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The Mechanism of Dietary Alterations in Rat Hepatic Xanthine Oxidase Levels

From the ¹ From Duke University, Durham, North Carolina and the University of Pennsylvania, Philadelphia, Pennsylvania 19104

Dietary protein depletion results in a decrease in rat hepatic xanthine oxidase activity to about 10% of control levels. Refeeding of a high protein diet leads, after a 6-hour lag phase, to a 5-fold increase in enzyme activity by 12 hours. The mechanism of this increase was studied utilizing actinomycin D, 5-fluorouracil, and puromycin. These agents all blocked the increase in enzyme activity, indicating that both ribonucleic acid and protein syntheses were involved. Following administration of ¹⁴C-leucine, specific radioactivities of xanthine oxidase were identical in the livers of protein-depleted and control animals, despite a 10-fold difference in enzyme activity levels. These results indicated that the fractional turnover rates were the same in the two groups of rats and suggested that the rate of xanthine oxidase synthesis was reduced 10-fold in the depleted rats prior to protein re-feeding. Greatly increased isotopic labeling of highly purified enzyme with ¹⁴C-leucine during the phase of restoration of enzyme levels indicated that the increase in activity was the result of accelerated apoenzyme synthesis de novo.

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