60. Proteoses, Peptones

When various proteins, such as an albumin or globulin, are subjected to the action of a weak acid or of certain enzyms, they undergo what is known as hydrolysis. This change involves a cleavage (splitting) of the protein body, accompanied by the taking up of the elements of water. In this way are formed proteoses and peptones, the latter being proteins that are soluble in water. A proteose is an intermediate stage between the original protein and a peptone, and it receives a name according to its source, as albumose, globulose, and caseose, according as an albumin, a globulin, or casein is its source.

Peptone was formerly regarded as the final product of enzym action in digestion, but we now know that the digestion of the proteins proceeds much farther. These hydrolyzed bodies are found abundantly in the digestive tract during digestion, the proteoses as stated being an intermediate stage of digestion between the original proteins and the peptones. This means that the formation of the final products of protein digestion is a progressive step. Proteoses and peptones may also'be obtained by laboratory methods. It should be noted that commercial peptones are largely proteoses.

61. Important Properties Of The Proteins

The previous description of the various groups of proteins cannot be understood to its fullest extent excepting by those who have a good knowledge of the fundamentals of organic chemistry. Nevertheless, the facts given serve to impress the important chemical and physical properties which these bodies possess, and point to the necessity of studying them individually in their relation to foods and nutrition. It is not rational to speak of protein as if the term represents an individual entity; but the members of this general class of compounds must be considered by sub-classes at least, in discussing the use of raw material in cookery and in meeting dietary conditions.

There are several points that the dietician should keep in mind. One is the solubilities of the different proteins, another the effect produced upon them by heat, and another their relations to acids and ferments, - facts that will develop more fully as we proceed. A fact still more important is the varying constitution of the protein molecule, and consequently the possible variation in the nutritive function of the individual proteins.

62. The Unlike Constitution Of Proteins From Different Sources

We have already seen that certain proteins are particularized in part by containing phosphorus, others sulfur, and others iron. The significance of these differences will become evident as we discuss nutritive processes. A phosphorus-bearing protein may have, and undoubtedly does have, a nutritive function that cannot be exercised by an albumin not carrying phosphorus.

It is well known that when proteins are submitted to the action of acids, alkalies, and certain ferments (enzyms), they break up into simpler compounds, which we speak of as cleavage products. It is very significant that the kind, and especially the proportions, of these products differ greatly with different proteins. For instance, the purin bases, which certainly sustain important physiological relations, are present in beef and certain glands used as food, but absent in milk and eggs. The variations in the decomposition products of certain vegetable proteins are striking, as also are the differences in this respect between vegetable and animal proteins. These cleavage products are sometimes spoken of as the "building stones" of the proteins. The following table is worthy of attention: -

Table XV. Compounds1 Into Which Various Proteins Are Broken By Cleavage

As these compounds into which the several proteins are split may be regarded as the building stones out of which the animal proteins are constructed, the foregoing figures are significant.

In this connection it should be noted that a comparison of vegetable and animal proteins shows a close resemblance in the kind of building stones out of which they are constructed, although the proportions are unlike.

1 There is no popular terminology with which to describe these compounds, that are known only to the chemist. They are distinguished from one another by their structure and chemical relations, and are stated in this connection simply to show that important structural differences exist between the proteins named.