RESUMEN
Milk xanthine oxidase oxidizes xanthine at pH 9.6 and reduces nitrates at pH 5.2. It is shown that the nitrate reductase activity requires molybdenum and sulfur-containing sites in the enzyme, whereas oxidation of xanthine also requires iron-containing sites and FAD. As the pH changes from 5.2 to 9.6, the conformation of the enzyme molecule is modified as demonstrated by changes in the absorption, fluorescence, and circular dichroism spectra. When the enzyme is treated with dithioerythritol, it may pass from the oxidase to the dehydrogenase form with a marked increase in the nitrate reductase activity.
Asunto(s)
Leche/enzimología , Nitrato Reductasas/metabolismo , Xantina Oxidasa/metabolismo , Animales , Cloruro de Calcio/farmacología , Bovinos , Cloromercuribenzoatos/farmacología , Ditioeritritol/farmacología , Flavina-Adenina Dinucleótido/metabolismo , Calor , Concentración de Iones de Hidrógeno , Conformación Proteica , Ácido p-CloromercuribenzoicoRESUMEN
Milk xanthine oxidase possesses the nitrate reductase activity at pH 5.2; the pH optimum of the xanthine oxidase activity for the enzyme lies at 9.6. After removal of FAD and binding of Mo and Fe with a simultaneous measurement at the pH optima of the above activities it was found that only the Mo-containing site is necessary for the nitrate reductase activity. The switch-over of the enzyme from the xanthine oxidase to the nitrate reductase activity is associated with considerable conformational changes of the enzyme molecule.