Abstract
In a previous communication 1 we reported that the active agent in the Rous chicken sarcoma filtrate could be concentrated and partially purified by adjusting the filtrate, normally about pH 7.2, to a reaction of pH 4. When brought to this reaction with a phthalate buffer, the tumor-producing agent is carried down with the precipitated portion. When this precipitate is extracted at pH 8, the carcinogenic agent is not only set free, but the activity of the extract seems definitely greater than that of the original filtrate.
To account for the increased activity of the first extract at pH 8, several possibilities suggested themselves; among others, that the active agent in the filtrate may be in combination with substances that restrain its activity to some extent, and that in the process of precipitation by the method described above, there is brought about dissociation between the restraining or protective agent and the tumor-producing element. If true, this would explain why the extract of the first precipitate is considerably stronger than the filtrate from which it was prepared.
To test for the presence of a protective agent, we examined the supernatant fluid obtained after precipitating at pH 4 the active agent from a 10% filtrate. After centrifuging, the supernatant fluid was decanted, neutralized and then concentrated in vacuo at room temperature to 1/2 its original volume. Two cubic centimeters of this concentrated supernatant fluid were now mixed with 0.5 cc. of the original filtrate, containing approximately 2 infective doses, and allowed to stand for 30-40 minutes prior to injection. Chicks, 10-14 days old, were used in all the experiments, for they have been found to give more uniform results. Of 80 chickens inoculated with this mixture, 75% failed to develop tumors, while tumors developed in all the controls which had received this amount of filtrate alone.
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