Joint Recommendations of the Committees on Protein Nomenclature of the American Physiological Society and American Society of Biological Chemists.

Since a chemical basis for the nomenclature of the proteins is at present not possible, it seemed important to recommend few changes in the names and definitions of generally accepted groups, even though, in many cases, these are not wholly satisfactory. The recommendations are as follows:

First

The word "proteid" should be abandoned.

Second

The word "protein" should designate that group of substances which consist, so far as at present is known, essentially of combinations of α-amino acids and their derivatives, e.g. a-amino acetic acid or glycocoll; α-amino propionic acid or alanine; phenyl-α-amino propionic acid or phenylalanine; guanidin-α-amino valerianic acid or arginine, etc., and are therefore essentially polypeptids.

Third

That the following terms be used to designate the various groups of proteins:

I. Simple Proteins

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

Although no means are at present available whereby the chemical individuality of any protein can be established, a number of simple proteins have been isolated from animal and vegetable tissues which have been so well characterized by constancy of ultimate composition and uniformity of physical properties that they may be treated as chemical individuals until further knowledge makes it possible to characterize them more definitely.

The various groups of simple proteins may be designated as follows:

(A) Albumins

Simple proteins soluble in pure water and coagulable by heat.

(B) Globulins

Simple proteins insoluble in pure water, but soluble in neutral solutions of salts of strong bases with strong acids.*

(C) Glutelins

Simple proteins insoluble in all neutral solvents but readily soluble in very dilute acids and alkalies. †

(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. ‡

(E) Albuminoids

Simple proteins which possess essentially the same chemical structure as the other proteins, but are characterized by great insolubility in all neutral solvents.§

(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 a large number of amino acids, among which the basic ones predominate.

(G) Protamins

Simpler polypeptids than the proteins included in the preceding groups. They are soluble in water, un-coagulable by heat, have the property of precipitating aqueous solutions of other proteins, possess strong basic properties, and form stable salts with strong mineral acids. They yield comparatively few amino acids, among which the basic amino acids greatly predominate.

* The precipitation limits with ammonium sulphate should not be made a basis for distinguishing the albumins from the globulins.

† Such substances occur in abundance in the seeds of cereals and doubtless represent a well-defined group of simple proteins.

‡ The subclasses defined (a, b, c, d) are exemplified by proteins obtained from both plants and animals. The use of appropriate prefixes will suffice to indicate the origin of the compounds, e.g. ovoglobulin, myoalbumin, etc.

§ These form the principal organic constituents of the skeletal structure of animals and also their external covering and its appendages. This definition does not provide for gelatin, which is, however, an artificial derivative of collagen.

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.

(B) Glycoproteins

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

(C) Phosphoproteins

Compounds of the protein molecule with some, as yet undefined, phosphorus-containing substance other than a nucleic acid or lecithin.*

(D) Hemoglobins

Compounds of the protein molecule with hematin or some similar substance.

(E) Lecithoproteins

Compounds of the protein molecule with lecithins (lecithans, phosphatids).

III. Derived Proteins

1. Primary Protein Derivatives

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

(A) Proteans

Insoluble products which apparently result from the incipient action of water, very dilute acids, or enzymes.

(B) Metaproteins

Products of the further action of acids and alkalies whereby the molecule is so far altered as to form products soluble in very weak acids and alkalies, but insoluble in neutral fluids.

* The accumulated chemical evidence distinctly points to the propriety of classifying the phosphoproteins as conjugated compounds, i.e. they are possibly esters of some phosphoric acid or acids and protein.

This group will thus include the familiar "acid proteins" and "alkali proteins," not the salts of proteins with acids.

(C) Coagulated Proteins

Insoluble products which result from (1) the action of heat on their solutions, or (2) the action of alcohols on the protein.

2. Secondary Protein Derivatives*

Products of the further hydrolytic cleavage of the protein molecule.

(A) Proteoses

Soluble in water, uncoagulated by heat, and precipitated by saturating their solutions with ammonium sulphate or zinc sulphate. †

(B) Peptones

Soluble in water, uncoagulated by heat, but not precipitated by saturating their solutions with ammonium sulphate. ‡

(C) Peptids

Definitely characterized combinations of two or more amino acids, the carboxyl group of one being united with the amino group of the other, with the elimination of a molecule of water.§

* The term "secondary hydrolytic derivatives" is used because the formation of the primary derivatives usually precedes the formation of these secondary derivatives.

† As thus defined, this term does not strictly cover all the protein derivatives commonly called proteoses, e.g. heterproteose and dysproteose.

‡ In this group the kyrins may be included. For the present we believe that it will be helpful to retain this term as defined, reserving the expression "peptid" for the simpler compounds of definite structure, such as dipeptids, etc.

§ The peptones are undoubtedly peptids or mixtures of peptids, the latter term being at present used to designate those of definite structure.