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Effect of Dietary Protein and Iron on the Fractional Turnover Rate of Rat Liver Xanthine Oxidase^1,2,

Department of Nutritional Sciences, University of California, Berkeley, CA 94720

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.

KEY WORDS: • xanthine oxidase • enzyme turnover • dietary protein • dietary iron

¹ Presented in part at the 70th Annual Meeting of the Federation of American Societies for Experimental Biology, St. Louis, MO, April 1986 [Cherry, D. M. & Amy, N. K. [1986] Effect of dietary iron on the turnover rate of rat liver xanthine oxidase. Fed. Proc. 45: 1080 (abs.)].

² This research has been supported by the California Agricultural Experiment Station.

³ To whom correspondence should be sent.

Manuscript received 24 April 1987. Revision accepted 12 August 1987.

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