Isolation and Partial Characterization of Xanthine Oxidase Associated with the Milk Fat Globule Membrane of Cows’ Milk

Abstract

Xanthine oxidase was isolated directly from fat globule membranes by freeing the bound enzyme with sodium deoxycholate, obviating the use of proteolytic enzymes in the purification procedure. The isolated enzyme possessed a Michaelis constant of 1.29±.08×10−5 M and a maximum initial velocity of 3.62±.07μmoles/mg/min. A molecular weight of ∼153,000 was determined by electrophoresis in sodium dodecyl sulfate-containing polyacrylamide gels. Upon storage at 4C for 30 days or 37C for 24h, the molecule degraded into smaller molecular units. The breakdown appears to be independent of microbial load and temperature-dependent association-dissociation phenomena. A membrane-associated protease endogenous to the milk system was implicated as the degrading agent.