1. A method is described for the preparation of pepsinogen from swine gastric mucosae which consists of extraction and fractional precipitation with ammonium sulfate solutions followed by two precipitations with a copper hydroxide reagent under particular conditions. Crystallization as very thin needles takes place at 10°C., pH 5.0 and from 0.4 saturated ammonium sulfate solution containing 3–5 mg. protein nitrogen per milliliter.

2. Solubility measurements, fractional recrystallization, and fractionation experiments based on separation after partial heat or alkali denaturation and after partial reversal of heat or alkali denaturation failed to reveal the presence of any protein impurity.

3. The properties of the enzymatically inactive pepsinogen were studied and compared with the properties of crystalline pepsin. The properties of pepsinogen which are similar to those of pepsin are: molecular weight, absorption spectrum, tyrosine-tryptophane content, and elementary analysis. The properties in which they differ are: enzymatic activity, crystalline form, amino nitrogen, titration curve, pH stability range, specific optical rotation, isoelectric point, and the reversibility of heat or alkali denaturation.

4. Conversion of pepsinogen into pepsin at pH 4.6 was found to be autocatalytic; i.e., the pepsin formed catalyzes the reaction. Conversion of pepsinogen into pepsin is accompanied by the splitting off of a portion of the molecule containing 15–20 per cent of the pepsinogen nitrogen.

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