Fat globule membrane material has been separated into two discrete fractions, particulate fraction BM1 and soluble fraction BM2, each containing membrane-bound xanthine oxidase. Substrate and electron acceptor specificities of free and membrane-bound xanthine oxidase have been compared. The ratio of xanthine oxidase activity to NADH oxidase activity (X/N) was found to be a convenient parameter for distinguishing membrane-bound xanthine oxidase. Membrane-bound xanthine oxidase has a lower Km for NADH and a higher Km for xanthine as compared with the free enzyme. Titration of free xanthine oxidase with fraction BM1, heated to remove associated enzyme activities, resulted in a progressive decrease in the X/N value to that characteristic of the membrane-bound enzyme. The binding parameters of this association were determined using the alteration in the X/N value as an indicator of enzyme binding. Xanthine oxidase represents 8% of the total membrane protein of fraction BM1. Most of this activity is firmly bound to the membrane. Less than 25% of the xanthine oxidase activity of fraction BM1 was solubilised by washing with buffer, and only about half of the total activity was solubilised by Triton x-100. The buffer- or Triton-solubilised enzyme is still membrane-bound and resembles fraction BM2. Free xanthine oxidase could only be liberated from the membrane by tryptic digestion. The effect of temperature, pH and solvent polarity on the X/N value of the free enzyme were investigated to elucidate the nature of the enzyme-membrane interaction. A model is proposed for the association of xanthine oxidase with the fat globule membrane.
|Date of Award||1973|