Other evidence of the constant production of amino-acids in the tissues in fasting is offered by the experiments of Turner, Marshall, and Lamson.5 In these important investigations one-third the blood of a dog was withdrawn, the blood corpuscles were washed with normal saline, and then the washed corpuscles were returned to the body. This process is called by Abel plasmapharesis. Three such bleedings, with the return of the corpuscles in a volume of saline solution equal to that of the serum removed, should theoretically reduce the serum protein to 30 per cent, of that originally present, provided there were no renewal of the plasma protein. But there is a fairly rapid flow of protein into the plasma from supplies existing in other tissues, so that the serum protein after three successive bleedings amounts to about 50 per cent, of the quantity ordinarily present. Notwithstanding the fall in protein in the blood-plasma, the quantity of urea increases and the amino-acid nitrogen remains constant. These relations are shown in the following table which compares the analysis of the normal blood and that obtained after five days of plasmapharesis, during which time one-third of the blood was withdrawn fifteen times, a total amount of bleeding equal to more than fivefold the quantity of blood in the fasting animal.

1Abderhalden: "Zeitschrift f. physiolog. Chem.," 1913, lxxxviii, 478.

2 Abel: First Mellon Lecture, University of Pittsburgh, 1915, p. 22.

3 Van Slyke and Meyer: "Journal of Biological Chemistry," 1913-14, xvi, 231.

4 Kossel: "Biochemisches Zentralblatt," 1906, v, 33.

5 Turner, Marshall and Lamson: "Journal of Pharmacology and Experimental Therapeutics," 1915, vii, 129.

Effect Of Plasmapharesis Upon The Composition Of Dog's Blood

The reaction of the organism which causes an increase in the amount of its protein metabolism after reducing the amount of serum protein is also shown in the experiments of Taylor and Lewis,1 who withdrew blood repeatedly at hourly intervals and substituted saline for it in a dog. In this manner the quantity of serum proteins was reduced to only 2.7 per cent., although the quantity of amino-acid nitrogen and of urea increased in the serum.

1 Taylor and Lewis: "Journal of Biological Chemistry," 1915, xxii, 72.

These facts accord with the older work of Bauer1 in Voit's laboratory, who found an increased nitrogen elimination in the urine following bloodletting.

Summarizing this discussion, it becomes clear that though the body is built up of proteins which are aggregates of amino-acids, these same amino-acids occur free in only minimal amounts, 4 parts in 100,000 in blood, for example, and 40 to 80 parts in 100,000 in muscle. These small amounts are constantly present and apparently are the precursors of urea. After giving meat in large quantity the amino-acid concentration rises in the blood, but not in the tissues, for in the tissues the amino-acids are either destroyed with the production of urea or they are reconstructed into body protein, thus becoming "deposit protein" (see p. 286). When starvation takes place it is obvious that the quantity of protein destroyed may depend upon the protein condition of the cells themselves, and that in the presence of much "deposit protein" this may be metabolized in large quantity during the first few days, as is indicated by a high nitrogen elimination in the urine.

This principle appears not only in the dog, as before stated, but also in man. This is shown in the experiments of Karl Thomas (see p. 275), narrated by Rubner,2 although in this work carbohydrates were ingested. The daily diet of a man contained 89 grams of protein nitrogen or 4.5 per cent, of the total protein nitrogen content of his organism. During the last day of this diet the man eliminated 77.7 grams of nitrogen in the urine. Then the man was given a diet of starch and sugar, both of which were free from protein, and the nitrogen elimination in the urines of successive days was determined as follows: 28.3; 10.7; 5.15; 5.16; 4.72; 3.93; 3.46; nine-day interval: 3.06; 2.31; 2.16. The gradual elimination of "deposit protein" with the tendency of the total protein metabolism to fall to lower and lower levels is, therefore, a concomitant of protein starvation. It seems that it is this gradual metabolism of "deposit protein," in addition to the constant and necessary metabolism of the protein built into living substance of the cells, which determines the higher level of the protein metabolism during the early days of fasting.

1 Bauer: "Zeitschrift fur Biologie," 1872, viii, 567. 2 Rubner: "Archiv fur Physiologie," 1911, p. 61.

During true fasting it is quit possible that the full extent of protein metabolism is not measured by the nitrogen in the urine, for it may be that muscle proteins are con-verted into amino-acids which are transported to other organs, to the heart, for example, for the replenishment of an organ which scarcely loses weight during the ordeal of life without food. Such a procedure would be akin to the development of the genital organs of the salmon already described.

It will be perceived that although Voit's term "circulating protein" is, generally speaking, a misnomer, yet it served the useful purpose of sharply differentiating the more resistant behavior of living tissue protein from that of ingested protein and from the material now known as "deposit protein," ingested protein being very readily, and deposit protein quite readily, metabolized.

This point is furthermore well illustrated by the behavior of gelatin. Voit has demonstrated that although gelatin can never be converted into tissue protein nor retained in the body, its ingestion may in part prevent the combustion of the living protein tissue of the body (see page 156).

The amount of protein metabolized by a starving animal in good condition bears a constant relationship to the total metabolism involved. Even in different animals this constancy is observed. E. Voit1 calls attention to the fact that the nitrogen elimination is not dependent on the weight of the animal, since a pig of 115 kilos produces 0.06 gram per kilo, whereas a guinea-pig weighing but 0.6 kilo eliminates 0.65 gram of nitrogen per kilo, or ten times as much. However, a comparison of the percentage of the total energy derived from protein in fasting animals in good condition (i. e., with considerable fat) varies within much narrower limits . between 7.3 and 16.5 per cent. This is shown in the following table:

1 E. Voit: "Zeitschrift fur Biologie," 1901, xli, 188.

Nitrogen Metabolism Of Different Animals In Starvation

It is evident from the above that an average of 90 per cent, of the energy of the fasting metabolism may be supplied by non.protein material. This material is fat (see page 27).

If a fasting organism be kept at the same temperature and under the same conditions as regards the performance of external work, the metabolism is remarkably even from day to day.