2. Conjugated Proteins.—There is no conclusive evidence of the existence in plants of any of the conjugated proteins, other than the nucleoproteins and the chromoproteins, the composition and properties of which have been discussed in previous chapters. The nucleoproteins undoubtedly occur in the embryos of many, if not all, seeds.
3. Derived Proteins.—Representatives of the various types of derived proteins are undoubtedly found as temporary intermediate products in plants, both as products of hydrolysis produced during the germination of seeds and as intermediate forms in the synthesis of proteins. So far as is known, however, they do not occur as permanent forms in any plant tissues. They have been prepared in large numbers and quantities, by the hydrolysis of the natural proteins and the artificial synthesis of polypeptides.
In the present state of our knowledge concerning the functioning of the proteins, no significance in the physiology of plant life, or metabolism, is to be attached to the particular type of protein material which it contains, at least so far as the simple proteins of the cytoplasm are concerned.
DIFFERENCES BETWEEN PLANT AND ANIMAL PROTEINS
A much larger variety of protein materials is found in animal tissues than in plants. This is undoubtedly because different animal organs perform so much more varied physiological functions than do those of plants. Three groups of simple proteins, the histones, the protamines, and the albuminoids, which are quite common in animal tissues, are entirely unknown in plants. Further, conjugated proteins of greater complexity and more varied structure are found in animal tissues, especially in the brain, nerve-cells, etc., than in plants.
Plant proteins, in general, usually contain larger proportions of proline and of glutamic acid than are found in animal proteins; also more arginine than is found in any of the animal proteins except the protamines, which contain as high as 85 per cent of this amino-acid.
Of the twenty-five plant proteins which have thus far been hydrolyzed and studied from this standpoint, all contained leucine, proline, phenylalanine, aspartic acid, glutamic acid, tyrosine, histidine, and arginine; two gave no glycine; two others, no alanine; four contained no lysine; and one, no tryptophane. Zein, the principal protein of corn contains no glycine, lysine, or tryptophane. It is not sufficient to support animal life and promote growth, if used as an exclusive source for protein for food.
THE EXTRACTION OF PROTEINS FROM PLANT TISSUES
Since proteins are indiffusible, it is essential that the cell-walls of the tissue shall be thoroughly ruptured as the first step in any process for the extraction of these compounds from plant tissues. This is usually accomplished by grinding the material as finely as possible, preferably with the addition of sharp quartz sand, or broken glass, to aid in the tearing of the cell-wall material.
The solvent to be used in extracting the proteins from this finely ground material depends upon the nature and solubility of the proteins which are present, and also upon whether it is desired to separate the proteins which may be present in the plant, during the process of the extraction. A glance at the scheme of classification of the proteins will show the following solubilities which serve as a guide to the procedure to be followed: (a) proteoses, albumins, and some globulins may be extracted with water; (b) globulins and most of the water-soluble proteins may be extracted by using a 10 per cent solution of common salt; (c) prolamines are extracted by 70-90 per cent alcohol; glutelins and prolamins dissolve in dilute acids or dilute alkali.