The Milk-clotting Action of Papain*

Although the milk-clotting action of papain has been recognized for many years, this property of the enzyme has received little quantitative attention. It appears to be, however, a characteristic property of the proteolytic component of this enzyme system. Various workers have brought forth evidence as to the composite nature of the papain system, which has at least two components. In the most recent work Bergmann and coworkers (l-3) have tentatively designated them as Papain Peptidases I and II. The first one is characterized as splitting benzoylisoglutamine and hippurylamide, and is completely inhibited by phenylhydrazine and other carbonyl reagents. The second splits peptone ex a&mine and gelatin, and is activated by phenylhydrazine. Controlled oxidation with iodine inactivates both components; subsequent reduction with HCN restores only the activity of Peptidase II.