黃嘌呤氧化? ( xanthine oxidase；XO ) 為一種含鉬酵素，在哺乳動物乳汁中，其主要係存於乳脂肪球膜 ( milk fat globule membrane；MFGM ) 上，並可能因乳品在加工時之許多程序而改變乳中黃嘌呤氧化?之活性。本研究旨在探討生牛乳經熱處理、均質化 ( homogenization ) 與乳脂肪含量對牛乳中黃嘌呤氧化?活性之影響。藉硫酸銨分離法純化出牛乳之黃嘌呤氧化?，再由光電比色儀偵測單位時間內黃嘌呤轉換為尿酸之氧化速率，以代表黃嘌呤氧化?之活性。結果顯示，未均質而經HTST和LTLT熱處理後之牛乳與生乳中黃嘌呤氧化?活性未有顯著差異；而經均質後之牛乳，不論何種熱處理其黃嘌呤氧化?活性較未均質之牛乳高；此外，當乳脂含量愈高時，其黃嘌呤氧化?活性亦愈高。總體而言，因黃嘌呤氧化?主要存於乳脂肪球膜中，故乳脂肪含量及當牛乳經均質後增加乳脂肪球表面積時，均會對其黃嘌呤氧化?之活性造成影響。 Xanthine oxidase (XO) is one member of the molybdenum family. In mammal milk, the XO exists in a milk fat globule membrane (MFGM). In milk processing, many process steps change the status of the MFGM and can affect to the XO activity of bovine milk. This study aims to investigate the effects of heat treatment, homogenization and milk fat content of bovine milk on the XO. The XO was purified from bovine milk by ammonium sulphate fractionation and the activity of XO was determined by measuring the rate of oxidation from xanthine to uric acid with UV-spectrophotometer. The results show that the XO activity of HTST milk, LTLT milk and raw milk were not significantly changed. The XO activity in homogenized milk was higher than that in unhomogenized milk no matter what kinds of heat treatments. Furthermore, when the milk fat content has been increasing in the fresh milk, it would also increase the XO activity to increase. In conclusion, XO exists in the MFGM, the homogenization and different fat content of the bovine milk can change the XO activity.
