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Fig. 1 | Nutrition Journal

Fig. 1

From: A simple method for plasma total vitamin C analysis suitable for routine clinical laboratory use

Fig. 1

Vitamin C metabolism. Ascorbic acid participates both in acid-base (ascorbic acid-ascorbate, pK 4.2) and oxidoreduction reactions (ascorbate-dehydroascorbic acid). The latter reaction involves transient creation of the ascorbyl radical (not shown). Vitamin C (or total vitamin C) refers to the sum of ascorbic acid, ascorbate and dehydroascorbic acid. Under the strongly reducing, neutral pH conditions in the bloodstream, almost all vitamin C circulates as ascorbate. As soon as the blood cells are removed from plasma, ascorbate progressively oxidizes to dehydroascorbic acid, which is highly prone to irreversible hydrolysis to 2,3-diketogulonic acid. Diketogulonic acid is oxidized to smaller molecules, including oxalate, which are excreted in the urine. In order to minimize loss of vitamin C, freshly obtained plasma must immediately be acid-deproteinized and flash frozen, and strongly acidic conditions maintained as much as possible throughout sample storage and processing

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