Search

Sulfite oxidase catalyzes the oxidation of sulfite to sulfate. To investigate whether or not sulfite oxidase activity (EC 1.8.3.1) is regulated by the amount of sulfur from dietary protein or excess methionine, we fed rats diets containing 5, 10, 20 and 50% casein with or without excess methionine and measured sulfite oxidase activity in liver and intestinal mucosa. Hepatic sulfite oxidase activity was significantly lower in rats fed 5 or 10% casein diets and significantly higher in rats fed 50% casein than in rats fed the control diet containing 20% casein, but activity did not change in response to the addition of methionine at any level of protein. Sulfite oxidase activity in the intestinal mucosa was only 5% of that seen in liver and did not change in response to dietary protein or methionine. Activity did not change in rats fed low iron diets (5 mg Fe/kg diet) at any level of protein tested or in response to glycine. These results show that sulfite oxidase activity can adapt to different levels of dietary protein but is unaffected by the level of methionine, total amino nitrogen or iron in the diet.

Rat liver xanthine oxidase activity is regulated in response to dietary protein and iron. To investigate whether the change in activity was mediated by a change in the rate of protein degradation, we measured the fractional turnover rate using the double-isotope technique with [3H]- and [14C]leucine and calculated the apparent half-life of xanthine oxidase in rats fed diets containing either 20 or 5% casein with either 35 or 5 mg iron/kg diet. Under control conditions, xanthine oxidase had an apparent half-life of 4.8 d and approximately 65% of the enzyme subunits were active. Rats fed diets with low dietary protein had lower xanthine oxidase activity, but the enzyme had a slower fractional turnover rate, resulting in an apparent half-life of 6.4 d, and only 15–20% of the enzyme was active. The apparent half-life of xanthine oxidase increased to 7.5 d in rats fed diets with low dietary iron, but dietary iron did not affect the specific activity of the enzyme or the percentage of active subunits. These results suggest that the loss of enzyme activity is not due to loss of enzyme protein by increased degradation, but rather to inactivation of the enzyme.

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.

We tested the hypothesis proposed by Topham, Woodruff and Walker that intestinal xanthine oxidase is important for iron absorption. We made weanling rats xanthine oxidase-deficient and measured their growth and iron status. There were no significant differences between control and xanthine oxidase-depleted rats in growth or iron absorption or a variety of measures of iron metabolism, except that xanthine oxidase-depleted rats accumulated nonheme iron in the liver. Iron deficiency caused a loss in intestinal xanthine oxidase activity, but also caused an increase in hepatic xanthine oxidase activity. This result may be important for understanding changes in purine and protein metabolism during iron deficiency.