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ORIGINAL RESEARCH COMMUNICATION |
1 From the Institute of Human Nutrition, University of Southampton, Southampton General Hospital, Southampton, United Kingdom (AAJ, NRG, and YL), and the Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston (NRG and FJ).
Background: The finding that plasma glutathione turnover decreases as dietary protein intake decreases suggests that the safe amount of dietary protein, although sufficient for maintenance of nitrogen balance, may be insufficient for maintenance of cellular glutathione.
Objective: Our objective was to determine the effect of the safe protein intake on the erythrocyte glutathione synthesis rate and its relation with urinary 5-L-oxoproline excretion.
Design: Erythrocyte glutathione synthesis and urinary 5-L-oxoproline excretion were measured in young adults (6 men and 6 women) by using an infusion of [13C2]glycine on 3 occasions: initially during the subjects' habitual protein intake (1.13 g · kg–1 · d–1) and on days 3 and 10 of consumption of a diet providing the safe protein intake (0.75 g · kg–1 · d–1).
Results: Compared with baseline values, the fractional synthesis rate of erythrocyte glutathione was significantly lower (P < 0.05) on days 3 and 10 of the diet with the safe protein intake. Urinary 5-L-oxoproline excretion increased significantly (P < 0.05) above baseline by the third day of the diet with the safe protein intake and remained elevated. Erythrocyte glutathione concentrations and absolute synthesis rates decreased by day 3 but recovered to baseline values by day 10. Erythrocyte concentrations of cysteine, methionine, and serine remained unchanged, whereas erythrocyte concentrations of glycine, glutamic acid, and glutamine increased significantly by day 10.
Conclusion: During adaptation to the safe amount of dietary protein, there are changes in the concentration and kinetics of erythrocyte glutathione that suggest a reduced antioxidant capacity and possible increased susceptibility to oxidant stress.
Key Words: Glutathione • glycine • safe amount of dietary protein • amino acids • nitrogen balance • 5-L-oxoproline
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