On the Further Purification of the Xanthine Oxidase.

SINCE the discovery of the xanthine oxidase of milk by Morgan, Stewart and Hopkins [1922] two main attempts have been made to purify this enzyme further; namely, by Dixon and Thurlow [1924, 1], and later by Sbarsky and Michlin [1925]. Dixon and Thurlow obtained an active and stable oxidase preparation by precipitating the enzyme, together with the caseinogen and fat, by halfsaturation of the milk with ammonium sulphate. The fat was removed from the dried precipitate by thorough extraction with ether. The resulting preparation is soluble, contains the whole amount of the enzyme, and is free from other milk enzymes such as peroxidase and catalase. Since the caseinogen content of milk is about 3 % it will be clear that this caseinogen preparation is about thirty times as concentrated as milk, i.e. 1 g. of the solid preparation contains as much enzyme as 30 g. of milk". Sbarsky and Michlin's method of preparation is essentially similar. They use acetone instead of ammonium sulphate to precipitate the protein, and light petroleum instead of ether to extract the fat. The protein is, however, precipitated in an insoluble form by the acetone, and it is necessary to digest the preparation with dilute acid in order to get the enzyme into solution. The