Proteins
a class of complex substances occurring in plants and animals, and differing from the other main constituents (carbohydrates and fats) in that they contain nitrogen. They are also known under the name of proteids, albumens and albumenoids, but there is a tendency to call the nitrogenous constituents, as a class, "proteins," restricting the terms "albumen" and "albumenoids" to sub-groups. Of all the constituents of food the proteins are the most important. Without them life is impossible, for it is the proteins which build the tissues and repair the waste of daily life. On the other hand, with protein, water and a little mineral matter, life can be maintained indefinitely.
Flesh foods of all kinds, including fish, are high in protein, as are also eggs, nuts, cheese and peas and beans. It is estimated that an adult requires, on the average, 120 grams of protein daily. In the body, proteins are decomposed into peptones and then to amino acids, in which form they are carried by the blood to the tissues, any excess being converted into urea by the liver, this, in turn, being excreted by the kidneys.
The chemical nature of proteins has been investigated by the famous German chemist, Emil Fischer, and his pupils.
More than fifty different varieties have been identified. A classification of these varieties was made by a joint committee of the Chemical and Physiological Societies of London, and was submitted to American scientific bodies for criticism.
As amended and completed by the latter, the classification divided the proteins into the following groups.
A. Simple Proteins, in which group are included the protamines, histones, albumins, globulins, prolamines, glutelins and sclero-proteins (or albumenoids). B. Conjugated Proteins, consisting of compounds derived from proteins by hydrolysis.
D. Proteans or Metaproteins, includ< ing coagulated proteins, proteoses, peptones, etc.
Proteins as a class are distinguished by the large size of their molecules, which renders possible their separation from simpler substances by dialysis, the smaller molecules passing through the dialyzer, while the larger protein molecules are retained. In other respects the proteins of the various groups differ greatly in their chemical and physical characteristics. Some (e. g. horn) are insoluble in all solvents; others, such as egg albumin, are soluble in water; still others, of which the globulins are an soluble in salt solutions. Nearly all the example, are insoluble in pure water, but soluble proteins can be precipitated from their solutions by the addition of salts, although, here again, there is great variation in their behavior with different salts, and in the amount of the salt necessary to bring about the precipitation. It was formerly believed that no proteins could be obtained in crystalline form, but of recent years, egg albumin and serum have both been crystallized, the method employed being to mix the protein with an equal bulk of concentrated ammonium sulphate solution, filter, and acidify the filtrate with acetic or sulphuric acid.