Effect of haptoglobin on the metabolism of vitamin C.

Haptoglobin is a hemoglobin-binding antioxidant showing a genetic polymorphism with three types: Hp 1-1, Hp 2-1, and Hp 2-2. The Hp 2-2 type has been associated with an increased risk of atherosclerosis. We investigated vitamin C metabolism in vivo and in vitro according to haptoglobin type in a study group of 135 healthy volunteers. Serum vitamin C concentrations were associated with haptoglobin type, showing lowest values in serum from Hp 2-2 subjects (P < 0.01). Renal threshold for L-ascorbic acid was within the normal range and metabolization to oxalate was not different among haptoglobin-type groups. Serum concentrations of other endogenous antioxidants (uric acid, bilirubin, albumin, ceruloplasmin, and total antioxidative status) were not different among haptoglobin-type groups. In vitro experiments showed a lower stability of L-ascorbic acid in blood from subjects with the Hp 2-2 type (P < 0.01). L-Ascorbic acid depletion in vitro was inversely related to haptoglobin concentration (r = -0.738). The results of this study indicate a higher rate of L-ascorbic acid oxidation in Hp 2-2 carriers because they have less protection against hemoglobin-iron driven peroxidation.