There was formerly considerable confusion in the classification and terminology of the proteins and some differences of usage will still be met in the literature. At present, however, the majority of writers follow the recommendations made by a joint committee of the American Physiological Society and the American Society of Biological Chemists in December, 1907. The full text of these recommendations will be found in the appendix. The following is an outline of the classification thus recommended; to which have been added examples covering most of the food proteins thus far described as chemical individuals.

I. Simple Proteins

Protein substances which yield only amino acids or their derivatives on hydrolysis.

(A) Albumins

Simple proteins soluble in pure water and coag-ulable by heat. Examples: egg albumin, lactalbumin (milk), serum albumin (blood), leucosin (wheat), legumelin (peas).

(B) Globulins

Simple proteins insoluble in pure water, but soluble in neutral salt solutions. Examples: muscle globulin, serum globulin (blood), edestin (wheat, hemp seed, and other seeds), phaseolin (beans), legumin (beans and peas), vignin (cow peas), tuberin (potato), amandin (almonds), excelsin (Brazil nuts), arachin and conarachin (peanuts).

(C) Glutelins

Simple proteins insoluble in all neutral solvents, but readily soluble in very dilute acids and alkalies. The best-known and most important member of this group is the glu-tenin of wheat.

(D) Alcohol Soluble Proteins

Simple proteins soluble in relatively strong alcohol (70-80 per cent) but insoluble in water, absolute alcohol, and other neutral solvents. Examples: glia-din (wheat), zein (corn), hordein (barley), kafirin (kafir corn).

(E) Albuminoids

These are the simple proteins characteristic of the skeletal structures of animals (for which reason they are also called scleroproteins) and also of the external protective tissues, such as the skin, hair, etc. None of these proteins is used for food in the natural state, but collagen when boiled with water yields gelatin.

(F) Histones

Soluble in water, and insoluble in very dilute ammonia, and in the absence of ammonium salts insoluble even in an excess of ammonia; yield precipitates with solutions of other proteins and a coagulum on heating which is easily soluble in very dilute acids. On hydrolysis they yield several amino acids, among which the basic ones predominate. The only members of this group which have any considerable importance as food are the thymus histone and the globin derived from the hemoglobin of the blood.

(G) Protamins

These are simpler substances than the preceding groups, are soluble in water, not coagulable by heat, possess strong basic properties, and on hydrolysis yield a few amino acids among which the basic amino acids greatly predominate. They are of no importance as food.

II. Conjugated Proteins

Substances which contain the protein molecule united to some other molecule or molecules otherwise than as a salt.

(A) Nucleoproteins

Compounds of one or more protein molecules with nucleic acid. Examples of the nucleic acids thus found united with proteins are thymo-nucleic acid (thymus gland), tritico-nucleic acid (wheat germ).

(B) Glycoproteins

Compounds of the protein molecule with a substance or substances containing a carbohydrate group other than a nucleic acid. Example: mucins.

(C) Phosphoproteins

Compounds in which the phosphorus is in organic union with the protein molecule otherwise than in a nucleic acid or lecithin. Examples: caseinogen (milk), ovovitellin (egg yolk).

(D) Hemoglobins

Compounds of the protein molecule with hematin or some similar substance. Example: hemoglobin of blood. (The redness of meat is due chiefly to the hemoglobin of the blood which the meat still retains.)

(E) Lecithoproteins

Compounds of the protein molecule with lecithins or related substances.

III. Derived Proteins

1. Primary Protein Derivatives

Derivatives of the protein molecule apparently formed through hydrolytic changes which involve only slight alterations.

(A) Proteans

Insoluble products which apparently result from the incipient action of water, very dilute acids, or enzymes. Examples: casein (curdled milk), fibrin (coagulated blood).

(B) Metaproteins

Products of the further action of acids and alkalies whereby the molecule is sufficiently altered to form proteins soluble in very weak acids and alkalies, but insoluble in neutral solvents. This group includes the substances which have been called "acid proteins," "acid albumins," "syntonin," "alkali proteins," "alkali albumins," and "albuminates."

(C) Coagulated Proteins

Insoluble products which result from (1) the action of heat on protein solutions, or (2) the action of alcohol on the protein. Example: cooked egg albumin, or egg albumin precipitated by means of alcohol.

2. Secondary Protein Derivatives

Products of the further hydrolytic cleavage of the protein molecule.

(A) Proteoses

Soluble in water, not coagulable by heat, precipitated by saturating their solutions with ammonium sulphate or zinc sulphate. The products commercially known as "peptones" consist largely of proteoses.

(B) Peptones

Soluble in water, not coagulable by heat, and not precipitated by saturating their solutions with ammonium sulphate or zinc sulphate. These represent a further stage of cleavage than the proteoses.

(C) Peptids

Definitely characterized combinations of two or more amino acids. An anhydride of two amino acid radicles is called a "di-peptid"; one having three amino acid radicles, a "tri-peptid"; etc. Peptids result from the further hydrolytic cleavage of the peptones. As was mentioned above, many peptids have also been made in the laboratory by the linking together of amino acids.

Substances simpler than the peptones but containing several amino acid radicles are often called "polypeptids."