Albumins and globulins are very often associated, as, for example, in blood serum and in the cell substance. As a rule the albumins are the more abundant in animal fluids, while the globulins predominate over albumins in animal tissues and in plants. There appears to be no sharp dividing line between the albumins and the globulins. While the globulins are insoluble in pure water, a water extract of animal tissue (muscle, for example) will contain, in addition to albumin, a considerable amount of globulin carried into solution by the salts present in the tissue, and if the salts are removed as completely as possible by dialysis, some of the globulin still remains in solution; separations based upon saturation with neutral salts are also apt to be unsatisfactory (Howell).

Notwithstanding these difficulties, a considerable number of individual albumins and globulins have been isolated, purified, and analyzed. In ultimate composition they show a general similarity except that the albumins are richer in sulphur than the globulins.

Several members of each group have also been studied to determine the kinds and amounts of amino acid radicles which they contain, with the results shown in the table on pages 60 and 61. It is of interest to compare the amino acid make-up of typical proteins with their adequacy in nutrition. A few studies of this sort, notably those of Kauffmann with gelatin and Willcock and Hopkins with zein, had been made some years earlier, but much the greater part of our knowledge in this field is due to the recent investigations of Osborne and Mendel (1911 et seq.). Rats have been chiefly used as the experimental animal.

Egg albumin, perhaps the most familiar of all proteins and the one most often chosen to illustrate, in the laboratory, the properties of proteins in general, will be seen to yield no glycine but to furnish all the other usual amino acids in quite appreciable proportions. The feeding experiments show that with a diet adequate as regards all other factors animals may be maintained in normal nutrition and young animals may make normal growth with egg albumin as the sole protein food.

Lactalbumin shows this same property in even greater degree. It appears to be the most efficient in supporting growth of all the proteins which have been studied, and this is believed to be due primarily to its high lysine content (see table beyond).

Legumelin and leucosin have not yet been studied in feeding experiments of this kind, nor have such experiments been made with amandin or vicilin.

Only preliminary feeding experiments not entirely successful as regards growth have been reported for legumin, phaseolin, and vignin; but each of the other three vegetable globulins shown - edestin, excelsin, and glycinin - has been found to suffice for maintenance and normal growth when fed as the sole protein in a diet adequate in other respects.* In fact Osborne and Mendel have kept one family of rats through three generations with edestin as a sole protein food.

Glutelins and the alcohol-soluble proteins (prolamins) are important as constituents of the cereal grains. The best-known examples of the respective groups are glutenin and gliadin of wheat flour. These proteins resemble each other in ultimate composition, but differ not only in solubilities, but also in their cleavage products. They are much the most important of the proteins of the wheat kernel, the gliadin making up about 50 per cent and the glutenin about 40 per cent of the total protein present. The gliadin and glutenin together constitute the gluten of wheat flour.

Glutenin (wheat glutelin) and maize glutelin have each been shown capable, in the rat-feeding experiments cited above, of meeting the requirements not only of maintenance but also of normal growth when fed as the sole protein food in diets adequate in other respects.

Gliadin, hordein, and the prolamin of rye, when fed singly in the same manner, are found capable of maintaining grown rats but not of supporting normal growth. Zein, fed alone in similar experiments, did not suffice either for maintenance or for growth. Osborne and Mendel concluded from these experiments that the failure even to maintain the grown animals was due to the absence of tryptophane; while the failure of the rats to grow on gliadin, hordein, or rye prolamin was due to the fact that these proteins either lack lysine or contain it in insufficient quantity. This interpretation was confirmed by later experiments in which they found that adding tryptophane to the zein food made it adequate for maintenance and adding lysine to the gliadin food made it adequate to support growth.

* Factors necessary to make a diet adequate will be discussed in Chapters XII and XIII, where experiments upon growth will be considered in greater detail.

Gelatin, the only member of the albuminoids (scleroproteins) which is of practical importance as food, has long been known to be unable to support protein metabolism when fed as the sole protein food. This inadequacy now appears to be due to the absence of tryptophane and tyrosine and perhaps in part also to the fact that some of the other amino acids, cystine and histidine, are furnished by gelatin in only very small proportion. As early as 1905 Kauffmann tried the experiment of living upon a diet in which gelatin was the sole protein, but was supplemented by additions of tyrosine, tryptophane, and cystine. So far as could be determined by a short experiment the addition of these amino acids seemed to make good the deficiencies of the gelatin.

Nucleoproteins are the characteristic proteins of cell nuclei, and are therefore especially abundant in the highly nucleated cells of the glandular organs, such as the thymus, the pancreas, and the liver. They are compounds of simple proteins with nucleic acid or nuclein. The chemical nature of the latter and their behavior in metabolism will be considered in Chapter V (The Fate Of The Foodstuffs In Metabolism Carbohydrates. Oxidation Of Carbohydrate).