Regulation of xanthine oxidase activity and immunologically detectable protein in rats in response to dietary protein and iron

To investigate the mechanism for changes in xanthine oxidase activity in response to dietary protein and iron, we fed rats diets containing 50, 20 or 5% casein with either normal iron (35 mg Fe/kg diet) or low iron (5 mg Fe/kg diet). Xanthine oxidase activity changed in liver and intestinal mucosa in response to protein and iron, but immunologically detectable xanthine oxidase protein did not change. When total liver RNA isolated from these rats was translated by a rabbit reticulocyte lysate, we found no difference in the amount of xanthine oxidase that was translated. These results demonstrated that the changes in xanthine oxidase activity were not accompanied by changes in the amount of protein. Since xanthine oxidase can exist in an inactive desulfo form, we asked if xanthine oxidase activity was changed by the content of sulfur-containing amino acids in the diet. Xanthine oxidase activity in intestinal mucosa of the rats fed the 5% casein + methionine diet was significantly greater than that of the rats fed the 5% casein diet alone. These findings suggest that xanthine oxidase activity may be regulated by interconversion of active and inactive desulfo enzyme.