NITROGEN METABOLISM 225 



are not available to the orgaiiisin as such but must first 

 be hydrolysed to molecules small enough to penetrate the 

 cell wall of the bacteria. All the proteolytic organisms 

 liberate exo -enzymes into the medium where the proteins 

 are attacked. In other words, it is possible to obtain cell- 

 free solutions by the filtration of fluid cultures of such 

 organisms as Proteus vulgaris, B. siibtilis, Serratia ynar- 

 cescens and most anaerobes, which will hydrolyse proteins, 

 as shown by the liquefaction of gelatin or the breakdown 

 of casein. The enzymes responsible are constitutive, that 

 is, they are produced even when the organism grows on 

 a synthetic medium containing no protein. In some 

 cases it has been claimed that they are adaptive enzymes, 

 only being formed when protein is present in the medium, 

 although the evidence for this is somewhat doubtful ; 

 it is very improbable that such proteolytic enzymes 

 could be adaptive since the protein which would provoke 

 them cannot penetrate into the cell, where alone it 

 could influence enzyme synthesis. 



Proteoljrtic endo -enzymes also occur in some organisms, 

 from which they may be extracted after destroying the 

 cell structure by an appropriate means, such as solution 

 in bile, or in sodium hypochlorite, or by grinding, or 

 repeated alternate freezing and thawing. Usually these 

 endo -enzymes attack peptones and partially degraded 

 proteins more readily than they do the complex proteins. 

 For instance, endo-enzymes have been obtained from the 

 pneumococcus, which will break down peptone to amino - 

 acids but will not attack gelatin or egg-albumin ; haemo- 

 Ijrtic streptococci yield an endo -enzyme which destroys 

 peptone and casein but not serum albumin. Conversely 

 the proteolytic exo -enzymes do not break down the 

 proteins completely, but only sufficiently to enable the 

 fragments to enter the cell, where they undergo further 

 degradation where the products are of use to the organism. 

 As we have already mentioned, Esch. coll can grow on 

 and break down amino -acids but has no effect on proteins, 



