[Sidenote: Proteids defined]
The food substances which contain nitrogen are commonly called
proteids, 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.
[Sidenote: Only proteid foods contain nitrogen]
Protein is the most important group of nutrients in the animal body. The
proteid substances in the body must be formed from proteids 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.
[Sidenote: Formation of organic nitrogen]
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.
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:
_Simple Proteids_
Albumins
Globulins
Nucleo albumins
Albuminates
Coagulated proteids
Proteoses (Albumoses)
Peptones
_Compound Proteids_
Respiratory pigments
Gluco Proteids
Nucleins
Nucleo proteids
Lecith albumins
_Albuminoids_
Collagen
Gelatin
Elastin
Reticulin
Keratin
[Sidenote: Amido compounds]
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.
[Sidenote: Ptomains--how formed]
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, under "Metabolism of Proteids." (See p. 209.)
[Sidenote: Sources, coagulation and solubility of albumin]
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.
[Sidenote: Sources and properties of globulins]
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.
[Sidenote: Sources of casein]
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.
[Sidenote: Sources of proteoses and peptones]
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).