Redox Properties of Human Erythrocytes Are Adapted for Vitamin C Recycling

Abstract

Ascorbic acid (AA; or vitamin C) is an important physiological antioxidant and radical scavenger. Some mammalian species, including homo sapiens, have lost the ability to synthetize AA and depend on its nutritional uptake. Erythrocytes from AA-auxotroph mammals express high amounts of the glucose transporter GLUT1. This isoform enables rapid uptake of glucose as well as dehydroascorbate (DHA), the fully oxidized form of AA. Here, we explored the effects of DHA uptake on the redox metabolism of human erythrocytes. DHA uptake enhanced plasma membrane electron transport (PMET) activity. This process is mediated by DCytb, a membrane bound cytochrome catalyzing extracellular reduction of Fe3+ and ascorbate free radical (AFR), the first oxidized form of AA. DHA uptake also decreased cellular radical oxygen species (ROS) levels. Both effects were massively enhanced in the presence of physiological glucose concentrations. Reduction of DHA to AA largely depleted intracellular glutathione (GSH) and induced the efflux of its oxidized form, GSSG. GSSG efflux could be inhibited by MK-571 (IC50 = 5 μM), indicating involvement of multidrug resistance associated protein (MRP1/4). DHA-dependent GSH depletion and GSSG efflux were completely rescued in the presence of 5 mM glucose and, partially, by 2-deoxy-glucose (2-DG), respectively. These findings indicate that human erythrocytes are physiologically adapted to recycle AA both intracellularly via GLUT1-mediated DHA uptake and reduction and extracellularly via DCytb-mediated AFR reduction. We discuss the possibility that this improved erythrocyte-mediated AA recycling was a prerequisite for the emergence of AA auxotrophy which independently occurred at least twice during mammalian evolution.

Supplementation with uric and ascorbic acid protects stored red blood cells through enhancement of non-enzymatic antioxidant activity and metabolic rewiring

“…DHA is an unstable molecule that can be rapidly degraded irreversibly both inside and outside cells [ 56 ], a conversion that is avoided through the activity of reducing agents such as GSH [ 57 ], glutathione S-transferases [ 58 ] and the duodenal isoform of cytochrome b 561 [ 59 ]. Interestingly, a recent study reported a model regarding the adaptation of RBCs to carry out recycling of intracellular and extracellular AA [ 60 ].…”

ZOOMICS: Comparative Metabolomics of Red Blood Cells From Guinea Pigs, Humans, and Non-human Primates During Refrigerated Storage for Up to 42 Days

“…This concept will need to be evaluated in tracing studies with isotope-labeled ascorbate. Alternatively, interspecies divergencies in this pathway may be explained, at least in part, by the evolution of a transport and extracellular reduction system for ascorbate in primates, but not in guinea pigs; this could depend on GLUT1 and Cytochrome B, a membrane-bound cytochrome catalyzing extracellular reduction of Fe 3+ and ascorbate free radical, the first oxidized form of ascorbic acid ( Eigenschink et al, 2021 ). Direct comparison to other rodents is currently missing, though severe storage hemolysis was reported in mice and rats after 14 ( Zimring et al, 2014 ) and 21 days ( Williams et al, 2019 ), respectively, which would preclude examining the 42 day storage period tested here.…”

Erythrocyte metabolism

“…In panel A, a minimal conceptual model showing the relationship between glycolysis and glutathione concentration is presented. In panels B1 and B2, experimental data from two independent studies are presented 362,363 . In panel B1, higher glucose levels preserve glutathione concentration after an oxidative insult (2 mM dehydroascorbic acid).…”