Page:Popular Science Monthly Volume 72.djvu/397

 place whereby cell protein is broken down into its component parts, while at the same time a synthesis of protein may be occurring from other amino-acids brought by blood or lymph with a possible utilization of some of the fragments liberated by the autolysis. In other words, our conception of nutritional changes to-day embodies the hypothesis of a synthesis of protein throughout the body; that it is a function of every living cell, "each one for itself, and that the material out of which all proteins in the body are made is not protein in any form, but the fragments derived from proteins by hydrolysis, probably the amino-acids, which in different combinations and different proportions are found in all proteins, and into which they are all resolved by the processes, autolytic or digestive, which can be carried out in every cell of the body" (Leathes).

Here, we have presented several points of view which are radically unlike the old-time traditions concerning nutrition. Objects and methods are both out of harmony with the long prevalent conception of the plan of the living organism in which oxidation was confessedly the ruling power. Profound and progressive hydrolytic cleavage is now seen to be the purpose of digestion, as well as of the autolytic processes which are associated with all the tissues and organs of the body; a cleavage which proceeds until the complex protein is split apart into its simplest chemical groups or components. In this process there is no sign of direct oxidation, but hydrolysis appears in the foreground, and by this means the rock protein is broken asunder into small fragments of definite shape, each of which can be used in the construction of fresh protein. Especially noteworthy is the harmony of action between the enzymes of the digestive tract and those of the living tissues of the body. Both have the same object in view; there may be differences in the rate of action, but in the end essentially the same simple fragments result. Again, what a suggestion of broad constructive power in the cells of the animal body; what a powerful synthetical process that by which the animal cell manufactures the most complex substance of its body protoplasm! For years the chlorophyll-containing cell of the plant world enjoyed the distinction of being the main laboratory in nature for the synthesis of organic compounds. The animal body could transform and modify, it could even accomplish a mild form of synthesis, such as a combination of two large molecules to form a still larger conjugate, but anything like a true synthesis, i. e., the formation of a well-defined complex, such as protein, out of simple amino-acids was far beyond our imagination, until now accumulated facts seem to open up a new point of view.

Are we really warranted in accepting this modern conception of protein synthesis in the animal body? Are the facts available sufficient to substantiate the claim advanced? Physiologists quickly recognized