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Diurnal variation in glutathione and cysteine redox states in human plasma^1,2,3

¹ From the Division of Pulmonary, Allergy and Critical Care Medicine (YP and DPJ); the Division of Endocrinology, Metabolism and Lipids (RAB, TRZ, and CJA); and the Clinical Biomarkers Laboratory (P-YC, YP, CJA, and DRJ), Department of Medicine, Emory University, Atlanta, GA; the Graduate Program in Nutrition Health Sciences, Emory University, Atlanta, GA (BAC); and the Department of Biostatistics, Emory University, Atlanta, GA (GAC)

Background: Plasma glutathione/glutathione disulfide (GSH/GSSG) and cysteine/cystine (Cys/CySS) couples are oxidized in humans in association with oxidative stress and cardiovascular disease risk. Animal studies show that both pools undergo diurnal variations associated with dietary intake of sulfur amino acids.

Objective: The objective of this study was to determine whether the redox state of GSH, Cys, GSH/GSSG, or Cys/CySS undergoes diurnal variation in healthy adults.

Design: Plasma samples were collected every hour for 24 h from 63 persons aged 18–86 y who were consuming normal food (protein, 0.8 g · kg^–1 · d^–1; sulfur amino acids, 20 mg·kg^–1·d^–1) at standardized mealtimes. Measurements of Cys, CySS, GSH, and GSSG were used with the Nernst equation to calculate the redox states.

Results: Plasma Cys and GSH concentrations varied with the time of day. The highest values for plasma Cys occurred 3 h after meals. Glutathione was maximal 6 h after peak plasma Cys. The calculated redox states of the GSH/GSSG and Cys/CySS couples varied in association with the concentrations of the thiol forms. Maximal reduction and oxidation of the Cys/CySS couple occurred at 2130 and 0630, whereas the respective values for the GSH/GSSG couple occurred at 0330 and 1330. The mean diurnal variation for Cys/CySS redox in persons aged 60 y was 1.8-fold that in persons aged <40 y.

Conclusions: Cys/CySS and GSH/GSSG redox states in human plasma undergo diurnal variation with an increased magnitude of variation in Cys/CySS redox state in older persons. This variation could alter sensitivity to oxidative stress over a course of hours.

Key Words: Amino acids • oxidative stress • aging • sex-dependent differences • thiol

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