332 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 
addition of hydrochloric acid, because the compound is not re-formed, 
meets a simple answer in the statement that the pepsin is permanently 
destroyed by the strong acid or alkali used. A strong argument against 
any such compound is that the concentrations of different acids causing 
equal activity in pepsin are not proportional to the chemical equivalents 
of the acids, as might be expected if the acids entered into chemical 
combination with the pepsin. 1 
Pepsin is very rapidly destroyed by solutions of alkalies or alkaline 
salts. 2 The principal conditions which influence the rate of destruction 
of pepsin by sodium carbonate are — the strength of the solution of the 
alkaline salt, the time during which it is allowed to act, the temperature 
of the mixture, and the amount of proteid present. The mere act of 
neutralising an acid pepsin solution may destroy a considerable part of 
the pepsin. When equal volumes of a fluid containing pepsin and of a 
1 per cent, solution of sodium carbonate are well mixed, the greater part 
of the pepsin is destroyed in fifteen seconds; in a neutralised acid 
extract of the gastric mucous membrane of a cat, the amount thus 
destroyed may be |^ of the whole. Even very dilute sodium carbonate 
(•005 per cent.) will cause an appreciable destruction of pepsin in one 
or two hours at the body temperature, provided proteids are present 
in small amount only. 
Proteids lessen the rate of destruction of pepsin, probably by 
combining with the alkali or alkaline salts, for the greater the amount 
of sodium carbonate present the greater must be the amount of proteid 
to lessen appreciably the destruction. In the presence of '5 per cent, 
sodium carbonate, less than "25 per cent, of peptone has very little effect, 
and even 2 - 5 per cent, of peptone does not prevent the greater part of the 
pepsin from being destroyed. Thus, in the presence of 2*5 per cent, 
peptone, seven-eighths of the pepsin in an extract of a cat's gastric 
mucous membrane may be destroyed at 17° C. by - 5 per cent, sodium 
carbonate in sixty seconds. Pepsin prepared from a frog is less rapidly 
destroyed than pepsin prepared from a mammal. Carbonic acid destroys 
pepsin also, but less rapidly than it destroys pepsinogen. 3 
Solutions of salts of the heavy metals weaken or entirely remove the 
activity of pepsin solutions, according to the amount added. This effect 
is probably due to the enzyme being mechanically carried down in the 
usual fashion by the precipitate formed between heavy metal and pro- 
teid. The neutral salts of the alkalies and alkaline earths, when added 
(even in small quantity) to solutions of pepsin, decrease the activity. 
Thus Al. Schmidt 4 found that salt-free proteid dissolved in a few seconds 
in salt-free pepsin solution, but on the addition of - 5-0 - 6 per cent, of 
sodium chloride the time of solution was increased three to ten times. 
Hydrobromic and hydriodic acids in large doses, and to a still greater 
extent their potassium salts, delay peptic digestion. Sulphurous acid stops 
peptic digestion, but arsenious and hydrocyanic acids, except in large 
amounts, have little effect. Carbolic acid in small quantities has also 
little effect, but acts injuriously in greater concentration. Salicylic acid 
1 Davidson n. Dietrich, Arch. f. Anat. u. Physiol., Leipzig, 1860, S. 688; Putzeys, 
Jahrcsb. it. d. Fortschr. d. Thier-Ghem., Wiesbaden, 1877, Bd. vii. S. 279; Halm, Virchoiv's 
Arehiv, 1894, Bd. exxxvii. S. 597. 
2 Langley, Journ. Physiol., Cambridge and London, 1882, vol. iii. pp. 253, 283 ; 
Langley and Edkins, ibid., 1886, vol. vii. p. 371. 
:i Quoted from Langley and Edkins, Inc. cit. 
4 Jahresb. u. d. Fortschr. d. Thicr-Chem,., Wiesbaden, 1876, Bd. vi. S. 23. 
