Chemistry Reference
In-Depth Information
(Thurnham, 2000). The upper limit of plasma ascorbic acid concentration is
controlled by the gastrointestinal absorption and renal reabsorption mechan-
isms, and fasting plasma concentration rarely exceeds 100 mmol/l, even with
dietary supplementation (Graumlich et al., 1997). However, following a large
(
>
500 mg) oral dose, plasma level can increase several fold, and may
approach 200 mmol/l (Benzie and Strain, 1997).
Vitamin C concentration varies widely in different blood cell types.
About 70% of blood-borne ascorbate is in plasma and erythrocytes (Bender,
1999). The remainder is in white cells, which have a marked ability to
concentrate ascorbate; mononuclear leucocytes achieve 80-fold concentra-
tion, platelets 40-fold and granulocytes 25-fold, compared with plasma con-
centration. Many tissues also appear to have an increased need to conserve
ascorbate (Bates and Heseker, 1994). Tissue-specific cellular mechanisms of
transport and metabolism allow for wide variation of tissue ascorbate con-
centration in order to enhance its function as an enzyme cofactor and anti-
oxidant (Wilson, 2005). Specific proteins mediate the entry and exit of vita-
min C in cells by facilitated diffusion or active transport. These cellular
transport systems are responsible for high inter-tissue ascorbate levels
found in the pituitary and adrenal glands (30-400 mg/100 g tissue), followed
by the brain, spleen, pancreas, kidney, liver and in the tissues of the eye with
between 10 and 50 mg/100 g of tissue. Intracellularly, and in plasma, vitamin
C exists predominantly in the free form as AscH
-
. DHA is either not detect-
able or found only at very low levels in the circulation of healthy people
(Rumsey and Levine, 1998).
In all species studied, ascorbate is filtered at the glomerulus and is
actively reabsorbed at the proximal tubules, in a concentration-dependent
manner, by an ascorbate transport protein. When the transport protein is
saturated, the remaining vitamin C is not transported, but excreted in the
urine. Ascorbate is excreted in the urine at an ingested dose above 100 mg/
day (i.e. at plasma concentrations above
60 mmol/l, with plasma saturated
at 70% and white blood cells saturated at 100%). At 100 mg/day, approxi-
mately 25% of AA is excreted, while above 500 mg/day almost all AA is
excreted (Levine et al., 1999).
12.6.3.
Antioxidant Activity of Ascorbic Acid
Ascorbate is often called the outstanding antioxidant. In chemical
terms, this is simply a reflection of its redox properties as a reducing agent.
Ascorbate is a reversible biological reductant and as such provides reducing
equivalents for a variety of biochemical reactions and is considered the most
important antioxidant in extracellular fluids (Sies et al., 1992). Ascorbate is
thermodynamically close to the bottom of the list of one-electron reducing
