Together with sucrase and maltase may be grouped other enzymes that split up the higher sugars into those of lower molecular weight, such as lactase which converts lactose or milk-sugar into dextrose and galactose, trehalase which splits up trehalose, a sugar obtained from Syrian manna, into two molecules of dextrose, and raffinase and melizitase which act upon certain of the higher polysaccharides.
Coagulating enzymes. The group of clotting or coagulating enzymes includes two comparatively well-known enzymes, rennet and plasmase or fibrin ferment. The use of rennet in setting curd for cheese and in preparing the delicate dessert known as junket is generally familiar. The source of most of the commercial rennet is the extract of the mucous membrane of the stomach of the calf; this enzyme is found also in many other young mammals during the period of lactation. Rennet has been obtained likewise from several vegetable sources; parts of the plant Galium are used in the country districts of England to aid in the formation of curd in cheese-making, and the peasants of the Italian Alps use the leaves of the butterwort (Pinguicula) for a similar purpose. The curdling or precipitation of the casein by rennet is singularly dependent upon the presence of salts of lime. A very minute quantity of rennet in the presence of calcium salts will curdle a prodigious quantity of casein. It is in fact uncertain whether rennet can act at all in the entire absence of calcium. In the presence of calcium the potency of rennet ranks higher than that of any other enzyme yet studied, one part of rennet being able to coagulate more than 250,000 times its own weight of casein.
The phenomenon of the clotting of blood is dependent upon a variety of factors as yet imperfectly understood. That the fibrin or solid portion of the clot is separated out from the blood plasma by the action of an enzyme is, however, solidly established. The character and mode of action of this enzyme—termed plasmase, or fibrin ferment—are still quite obscure, although the fact that in mammalian blood the enzyme originates from the leucocytes, or white blood corpuscles, seems to be generally admitted. In birds the enzyme exists in the cells of the tissues and not in the blood corpuscles. The blood of all vertebrates with nucleated red corpuscles presents a marked resistance to spontaneous coagulation; clotting, on the contrary, is almost immediate among the mammals, which possess enucleated red corpuscles. As is the case with rennet, calcium salts favor coagulation; their presence seems, however, not to be necessary.
Other clotting phenomena have been shown to be due to enzyme action. The formation of jelly from the juices of various fruits and berries is due to the gelatinizing or coagulating effect of an enzyme, pectase, which acts upon pectose, a carbohydrate allied to cellulose and occurring in many fruits and vegetables.
