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外文期刊>The biochemical journal
>Association of xanthine oxidase with the bovine milk-fat-globule membrane. Catalytic properties of the free and membrane-bound enzyme
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Association of xanthine oxidase with the bovine milk-fat-globule membrane. Catalytic properties of the free and membrane-bound enzyme
p1. The catalytic properties of xanthine oxidase in bovine milk (EC 1.2.3.2) are dependent on the state of the enzyme, i.e. whether free or bound to the fat-globule membrane. Oxidase activity of the membrane-bound enzyme towards NADH is enhanced relative to that towards xanthine. This reflects a change in the relative iKsubm/sub/i values and enables the ratio of xanthine to NADH oxidase activities (X/N) to be used as a parameter for the relative amounts of free and membrane-bound xanthine oxidase in milk fractions. 2. Chromatography of buttermilk on Sepharose 2B yielded an excluded fraction, BMsub1/sub, with xanthine oxidase activity. The remaining xanthine oxidase activity was eluted as a single broad peak. This was further resolved on Sephadex G-200 into an excluded fraction, BMsub2/sub, and free xanthine oxidase. Fractions BMsub1/sub and BMsub2/sub had X/N values in the range 45–65, which is characteristic of membrane-bound xanthine oxidase. Purified xanthine oxidase has a mean X/N value of 110.3. Addition of fraction BMsub1/sub, heated to remove associated enzyme activities, to purified xanthine oxidase progressively enhanced its NADH oxidase activity to a value where its X/N value was characteristic of membrane-bound xanthine oxidase. This was shown to be due to binding of free enzyme to heated fraction BMsub1/sub. The binding constant and stoicheiometry were determined. 4. Proteolytic digestion of fraction BMsub1/sub liberated free xanthine oxidase from the fat-globule membrane with a corresponding alteration in X/N value./p
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