phils, platelets, and lymphocytes, respectively (Evans et al., 1982; Jacob et al., 1992; Levine et al., 1996a). Metabolic priority for maintenance of intracellular lymphocyte ascorbate levels was demonstrated by its lower depletion rates compared to plasma and semen ascorbate levels during controlled vitamin C deficiency (intake of 5 mg/day) and faster recovery during vitamin repletion at 60 mg/day (Jacob et al., 1992). Intracellular ascorbate recycling (the intracellular regeneration of oxidized extracellular ascorbate) provides a cellular reservoir of reducing capacity (electrons) that can be transmitted both into and across the cell membrane (May et al., 1999).
The high intracellular concentration of ascorbate in leukocytes provides cellular protection against oxidant damage associated with the respiratory burst. In isolated neutrophils, ascorbate recycling is increased up to thirtyfold upon exposure of the cells to microbial pathogens (Wang et al., 1997b). Ascorbate effectively neutralizes phagocyte-derived oxidants without inhibiting the bactericidal activity of the phagosome (Anderson and Lukey, 1987). Evidence that ascorbate modulates leukocyte phagocytic action, blastogenesis, immunoglobulin production, chemotaxis, and adhesiveness has been reported in vitro, although evidence for the latter two functions has been mixed (Evans et al., 1982; Jariwalla and Harakeh, 1996).
Concentrations of ascorbate normally found in plasma (22 to 85 µmol/L [0.4 to 1.7 mg/dL]) were shown to neutralize hypochlorous acid (HOCl), one of many powerful oxidants generated by myeloperoxidase in activated neutrophils and monocytes (Halliwell et al., 1987; Heinecke, 1997). This action was hypothesized to protect α-1-antiprotease against inactivation by HOCl and thereby prevent proteolytic damage at inflamed sites such as the rheumatoid joint (Halliwell et al., 1987). Indeed, the ratio of oxidized to reduced ascorbate was found to be increased in the knee synovial fluid of active rheumatoid arthritis patients, which suggests that ascorbate is acting to scavenge phagocyte-derived oxidants in this locally inflamed area (Lunec and Blake, 1985). Similarly, increased ascorbate oxidation in the plasma of patients with adult respiratory distress syndrome (Cross et al., 1990) and in smokers (Lykkesfeldt et al., 1997) indicates protection against oxidant damage from activated neutrophils and other sources in the lung. Exposure of nine apparently healthy adults to 2,000 parts per billion (ppb) of ozone, an environmental pollutant, for 2 hours resulted in increased myeloperoxidase and decreased ascorbate concentrations in bronchoalveolar lavage fluid. These results imply that ascorbate protects against inflammatory oxidative stress induced by ozone (Mudway et al., 1999).