SOLUBLE FERMENTS OR ENZYMES. 501 



The various phenomena of clotting and gelatinizing belong in the 

 debatable border land between physics and chemistry. Duclaux has 

 given in his treatise a clear exposition of his reasons for believing that 

 the changes involved in the various processes of coagiilation are due to 

 a disturbance of the physical equilibrium of the substances in solution 

 rather than to any chemical reaction. 



Enzymes acting upon proteid substances. One of the most im- 

 portant groups of enzjTues is that of the proteolytic enzymes, 

 characterized by their property of breaking down albuminous 

 or proteid compounds into simpler ones. Owing to the promi- 

 nent role they play in the human body in connection with 

 digestive processes they have been subjected to exhaustive study. Two 

 chief groups are recognized, the peptic enzymes, of which pepsin, the 

 enzjTne of the gastric juice, is the type, and the tryptic enzymes, the 

 best known of which is the trypsin secreted by the mammalian pan- 

 creas. The peptic enzymes are almost unique among known enzymes, 

 inasmuch as they can act only in an acid medium; they are further 

 characterized by their inability to carry the decomposition of proteid 

 substances beyond the 'peptone' stage. The tryptic enzymes, on the 

 other hand, are most potent in a slightly alkaline medium, and they 

 are able to push proteid decomposition to a point beyond that reached 

 by the pentic enzymes. Two of the most characteristic end-products of 

 tryptic digestion are the substances leucin and tryosin which, like 

 urea, are not assimilable by the tissues and are eliminated from the 

 body. Several tryptic enzymes of vegetable origin are known, among 

 which hromelin from the juice of the pineapple and papain from the 

 fruit of the papaw-tree have been thoroughly studied. It is probable 

 that the digestive enzyme secreted by insectivorous plants belongs to 

 the tryptic class. 



An interesting tryptic enzyme has been recently discovered in fresh 

 milk by Professors Babcock and Eussell. This enzyme, called galactase 

 by its discoverers, acts upon the proteids in milk and plays a most im- 

 portant part in the manufacture of cheese; it is probably responsible 

 for many of the phenomena of cheese-ripening that were formerly 

 ascribed to bacteria. 



Owing to the great chemical complexity of proteid substances and 

 to the fact that little is known about their chemical constitution, the 

 study of proteolytic enzymes is hampered by difficulties of an especially 

 serious nature. Since neither the initial composition of the proteid 

 compound nor the substances to which it gives rise on decomposition 

 can be accurately determined, the distinction of different kinds of 

 proteolytic enzymes is attended with greater difficulty than is ex- 

 perienced in the case of enzymes that attack chemical compounds so 

 comparatively well understood as the sugars. 



