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 coagulation 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 important groups of enzymes is that of the proteolytic enzymes, characterized by their property of breaking down albuminous or proteid compounds into simpler ones. Owing to the prominent rôle 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 enzyme of the gastric juice, is the type, and the tryptic enzymes, the best known of which is the trypsin secreted by the mammalian pancreas. 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 bromelin 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 Russell. This enzyme, called galactase by its discoverers, acts upon the proteids in milk and plays a most important 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 experienced in the case of enzymes that attack chemical compounds so comparatively well understood as the sugars.
