amino-acids containing an aromatic nucleus, amino-acids of heterocyclic compounds, sulphur-containing amino-acids.

Thus all the proximate constituents of simple proteins, as far as is known, are amino-acids. Of these the acids mentioned in the table, page 66, have been isolated.

These results show that all the proteins contain a very considerable proportion of the total number of amino-acids which have as yet been isolated from acid digests of proteins. The differences in various proteins cannot therefore be determined by qualitative differences in their constituent molecules, but must depend on the relative amounts of the amino-acids which are present and on their arrangement in the whole molecule.

DEAMINIZATION.-This process involves the splitting off of an NII, group from an amino-acid as ammonia, and its replacement by H or OH. Many tissues of the body appear to have this power. In most cases the nature of the change in the remaining moiety of the molecule has not yet been ascertained. If, for instance, some amino-acid, such as glycin, alanin or leucin, be added to a mass of liver cells, ammonia is set free in proportion to the amount of amino-acid which was added. The ammonia, therefore, is assumed to be derived from the amino-acid.

Certain investigations of Neubauer tend to show that deaminization is accompanied in the first place by oxidation, so that the first intermediate product formed is not an oxyacid but a ketonic acid. A second atom of oxygen is then taken up, and carbon dioxid is split off, with the production of the next lower acid of the series.

However, it would be superfluous to elaborate further the principle of deaminization. It will be sufficient to restate the hypothesis that practically all the proteins of the body are absorbed by the blood stream, removed from it by the tissue cells and dealt with either for the purpose of assimilation into protoplasm, or for the discharge as waste material by the kidney. Deaminization is essential to bring about the end. The arguments in support of this line of reasoning are as follows: In the first place there has been the discovery of erepsin, which assists the pancreatic juice in cleaving proteins into their simple crystalline products; in the second place, there are the experiments of Loewi and Abderhalden and of others which have shown that animals maintain their nitrogenous equilibrium and health when fed exclusively upon these simple materials; thirdly, there are the experiments of Schryver, of Leathes and of Howell, who have shown that the non-protein nitrogenous constituents of the blood and liver increase after the taking of a protein meal; and they have been

sucessful in the difficult task of detecting in the blood stream some of the individual amino-acids. Lastly, there has been the discovery of Vernon and others, of intracellular enzymes in the various tissues which render the cells capable of dealing with these substances which reach them via the blood and lymph.

Chemical Classification of Proteins.-The present chemical classification of proteins (see table on page 75) is based upon gross differences in structure, differences in solubility, etc., and does not recognize the subtle differences just described that have been demonstrated by biological methods. Nevertheless, the chemical classification is helpful and important. Native proteins are divided into simple and compound. The former include albumins, which are apparently soluble in water alone and are less easily precipitated than most proteins; and globulins, which require the presence of neutral salt to keep them in solution. While at one time it was thought that these made up a considerable amount of the cytoplasm of the cells, it is now recognized that they form a relatively insignificant part, the chief mass of the protein substances consisting of the more complex compound proteins, the albumins and globulins being nutritive materials for the cells or destruction products in the chemical transformations of the protoplasm.

ACTION OF PROTEINS.-The recent work of Abderhalden and the study of cytolysins afford evidence that the proteins of the liver, spleen, pancreas, and possibly other organs have their special peculiarities. Each kind of foodstuff, and indeed each individual protein fraction, follows its own special metabolic path. Moreover, there are paths for fats and paths for carbohydrates, and it is evident that even dextrose and levulose are dealt with in different manners, for a patient whose power of burning dextrose is grossly impaired, might deal with levulose with far better success. In the same way the power of catabolizing a single protein fraction. might be lacking in an individual who was able to dispose of other protein fractions in a normal manner. It is believed that these changes are wrought by specialized enzymes, many, if not all, of which are capable of reversed action.

The most prominent compound protein of the cell is nucleoprotein, a compound of simple protein with nucleic acid; this latter being a phosphoric acid compound of purin bases, pyrimidins and sugar. In the cell, nucleoprotein exists in the acid condition in the chromatin structure of the nucleus, while in the cytoplasm the acid is quite saturated with pro

tein.

