Summing up these results, we found that all the ten α-amino-acids used inhibited the growth of the cells and finally killed the cultures. This inhibition is preceded by a short period of activity. The typical effect on the cells is shown in Figs. 1 and 2. The first (Fig. 1) is a control culture showing the usual growth of cells and their typical spindle shape form. The second (Fig. 2) is a culture in plasma plus asparagine showing the cells rounded off and beginning to undergo dissolution.
We do not wish to draw too extensive conclusions from these experiments, but we believe that toxicity of α-amino-acids towards growing cells has been shown beyond a reasonable doubt; while we have found that compounds of higher molecular weight, namely, the peptones of egg yolk and proteins, are non-toxic. This toxicity depends upon the concentration and the time that the cells are exposed to their action. As these factors are reduced, the toxicity is decreased. In this respect, these substances are similar to all cell poisons.
Applying these results to the work done on the intravenous injection of digestion mixtures, we believe that we have found a reason for the death of the experimental animals when the hydrolyzed proteins were injected too rapidly. Buglia found that large amounts of α-amino-acids could be injected into the circulation without causing deep-seated changes in the renal and intestinal functions, provided they were injected slowly enough; in fact, that enough of these mixtures could be injected in this way to cover the nitrogen consumption of the body. This injection, however, was always accompanied by an α-amino excretion through the urine and an increase of the peristalsis, of the intestine, with resultant liquid stools. As is well known, a sudden great concentration of these substances in the blood of an animal causes death. These results agree with our findings
Folin and Denis demonstrated the fact that α-amino-acids probably pass into the circulation through the intestines. Van Slyke and Meyer, by means of Van Slyke's nitrogen method, have practically proven this, and Abel, Rowntree, and Turner, and Abderhalden have lately succeeded in obtaining α-amino-acids in crystalline form from the blood. Van Slyke and Meyer have shown that the tissues take up α-amino-acids to a certain point, but that after this the limit of saturation is reached. This is not so in the liver, which continually desaturates itself by metabolizing the α-amino-acids that it has absorbed, and consequently maintains indefinitely its power of removing them from the circulation, as long as they enter it no faster than the liver can metabolize them. Marshall and Rowntree have shown that there is an increase of the α-amino-acid concentration in the blood after injuries to the liver, which have caused deep-seated anatomical changes. Our experiments prove that tissue cells in general are unable to live in the presence of any great concentration of these acids.
At the present time we do not feel able to give an explanation of the significance of this evident toxicity. However, the fact in itself seems to indicate that we should expect stimulation from a certain increase of the α-amino-acid concentration in the body, or the concentration of any one of the acids, while a greater increase would lead to marked disturbances of the metabolism.