Mendel and Underhill — Papa'in-digestion. 13 



IV. General Conclusions. 



The observations recorded in this paper indicate that papain 

 belongs to a class of enzymes which differs somewhat in type from 

 the two proteolytic enzymes that have received most careful investi- 

 gation in the past, viz., pepsin and trypsin. While the products of 

 the papain digestion of proteids resemble quite closely those of pep- 

 sin so far as these have been examined in detail, the enzjnne differs 

 from ordinary animal pepsin in that it acts readily in both neutral 

 and alkaline media. On the other hand, although papain is com- 

 parable with trypsin in exerting a solvent action in fluids of various 

 reactions, the failure to form leucin, tyrosin and tryptophan in 

 appreciable quantities — at least under conditions in which they are 

 readily formed in large quantities by other tryptic enzymes — places 

 it in a class of its own for the present. 



The failure of papain to conform exactly with any of the standards 

 set in the past for proteolytic enzymes need not surprise us. The 

 more carefully such enzymes — especially those from vegetable 

 sources — are being examined with reference to their activities, the 

 more varied are found to be the manifestations which characterize 

 and distinguish them. We may refer, for example, to bromelin, the 

 proteolytic enzyme of the pineapple (Ananassa sativa), which has 

 been studied very thoroughly hj Chittenden. 1 Bromelin readily 

 forms leucin and tyrosin in large quantities in both acid and neutral 

 media, besides the characteristic proteoses and peptones. 2 This 

 recalls the proteolytic enzyme of the yeast, discovered by Salkowskr 

 and quite recently found by Halm and Geret 4 in the yeast juice 

 expressed by Buchner's method. It acts with intense vigor, giving 

 rise readily to leucin and t3ax>sin ; peptone is not obtained and albu- 

 moses occur only in traces ; acid reaction is favorable, while alkalies 

 retard digestion with it. The circumstance that the favorable reac- 

 tion corresponds with the one best for pepsin, while the products 

 formed resemble those resulting in trypsin proteolysis (the absence 

 of peptones being unique), has led Halm and Geret to classify this 

 yeast enzyme by itself and to give it a new name : yeast endotrypsin. 



'Chittenden : Journal of Physiology, 1893, xv, p. 249. 



2 In unpublished experiments by 0. H. Schell, Ph.B. and one of us, tryp- 

 tophan and other end-products were found in addition to those already described. 

 s Salkowski : Zeitschrift fur physiologische Chemie, 1889, xiii, p. 527. 

 4 Halm and Geret : Zeitschrift fur Biologie, 1900, xl, p. 117. 



