This fact has been made use of in the artificial bate “Oropon” (see Chapter [VII].), in which extract of pancreas is combined with ammonium chloride, and some inert material.
The effect of puering on the fatty matter in the skin is well known. The fat and grease are partially emulsified, and set free, so that they can be removed by scudding. This action is a most important one, and one in which artificial bates have hitherto been wanting. The emulsification of the fats is brought about by means of an enzyme either identical with, or closely resembling, lipase.[105] This enzyme is found in the pancreatic juice, and in the seeds of many plants. It brings about the emulsification of the fat by saponifying a portion of it, i.e. the fat is split up into glycerin[106] and a fatty acid, according to the equation—
C3H5(C18H35O2)3 + 3H2O
Stearin.
= C3H5(OH)3 + 3(C18H35OHO)
Glycerin. Stearic Acid.
Lipase was one of the first enzymes in which the reversibility of the reaction was shown, i.e. it is not only capable of hydrolysing a fat, but also of causing the formation of one by the combination of the fatty acid and glycerin.[107] This explains why the reaction of such an enzyme is never complete. An equilibrium is reached just as with ordinary reversible chemical reactions like the precipitation of magnesium hydroxide by ammonia.
Lecithin, and possibly other fatty compounds, are known[108] to be important auxiliaries in the ferment-like actions produced by toxins; cholesterol has a similar effect, and as this body is a constituent of dung, it may play some part in puering. Here, again, is a further problem awaiting investigation.
Loevenhart has shown that the bile salts, sodium cholate, and sodium glycocholate, greatly increase the activity of lipase, and Magnus found that synthetic bile salts have the same effect; such bodies are known as co-enzymes. I have shown that bile itself is not favourable to the bating action, but the bile salts, by stimulating the action of the fat splitting enzymes, are probably essential to the full action of the dung bate.
Another enzyme which may be of importance in puering is erepsin, the enzyme of the intestinal juice, which is responsible for the completion of the digestive process. The pancreatic enzymes act upon the peptones produced by the pepsin of the stomach, splitting them up into simpler compounds, while the erepsin acts further upon these products. It dissociates albumoses and peptones into amino acids, taking as it were the last traces of nutriment from the food passing through the intestine. It acts best in alkaline solutions.
This ferment is very widely distributed in the animal kingdom, and occurs in other organs and tissues besides the intestines. The quantity of erepsin in the fresh fæces must be considerable, since a dog secretes from 400 to 500 c.c. of intestinal juice per day. It remains to be shown whether this retains its properties after excretion, and for how long.
A most important point in connexion with the activity of enzymes is the reaction of the medium, i.e. its acidity or alkalinity, or, more strictly speaking, its hydrogen ion concentration. A very slight increase or decrease of the acidity or alkalinity of the liquor will diminish the rate of action of the enzyme by a large amount, and in some cases cause the action to cease. In all cases enzymes have an optimum acidity or alkalinity; in other words, for every enzyme there is a particular hydrogen ion concentration at which its activity is at a maximum. The work of Soerensen, to which I have already referred, gives a very complete account of this aspect of enzyme action and also of the methods he employed to investigate it. His work should certainly be carefully studied by anyone wishing to take up this part of the subject.
The same remarks as to optimum conditions apply to temperature, although the effect of this is better known. Most tryptic enzymes act best at the body temperature, viz. 98° to 100° F. (37° to 40° C.), and hence puering should be conducted at this temperature. In the case of the hen or pigeon-dung bate the enzymes have not yet been studied, so far as I am aware, so that it is not possible to give an account of them. At the same time it seems highly probable that, as these bates are employed at comparatively low temperatures, the enzyme action is kept back, and therefore the chief action would be a chemical one.