66 ENZYMES OR SOLUBLE FERMENTS. 



greatest, to the fact that the briefest exposure to 100 or the more 

 prolonged exposure to lower temperatures (70 or above) * suffices 

 to destroy its active properties, and to the fact that a minute 

 trace suffices to clot a relatively enormous amount of casein. 2 



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

 the gastric mucous membrane contain both rennin and pepsin. Harn- 

 marsten 8 has obtained it free from the latter enzyme by fractional pre- 

 cipitation 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, 4 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 con- 

 verted into the enzyme by the action of acids. 5 The preparation 

 of highly active and permanent solutions of rennin is of consider- 

 able 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. 6 



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 zymogen (Hammarsten). It occurs also in the stomach of chil- 

 dren 7 and of man ; 8 and Roberts has described a similar enzyme 

 in the pancreas of the pig, ox, and sheep. 9 Rennin is stated to 

 occur in traces in urine. 10 



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

 p. 259. 



1 Mayer, Die Lehre von dem chem. Fermenten, 1882. See also Maly's Ber. Bd. x. 

 (1880), S. 208. 



2 400,000800,000 times its own weight. Hammarsten. See Maly's Bericht. Bd. 

 vii. (1877), S. 166. 



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

 1888, p. 15. 



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

 Miinchen, 1875, Hft. 1. 



5 Hammarsten, loc. cit. Langley, Jl. ofPhi/siol. Vol. in. (1881), p. 287. 



6 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, J/. of Physiol. Vol. xn. (1891). Note 2, p. 164. 



7 Zweifel, Centralb. f. d. med. Wiss. 1874, No. 59. Hammarsten, Ludwig's 

 Festgabe, Leipzig, 1875. 



8 Schumberg, Virchow's Arch. Bd. xcvu. (1884), S. 260. Boas, Centralb. f. d 

 med. Wiss. 1887, No. 23. 



9 Proc. Roy. Soc. No. 29, 1879, p. 157. 



10 See Helwes, Pfliiger's Arch. Bd. XLIII. (1888), S. 384. 



