1. By means of certain chemical reagents, normal guinea pig serum can be brought to autodigestion without the presence of any foreign substrate. There exists in normal sera a highly characteristic protease.

2. The serum ferment survives heating at 55°C. for 30 minutes, but is completely inactivated at 60°C. for the same length of time.

3. The autodigestion of serum requires a temperature of about 37°C., and no noticeable digestion takes place at a temperature of 16°C. or lower.

4. Autodigestion of the serum may be brought about by chloroform and various saturated monovalent ketones and alcohols of the lower series.

5. The ketones and alcohols have a certain narrow limit of concentration for activating serum, beyond which the ferment is destroyed, even at room temperature.

6. The ketones and alcohols in concentrations regulated to activate serum at room temperature destroy the ferment when allowed to act on serum at 37°C. for 30 minutes. The elimination of the concentrated reagents from serum by evaporation or dialysis protects the ferment from their destructive action.

7. A certain length of time is required for the chemical activators to complete their action. In this respect chloroform is much slower than acetone.

8. The chemical activators may be removed from the activated serum by means of vacuum, dialysis, or extraction with certain indifferent chemicals without causing a return of the serum to its original non-autolytic state. Once activated by these reagents, the serum remains in the activated state, in spite of the removal of the activators.

9. The ferment is highly sensitive to the reaction of the medium, being readily inactivated when the reaction exceeds a certain narrow limit towards acid or alkaline. The optimal digestion is obtained with a faintly alkaline or neutral reaction.

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