Metabolism and Excretion

As shown in Figure 1, oxidation of ascorbic acid proceeds by a one-electron process, forming mono-dehydroascorbate, which disproportionates to ascor-bate and dehydroascorbate. Most tissues also contain monodehydroascorbate reductase (EC 1.6.5.4), a flavoprotein that reduces the radical back to ascorbate. Dehydroascorbate is reduced to ascorbate by dehydroascorbate reductase (EC 1.8.5.1), a glutathione-dependent enzyme; little is oxidized to diketogulonic acid in human beings.

dehydroascorbate reductase

dehydroascorbate reductase

monodehydroascorbate

Figure 1 The metabolism of ascorbate. Monodehydroascorbate reductase, EC 1.6.5.4; dehydroascorbate reductase, EC 1.8.5.1.

COOH diketogulonate monodehydroascorbate

Figure 1 The metabolism of ascorbate. Monodehydroascorbate reductase, EC 1.6.5.4; dehydroascorbate reductase, EC 1.8.5.1.

Both ascorbate and dehydroascorbate are filtered at the glomerulus, then reabsorbed by facilitated diffusion. When glomerular filtration exceeds the capacity of the transport systems, at a plasma concentration of ascorbate above about 85 mmol/l, the vitamin is excreted in the urine in amounts proportional to intake.

It has been reported that approximately 25% of the dietary intake of ascorbate is excreted as oxalate; this would account for about 40% of the total urinary excretion of oxalate. However, there is no known metabolic pathway for the synthesis of oxalate from ascorbate, and it is likely that all or most of the oxalate found in urine after loading doses of ascor-bate is formed nonenzymically, after the urine has been collected. Even in people at risk of forming oxalate renal stones it is unlikely that normal or high intakes of ascorbate pose any additional hazard.

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