(A.) Animal

The proteins are the best-known representatives of the nitrogenous class, and are composed of carbon, hydrogen, and oxygen, with nitrogen and sulphur. The first three elements are likewise the component parts of the fats and carbohydrates, so that proteins combine in themselves to a certain extent the properties of both classes of nutrients, and can to a certain extent replace them. It is, however, impossible for the fats and carbohydrates to replace or be built up into proteins, of which the protoplasm of both the plant and the animal cell is mainly composed.

There are many varieties of proteins, most of them being insoluble in water, but some, like the albumins of blood serum, egg, and milk, being quite soluble. One part of egg white and two parts water thoroughly shaken together make albumin water, an excellent food to administer to babies and others suffering from diarrhoea. There are other proteins in blood and white of egg called globulins, which are only soluble in water with the aid of a neutral alkaline salt. Egg yolk again contains vitellin, whilst milk contains caseinogen, both of which are phospho-proteins. The flesh of most animals is composed of proteins, chiefly myosin, and these are associated with nucleo-proteins which on decomposition yield the notorious substance known as uric acid - in its way quite a harmless body if it only be expelled rapidly enough from the system. Being however rather insoluble, especially in acid fluids, it has a way of getting into positions where it is not wanted and thus creating trouble.

(B.) Vegetable

So far I have only mentioned animal proteins, but there is an infinite variety of vegetable proteins - twenty or thirty different kinds at least being known - and these are not quite so acceptable to the human body as animal proteins, because the latter undoubtedly correspond more closely to the proteins of which our own tissues are composed. Not only is this the case, but the vegetable proteins are usually enclosed in an indigestible envelope of cellulose, just as we have seen in the case of the gluten of wheat in the wholemeal bread, and until this has been ruptured they are unable to be digested and absorbed into the blood. They are therefore in large measure expelled in the faeces quite unutilised, and Voit has shown that in this way as much as 42 per cent. of the nitrogen of the vegetarian's food is lost.

Proteins all agree in one particular, and that is, they undergo a curious transformation called coagulation in the presence of agents such as heat, alcohol, and astringents generally, like alum and tannin, and ferments such as rennet. The hardening of eggs, the clotting of blood, and the curdling of milk are familiar examples of this change.

Like all organic matter proteins may be burned, and when this takes place in the air they are decomposed into carbonic-acid gas, water, oxide of sulphur, and pure nitrogen, which passes off unchanged. A precisely similar process of oxidation - as it is called because oxygen is absorbed from the air - takes place in the body, but the various stages of decomposition are by no means so simple, another process termed hydrolysis being invoked to assist matters. Hence the proteins are not so thoroughly burned up as in the air, so that they leave cinders or ash, which is apt to accumulate in the body under certain circumstances. Urea is the best known of these residual products of the disintegration of proteins, and it is excreted in the urine, but there are many other much more complex and usually poisonous compounds which make their appearance in the lower bowel and are apt to be reabsorbed into the body and so set up a form of self-poisoning termed auto-intoxication.