Phosphoproteins occur especially in milk and eggs, which obviously function in nature to provide the material for growth and development of new animal tissue. The phosphorus, while probably present in the form of a more or less modified phosphoric acid radicle, appears to be more closely bound in these than in the nucleoproteins. Casein of milk and the vitellin of egg yolk (ovo-vitellin) are the most prominent members of the group. These are sometimes classed with simple proteins under the name nucleo-albumins. Phosphoprotein preparations show on analysis small amounts of iron, which has usually been neglected as an impurity but which is not improbably an essential constituent.

Casein and ovo-vitellin fed singly as the sole protein of the ration in the experiments by Osborne and Mendel described above have each been found capable of supporting both maintenance and normal growth, as their amino acid make-up and their place in nature would lead us to expect. The curves in Fig. I illustrate the rapid growth on casein as compared with the very slow growth on gliadin and the loss of weight when zein was the sole protein food. The rations were alike except for the nature of the protein fed; the percentage of protein in the ration was the same in each case.

Fig. 1. - Showing typical curves of growth of rats on diets otherwise similar and adequate but containing in each case only a single protein, casein, gliadin, or zein. Courtesy of Dr. L. B. Mendel and the Journal of the American Medical Association.

It will be seen that the rat receiving casein grew over 200 grams in 140 days while the one fed with gliadin grew only 20 grams during the same period. The third rat, which had been growing rapidly on mixed food, began at once to lose weight when put on a ration of which zein was the sole protein.

Hemoglobins, consisting of combinations of simple proteins with coloring matter, serve as carriers of oxygen from the air to the tissues. On boiling or heating with acids or alkalies they are split into their constituent parts: for example, ordinary hemoglobin yields about 4 per cent of hematin, C32H32N4FeO4, and a residue of globin which was formerly considered a globulin but is now assigned to the histone group.

Proteoses and peptones are products derived from other proteins by digestion or by simple hydrolysis. They are soluble in water and not coagulated by boiling their aqueous solutions. No sharp line can be drawn either between proteoses and peptones, or between peptones and the simpler nitrogen compounds which result from prolonged digestion. As the terms are generally used, peptones may be considered as the products of digestion or hydrolysis which still show the usual color reactions of proteins and are precipitated by strong alcohol; but are not precipitated by saturation of their solutions with zinc or ammonium sulphate, as is the case with proteoses. Proteoses (albu-moses) are intermediate products between metaprotein and peptones. In addition to the protein reactions shown by peptones, the proteoses are precipitated from aqueous solutions at ordinary temperatures by adding acetic acid and potassium ferrocyanide, or by saturating the solution with zinc or ammonium sulphate.

The term "peptone" was formerly applied to all digestion products not coagulated by boiling, and is still popularly used in the same sense, the best commercial "peptones" consisting largely of proteoses.

Percentages Of Amino Acids From Hydrolysis Of Various Proteins

* Not determined.

Percentages Of Amino Acids From Hydrolysis Of Various Proteins

* Not determined in these cases and presumably not, in the cases left blank. In other cases the figures given for cystine may be much too low because of decomposition of this amino acid in the process of hydrolyzing the protein. 1 Fischer and Abderhalden report also diamino-trioxy-dodecanic acid 0.75 %.