The food substances which contain nitrogen are commonly called proteid, or, if these compounds are considered together, the name protein may be given the group. Protein is not a single compound, but includes all substances which contain the element nitrogen in such combinations as are available for assimilation in the human body.

Rancid fats made edible.

Protein is the most important group of nutrients in the animal body. The pro-teid substances in the body must be formed from pro-teids taken in the form of food, because only proteid foods contain the element nitrogen. All proteids contain nitrogen, but all nitrogen does not contain protein. All proteids, therefore, are nitrogenous compounds.

The animal body does not possess the power of combining elementary nitrogen with other elements. Bacteria have the power to utilize the nitrogen of the air to form mineral salts or nitrates. Plants have the power to unite the nitrogen derived from these nitrates with carbon, oxygen, and hydrogen. In this way organic nitrogen, or proteids, are formed. The animal body may digest these proteids, however, and transform them into other proteid compounds. All proteids contain carbon, hydrogen, oxygen and nitrogen; most of them contain sulfur, and a few contain phosphorus, iron, copper, and bromid.

Proteids defined.

Only proteid foods contain nitrogen.

Formation of organic nitroge.

The percentage by weight of the various elements which form proteid matter is about as follows:

Carbon.........................52%

Hydrogen....................... 7%

Oxygen..........................22%

Nitrogen........................16%

Sulfur........................... 2%

Phosphorus...................... 1%

The following table gives three groups of proteid substances:.

Besides these real proteids there are a few substances known as amido compounds which exist in small quantities in vegetables, and a number of nitrogenous substances which exist in meat and meat extracts, which are not true proteids, as they have little or no nutritive value, but act as stimulants or irritants in the body. Ptomains are another class of substances which are often found in food products. They are formed by the growth of bacteria, and are in reality the nitrogenous waste-products of bacterial life. Ptomains develop in meats and dairy products held in cold storage, and are sometimes the cause of serious poisoning. Nitrogenous waste-products will be further discussed in Lesson VI (Chemistry Of Metabolism), under "Metabolism of Proteids." (See p. 209.)

Amido compounds.

Ptomains - how formed.

Albumin is one of the commonest and simplest forms of proteids known. It is found in the white of eggs, in milk, and in blood. It is coagulated by heat, and by-certain chemicals, such as acids, alcohol, and strong alkalis. Albumin is soluble in water and in weak solutions of salt, but it is not soluble in very strong salt solutions.

Globulins are much like albumin, but are not soluble in water. They are, however, soluble in dilute salt solutions. Globulins exist in considerable quantities in the yolk of eggs, and in the blood. The globulin in the body could not remain in solution if there were not always present a small quantity of salt in the blood. There are several types of globulins. The fibrinogen of the blood, which coagulates, forming clots, when the blood is exposed to the air, is a globulin. Hemoglobin, which is the chief component of red blood-corpuscles, and which unites with the oxygen in the lungs and carries it to the various tissues of the body, is another form of globulin, and one which contains a considerable amount of iron. Casein is the most important proteid substance in milk, and is familiar to all as the curd or white substance of clabbered milk. A related form of vegetable casein is found in leguminous seeds, such as beans and peas.

Sources, coagulation and solubility of albumin.

Sources and properties of globulins.

Proteoses and peptones are proteids that are formed by the digestion of other proteids. They exist in the alimentary canal in the partly digested food. Peptones are readily soluble, and for this reason are easily absorbed through the walls of the digestive organs. (See Lesson V, "Digestive Organs" - [The Stomach], p. 137; also "Composition of Gastric Juice," p. 147).

Sources of casein.

Sources of proteoses and peptones.