60 ENZYMES OR SOLUBLE FERMENTS. 



which suffices for demonstration and ordinary purposes 1 . When how- 

 ever a peptic extract is required for research purposes it is essential 

 to adopt some more elaborate method which yields a product as free as 

 possible from admixed substances ; one of the best is that of Maly 2 . 

 The mucous membrane is digested, as in Briicke's method, with 

 phosphoric acid and the fluid precipitated with lime-water. The 

 precipitate of calcium phosphate is then filtered off, washed, and 

 dissolved in dilute hydrochloric acid, and this solution is then dialysed 

 until it is free from chlorine and phosphates, and on acidulating with 

 hydrochloric acid is ready for use. 



Owing to the relatively slow diffusibility of albumoses and peptones, mere dialysis 

 of a solution of pepsin in which these substances are present does not, within any 

 reasonable time, suffice to yield an even comparatively pure solution of the enzyme. 



Many forms of commercially prepared pepsin are obtained by digesting the 

 gastric mucous membrane with dilute hydrochloric acid ; the solution thus obtained 

 is then saturated with some salt such as NaCl, MgS0 4 or CaCl 2 , whereupon a scum 

 rises to the surface, consisting chiefly of proteid matter to which the pepsin is 

 adherent. This scum is then removed, frequently mixed with milk-sugar and dried 

 at a low temperature 3 . 



Pepsin does not exist preformed in the cells of the gastric glands 

 but as a zymogen to which the name of pepsinogen is given ; this is 

 readily converted into pepsin by the action of hydrochloric acid 4 . 



Unlike ptyalin the hydrolytic activity of pepsin is manifested only 

 in presence of an acid. The most efficient acid in this respect for 

 artificial digestions is hydrochloric of a strength of *2 p.c. 5 The 

 average percentage of this acid may be stated as '2 p.c. in normal 

 gastric juice but it varies slightly in the case of different animals 6 . 

 Other acids may be substituted for the hydrochloric, the optimal per- 

 centage varying for the several acids 7 . 



A remarkable peptonising enzyme (papam), exists in the milky juice of an East 

 and West Indian plant, Carica Papaya. An} 7 description of this enzyme and its 

 properties lies outside the scope of this work ; all necessary information may be 

 obtained by referring to the papers quoted below 8 . 



1 See also Kiihne and Chittenden, Zt. f. Biol. Bd. xix. (1883), S. 184. 



2 Pfliiger's Arch. Bd. ix. (1874), S. 592. 



3 Scheffer. See abstract in Maly's Jahresbericht. Bd. in. (1873), S. 159. 

 Sellden (Swedish), Ibid. S. 159. 



4 Ebstein und Griitzner, Pfluger's Arch. Bd. vm. (1874), S. 122. Langley, JL of 

 Physiol. Vol. in. (1881), p. 278. Langley and Edkins, Ibid. Vol. vn. (1886), p. 371. 

 Podwyssozky, Pfluger's Arch. Bd. xxxix. (1886), S. 62. 



5 Ad. Mayer, Zt. f. Biol. Bd. xvn. (1881), S. 356. 



6 Bidder und Schmidt, Die Verdauungssdfte, Leipzig, 1852, S. 46. Heidenhain, 

 Pfluger's Arch. Bd. xix. (1879), S. 152. 



7 Davidson und Dieterich, Arch. f. Anat. u. Physiol. Jahrg. 1860, S. 688. 

 Petit. See ref. in Maly's Bericht. Bd. x. 1880, S. 308. Also Ad. Mayer, loc. cit. 



8 Wurtz et Bouchut, Compt. Rend. T. LXXXIX. (1879), p. 425. Wurtz, Ibid. T. 

 xc. p. 1379 ; T. xci. p. 787. Polak (Dutch). See Abst. in Maly's Jahresber. 1882, 

 S. 254. Martin, Jl. of Physiol. Vol. v. (1883), p. 213; vi. p. 336. 



