CHEMICAL BASIS OF THE ANIMAL BODY. 65 



may be conveniently given 1 . The enzymic nature of the active agent 

 in rennet is clearly shown by the typical relationship which it exhibits 

 in its activity to the reaction of the solution in which it is present 2 , to 

 the temperature at which its activity is greatest, to the fact that the 

 briefest exposure to 100 or the more prolonged exposure to lower 

 temperatures (70 or above) 3 , suffices to destroy its active properties 

 and to the fact that a minute trace suffices to clot a relatively enormous 

 amount of casein 4 . 



Nothing is known as to the chemical nature of rennin. Extracts of the gastric 

 mucous membrane contain both rennin and pepsin. Hammarsten 5 has obtained it 

 free from the latter enzyme by fractional precipitation with either magnesium 

 carbonate or normal lead acetate, by which pepsin is more readily precipitated than 

 is rennin. He further endeavoured to separate out the enzyme, after freeing it from 

 pepsin, by precipitation with the acetates of lead in presence of a trace of ammonia ; 

 this precipitate was then carefully decomposed with very dilute sulphuric acid and 

 the enzyme finally separated by means of cholesterin. (Vide preparation of pepsin 

 p. 59.) The reactions of the purified enzyme described by Heidenhain seem to 

 indicate that it is not a proteid. 



Aqueous and glycerin extracts of the gastric mucous membrane 

 are usually found to be active in clotting milk 6 , but the activity of a 

 faintly acid extract is in all cases greater. This is due to the existence 

 of a rennin zymogen (renninogen) which is readily converted into the 

 enzyme by the action of acids 7 . The preparation of highly active and 

 permanent solutions of rennin is of considerable commercial importance 

 in connection with the cheese-making industry. The most efficient 

 extractive is sodium chloride 5 15 p.c., and permanency is attained by 

 the addition of alcohol or in some cases thymol 8 . 



Although rennin is most copiously present in the gastric mucous 

 membrane of the calf, it may be obtained from the tissue of almost any 

 stomach, if not as ready-made enzyme at least in the form of a 



1 This name seems more convenient than the more commonly used expressions 

 * the rennet ferment ' or ' the milk-curdling ferment.' 



2 Hammarsten (Swedish). See Abst. in Maly's Bericht. Bd. n. (1872), S. 121. 

 Heintz, Jn. f. prakt. Chem. (N.F.) Bd. vi. (1872), S. 374. See also Al. Schmidt. 

 Maly's Bericht. Bd. iv. (1874), S. 159. Langley, Jl. of Physiol. Vol. in. (1881), 

 p. 259. 



3 Mayer, Die Lehre von den chem. Fermenten, 1882. See also Maly's Ber. Bd. 

 x. (1880), S. 208. 



4 400,000800,000 times its own weight. Hammarsten. See Maly's Bericht. Bd. 

 vii. (1877), S. 166. 



5 Maly's Bericht. Bd. n. S. 121. See also Friedberg, JL Amer. Ch. Soc. May, 

 1888, p. 15. 



6 Hammarsten, loc. cit. See also Erlenmeyer, Sitzb. d. k. b. Akad. d. Wiss. 

 Miinchen, 1875, Hft. 1. 



7 Hammarsten, loc. cit. Langley, Jl. of Physiol. Vol. m. (1881), p. 287. 



8 Soxhlet, Milchzeitung, 1877, Nos. 37, 38. Chem. Centralb. 1877, S. 745. 

 Nessler, Landwirth. Wochenblatt. f. Baden, 1882, S. 57. Friedberg, Jl. Amer. Ch. 

 Soc. May, 1888, p. 15. Kinger, JL of Physiol. Vol. xn. (1891). Note 2, p. 164. 



F. e 