The solution of a protein is in some respects hardly more than a sus

pension, so that relatively slight changes suffice to throw it out. Thus, many proteins may undergo changes whereby they lose their solubility and become inert masses. This state of "coagulation" may be brought about outside the body in the presence of water by the action of heat, of chemicals or of enzymes; it also may occur spontaneously within the body probably from enzyme action when the life of the tissue is extinguished. Certain natural changes are thus explained, such as coagulation of blood and rigor mortis.

DIGESTION OF PROTEINS.-The digestion of proteins, that is to say their preparation in alimentation for absorption, has long been a matter of investigation. The change is at first one of hydration and cleavage whereby soluble and slightly diffusible products are formed, namely, proteoses and peptones, and it was long thought that it was in this form that proteins were absorbed. It is now believed that the end accomplished by protein digestion is the breaking down of the complex protein molecule into its simple constituent amino-acids, or at least into relatively simple combinations of amino-acids, by which change the protein character is entirely lost. According to this conception, the reconstruction of the proteins in the organism starts from the simple amino-acid fractions, a view that furnishes an adequate explanation of how the transformation of the foreign food proteins into proteins that are specific for the individual organism, or at least for the species, is accomplished.

The protein breakdown in digestion is brought about by enzymes of the digestive juices. The reconstruction of tissue proteins after absorption is likewise accomplished by enzymes which are now working in a reverse direction, taking the simple blocks liberated by the digestive processes and recombining them into structures, that is to say protein molecules, of the nature that constitutes the particular tissues of the individual. There is thus the explanation of individuality in chemical composition as in structural morphology. Applied to germ plasm, it is readily seen how this conception bears upon the problem of hereditary transmissions.

Fischer has thrown much light on the structure of proteins by the experimental joining of amino-acids into combinations, so-called peptids, of greater or less complexity. Molecules have thus been synthesized that present many of the characters of native proteins.

The mechanism by which the regulation of the intricate metabolic changes are maintained, and the work of the regulating mechanism in the animal body, are made manifest in a variety of different ways. Thus we know that internal secretions are implements rather than originators of

control. The hormones are themselves but chemical products of certain specialized cells, and the activities of the glands which produce them are under the control of the vegetative nervous system which transmits to them impulses in response to the chemical demands of the tissues.

Thus we see that proteins are of very complex constitution, occurring in both animal and vegetable foods, and that chemically they may be regarded as peptids or combinations of amino-acids and their derivatives. They are colorless, odorless, in general tasteless, non-volatile and of varying degrees of solubility; they are putrefiable and readily undergo chemical change under the influence of the digestive ferments.

PROTEINS, AS CLASSIFIED BY AMERICAN BIOCHEMISTS (4)

I SIMPLE PROTEINS
A Albumins

B Globulins

C Glutelins

D Prolamins

E Albuminoids

F Histones

G Protamins

II CONJUGATED PROTEINS
A Chromoproteins

B Glyco- or glucoproteins
C Phosphoproteins
D Nucleoproteins

E Lecithoproteins

III DERIVED PROTEINS

A Primary Protein Derivatives a Proteans

b Metaproteins

e Coagulated proteins

B Secondary Protein Derivatives

a Proteoses

b Peptones

c Peptids

SIMPLE PROTEINS.-These are native proteins that apparently are made up entirely of amino-acids, or at least contain no distinct moiety. capable of being split off as such.

Coagulable proteins are the typical simple proteins. They are the albumins and globulins and occur together in most tissues and fluids of animals, as also in various parts of plants, especially fruits and seeds. They are changed into insoluble modifications when they are heated to boiling in a very slightly acid solution. They are built up of a rather large proportion of mono-amino-acids, in distinction to histon and especially protamin, in which the diamino-acids are conspicuous. Moreover, the mono-amino-acids include at least eight varieties. The important chemical distinction is that while albumins are soluble in water, globulins are only soluble in dilute salt solutions. Further, albumins are less readily salted out of solution than are globulins.

ALBUMINS.-Albumins are widely distributed. The molecule is highly complex and within certain limits varies greatly in different organs and conditions. The chief examples of albumins are serum albumin, egg albumin, myo-albumin, lactalbumin, vegetable albumin. The serumalbumin of the blood serum, plasma, lymph and other tissue fluids is probably a mixture of two albumins, from one of which amino-sugar was obtained by Langstein.