PROTEINS, a class of complex sub stances occurring in plants and animals, and differing from the other main con stituents (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 con stituents, as a class, "proteins," restrict ing the terms "albumen" and "albumen oids" to sub-groups. Of all the constit uents of food the proteins are the most important. Without them life is im possible, for it is the proteins which build the tissues and repair the waste of daily life. On the other hand, with pro tein, 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 con verted 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 Socie ties 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, glute lins and sclero-proteins (or albumen oids).
B. Conjugated Proteins, consisting of those proteins having, in combination, other groups, such as carbohydrates, phosphorus, etc., and including the nucleoproteins, glycoproteins, hremoglo bins, phosphoproteins and lecithoprotcins.
C. Derived Proteins, consisting of compounds derived from proteins by hydrolysis.
D. Protean or Metaproteins, includ ing coagulated proteins, proteoses, pep tones, 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 mole cules 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 example, are insoluble in pure water, hut soluble in salt solutions. Nearly all the soluble proteins can be precipitated from their solutions by the addition of salts, although, here again, there is great vari ation in their behavior with different salts, and in the amount of the salt necessary to bring about the precipita tion. It was formerly believed that no proteins could be obtained in crystal line form, but of recent years, egg al bumin and serum have both been crystal lized, the method employed being to mix the protein with an equal bulk of con centrated ammonium sulphate solution, filter, and acidify the filtrate with acetic or sulphuric acid.