The following useful classification of enzymes is due to Effront.[98]
A more recent classification based on chemical properties is that of Kossel and Dakin.[99] They divide ferments into two classes:—
(1) Oxylytic ferments capable of breaking the O-link by which the radicals are held together in fats and carbohydrates.
(2) Imino-lytic ferments, including the amino-lytic ferments which act on the amino groups of urea.
Group 2 is sub-divided into—
(a) Trypsin and erepsin, which separate the imide NH from the neighbouring carbonyl CO.
(b) Arginase, which separates off urea from arginin.
Although it is somewhat doubtful whether the enzymes contained in dog dung are of glandular origin,[100] it is quite certain that other enzymes are secreted by bacteria developing in the dung while it is kept prior to being used for puering. These enzymes may be separated by the following method. About 150 c.c. of puer is well mixed with an equal quantity of glycerin, and allowed to stand for seven days. It is then filtered through paper by means of a pump, and yields a clear filtrate of a deep golden-brown colour; the filtrate is poured in a thin stream into a tall vessel containing about 1500 c.c. of 98 per cent. alcohol. A copious flocculent precipitate of the albuminous matter and enzymes is thrown down, the solution is filtered and the precipitate washed on the filter with absolute alcohol, and then dried over sulphuric acid in vacuo. The resulting powder is, of course, a mixture of all the albumins in solution, and probably only a small portion of it consists of the pure enzymes. We have merely succeeded by this method in concentrating them. The property of albuminous bodies in the act of coagulation to carry down soluble matter is well known, and this also renders the preparation of any pure proteid extremely difficult. It may be mentioned here that recent evidence goes to show that enzymes are not of a proteid nature, since, by repeated purification, the proteid matter may be almost entirely got rid of, while the activity of the residue containing the enzyme becomes considerably greater.