Enzymes are retarded in their action in the first place by their own products, in a very similar manner to bacteria, in the second place the retarding or inhibiting action is brought about by the so-called anti-bodies. Of these the longest known and best studied are those which are produced by toxins, and which neutralize the action of the toxins upon the animal organism (anti-toxins).[109] Normal serum also inhibits the action of trypsin and many other enzymes. Another very important group of anti-bodies are the precipitins. If the serum of an animal be injected repeatedly into another animal of different species, a precipitin appears in the serum of the animal treated, which causes a precipitate when added to the serum of the first animal. The special importance of this fact is, that it can be utilized as a method of distinguishing between human blood and that of animals, which is often of importance in medical juris-prudence.[110]
This fact has been made use of to distinguish dog dung from other matters, with which it has been adulterated.[111] A perfectly clear extract of dog dung filtered free from bacteria was injected into a rabbit. The serum obtained from the rabbit was found to contain a precipitin, and on addition of the serum to the dog dung extract a precipitate was produced. When the serum was added to the extract from the dung of another animal no precipitate was formed. When added to the extract of the adulterated dung a very much smaller precipitate was produced than with the extract from dog dung alone.
The coli bacteria in the puer also produce an anti-body—agglutinin.[112] If a culture of B. coli be examined under the microscope, the bacteria are seen moving here and there in the liquid, evenly distributed. On the addition of a trace of the serum of an animal which has been previously injected with coli bacteria the bacteria on the slide cease their movements and collect together in clumps. They are said to be agglutinated. This property is used to diagnose bacteria in suspected cases of cholera and some other diseases.
The wonderful discoveries that have been made in this direction constitute one of the most marvellous chapters in the history of science. Serum diagnosis and serum therapeutics are now firmly established as invaluable aids to the physician in his fight against disease and death.
The extent of our present knowledge of the action of enzymes in puering may be summed up as follows. Active enzymes are produced by bacteria growing in the infusion of dung, in addition to digestive enzymes which may be originally present in the dung; the bacterial enzymes are produced more rapidly in a dilute infusion, as employed in the puer wheel, than in the dung itself. The enzymes are of various kinds, proteolytic, peptolytic, lipolytic, etc., but the proteolytic and lipolytic are the most important. These have a solvent action on the fibres of the skin, but little or no action on the hyaline layer, at the concentration usually found in the puer liquor. The fatty matters and soaps in the skin are acted upon by the lipolytic enzymes, and the fats to some extent emulsified, so that they may be easily removed from the skin by scudding or pressing.
It must be clearly understood that enzyme action alone is not sufficient, as has been previously explained, but that the dung enzymes, acting in conjunction with the chemical compounds present, produce the specific puering effect.
There is still much work to be done before the action of the enzymes in dung is fully understood, but from the above short account it will be seen that the part they play in the bate is of great importance.