PHYSIOLOGICAL CHEMISTRY 383 



the gland cells of the stomach mucosa. This precursor or zymogen 

 (see p. 388) is called pepsinogen. It differs from pepsin in that alkali 

 does not destroy it, whereas alkali destroys pepsin. 



ADVANCED EXPERIMENT. Scrape off the mucosa from about three square 

 inches of the stomach, grind it with some sand in a mortar, and gradually add 

 about 20 c.c. of 1 per cent, sodium carbonate solution. Filter. When about 

 10 c.c. of filtrate has collected which will take some time on account of the 

 mucilaginous nature of the extract place a piece of washed fibrin in the filtrate 

 and incubate at body temperature. No digestion occurs. In half an hour add 

 3 per cent. HC1, drop by drop, to the solution until it reacts faintly acid towards 

 litmus and again incubate. The fibrin soon digests. The acidity has converted 

 pepsinogen into pepsin. Divide the resulting solution into two parts. To one 

 of these add 1 per cent, sodium carbonate solution until faintly alkaline, and set 

 aside at warm temperature for 15 minutes, after which again render it faintly 

 acid with 3 per cent. HC1. To both test tubes now add similar pieces of fibrin 

 and warm to body temperature. It will be found that the fibrin becomes quickly 

 digested in the tube, which has been kept at acid reaction, but not in the other 

 tube, because of the alkalinity of the solution having destroyed the pepsin. 

 Pepsinogen, therefore, withstands an alkaline reaction, but pepsin is destroyed. 



The most favourable conditions for the action of pepsin may be studied 

 in the test tube as described in the following experiments : 



The Action of the Gastric Juice. The most convenient protein for 

 studying the action of pepsin is blood fibrin which has been very 

 thoroughly washed with boiling acidulated water so as to remove all 

 impurities. Cubes of coagulated egg white may also be employed, but 

 they digest more slowly than fibrin. 



EXPERIMENT VII. Label six test tubes A, B, C, D, E, F, and place 

 a small piece of fibrin in each. Half fill A with water, B with 0'2 per 

 cent. HC1, C with water and a few drops of peptic extract, D with 0'2 

 per cent. HC1 and a few drops of peptic extract, 1 E same as D, but boil 

 the mixture, and F with 1 per cent, sodium carbonate solution and a 

 few drops of the peptic extract. 



Place all these in a water bath kept constantly at body temperature 

 (37-38). Observe that in A the piece of fibrin remains unchanged, 

 whereas in B, D, and E, which all contain 0-2 per cent. HC1, it becomes 

 swollen and transparent. In F, which contains alkali, it does not swell. 



EXPERIMENT VIII. After about half an hour, remove a sample of 

 the contents of any of the tubes containing acid, colour it faintly with 

 a drop or two of litmus solution, and then carefully neutralise with 

 weak sodium carbonate solution (1 part 1 per cent, sodium carbonate + 

 2 parts of water). A precipitate of acid meta protein or syntonin is 

 usually produced (for reactions, see proteins, p. 310). 



1 Use larger quantities of fibrin and fluid in this test tube, because the products 

 of digestion will be required for succeeding experiments. 



