 required, biotin did not appear to be necessary [62\ 



