Chemiluminescent determination of xanthine oxidase activity in milk

A chemiluminescent method for determining xanthine oxidase (XOD) activity w as developed and applied to the assay of milk enzyme activity using a photo multiplier luminometer. Various kinds of milk and cream samples were analys ed for XOD content. In pasteurized milk, XOD activity depended on the fat c ontent and in UHT milk it disappeared owing to the heat treatment. Milk sam ple preparation was very simple, requiring only homogenization at 40 degree s C followed by a 1 :10 dilution with UHT ('XOD-free') milk. The assay was carried out at 25 degrees C. The response obtained from XOD standard soluti ons in milk was linear from 0.1, to 500 enzyme units (U) l(-1), but for the actual milk samples values ranged only from 1 to 135 U l(-1). The detectio n limit at 2 so was 0.1 U l(-1) in milk, while in buffer it was 100 times l ower. The intra-assay and interassay CV for XOD activity in milk were 6-12% .