The chemistry of xanthine oxidase. 11. Ultracentrifuge and gel-filtration studies on the milk enzyme

Abstract
The molecular weight of xanthine oxidase from cow''s milk has been determined as 274000[plus or minus]30000 from sedimentation and diffusion measurements, 265000[plus or minus]22000 by the Archibald method and 286000[plus or minus]35000 by gel filtration. Between pH 3-5 and 9-8 there were no indications of dissociation into sub-units. At pH 1-9 and also at pH 11-7 and above, breakdown of the enzyme into smaller units was observed. Dissociation was not brought about by calcium chloride, urea, or urea plus thioglycollate. The nature of contaminants present in small amounts in some preparations and detected in the ultracentrifuge or by gel filtration is discussed briefly.