ASCORBATE FUNCTION AND METABOLISM IN THE HUMAN ERYTHROCYTE

James M. May

Departments of Medicine and Molecular Physiology and Biophysics, 736 Medical Research Building II, Vanderbilt University School of Medicine, Nashville, TN 37232-6303 USA

Received 11/5/97 Accepted 11/9/97

3. REDOX CHEMISTRY OF ASCORBIC ACID

Ascorbic acid, or ascorbate, is an antioxidant because of the high reducing potential of its carbon-carbon double bond (figure 1), which readily donates one or two hydrogens and electrons to a variety of oxidants, including oxygen free radicals, peroxides, and superoxide (3). Each step of ascorbate oxidation is reversible, and this permits recycling back to ascorbate. The partially oxidized form of ascorbate, termed the (mono)ascorbyl free radical or AFR (figure 1), may serve as an electron acceptor or donor. The AFR is surprisingly stable, and can easily be detected by EPR at concentrations as low as 10 nM in biological fluids (4-6). Loss of the second electron results in dehydroascorbic acid, which is not an acid. This contrasts with ascorbate, which carries a negative charge at physiologic pH (the pK_a of the carbon-3 hydroxyl is 4.2). DHA is quite unstable at physiologic pH and temperature, with a half-life of about 6 min (7, 8). With hydrolysis of the lactone ring, DHA is converted to 2,3-diketo-1-gulonic acid (figure 1) (1, 9). This last step is probably irreversible in cells, although it can be reversed by mercaptoethanol in vitro (10). Loss of ascorbate through decomposition of DHA is obviously wasteful, and cells such as the erythrocyte have redundant mechanisms to recycle DHA back to ascorbate.

Figure 1. Oxidative metabolism of ascorbic acid.