PHYSIOLOGICAL CHEMISTRY 385 



(1) Boil the solution in a beaker or evaporating dish, cool and separate any 

 coagulated native protein by filtration. 



(2) Carefully neutralise the filtrate with 1 per cent, sodium carbonate solution ; 

 if a precipitate of metaprotein (syntonin) falls down, separate it by filtration. 



(3) The resulting filtrate is now shaken vigorously with an equal volume of a 

 saturated solution of ammonium sulphate. A precipitate of primary proteases 

 forms since these are insoluble in half saturated ammonium sulphate solution. 

 Collect the precipitate on a filter paper and preserve the filtrate. The further 

 investigations of the precipitate and filtrate are conducted simultaneously. The 

 precipitate is washed at least once with a half-saturated solution of ammonium 

 sulphate, the filter paper floated on to water in an evaporating dish stirred up 

 with a glass rod, warmed, and portions of the resulting solution of primary 

 proteoses (containing ammonium sulphate) employed for the following tests : 



1. Add a few drops of a saturated solution of salicyl sulphonic acid a white 

 precipitate forms which dissolves on heating and reappears on cooling. 



2. Add a few drops of pure concentrated nitric acid. The same result is 

 obtained as in 1. 



3. Apply the Biuret test using a sufficiency of 20 per cent, caustic soda solution 

 to permit of some excess after all the ammonium sulphate has been decomposed 

 (see p. 31 



4. Apply the various protein tests described on pp. 302, 303. T 



The filtrate still contains the secondary proteoses. These are precipitated by 

 adding sufficient sulphuric acid to the filtrate to render it distinctly acid towards 

 litmus, saturating while boiling with ammonium sulphate crystals, adding 

 ammonia till just alkaline and allowing to cool. The precipitate is then filtered 

 off, dissolved in water, and the tests described under primary proteoses applied 

 to the solution. The precipitates with nitric acid and salicyl sulphonic acid will 

 be much less marked than with primary proteoses. 



The filtrate after removal of the last traces of proteose is tested for peptones 

 by applying the Biuret test with excess of caustic soda, and other general protein 

 tests. 



For convenience in testing the large amount of ammonium sulphate present 

 may be removed by boiling the solution, adding barium carbonate as long as 

 ammonia continues to be evolved, and filtering. 



The table on page 386 shows the main reactions of the intermediate 

 products of peptic digestion (as occurring in Witte's Peptone) com- 

 pared with those of native protein. (See also p. 312.) 



The action of gastric juice in curdling milk is described in the section 

 on Milk (see p. 327). This action is usually attributed to the ferment 

 rennin, but it is probable that rennin and pepsin are identical, as proteo- 

 lytic ferments always have a rennin action, and the proteolytic activity 

 of a given ferment is proportional to its rennin activity. 



The gastric juice scarcely affects other foodstuffs. In the case of fat, 

 however, it dissolves the protein envelope of the fat cell, and liberates 

 the contents, which now float in the chyme as oil globules. On 

 emulsified fats the gastric juice can effect hydrolysis into fatty acid 



1 The alkaloidal reagents give precipitates with proteoses. 

 2B 



