Transport and metabolism of ascorbic acid in human placenta

Abstract

The role of human placenta in cellular transport and metabolism of the potentially toxic oxidized form and the useful reduced form of ascorbic acid was examined in surviving tissue fragments in vitro. At the end of a 60-min incubation with the 14C label nominally present in the reduced form, a tissue-to-medium ratio in excess of unity was reached. The importance of evaluating uptake of the ascorbic acid metabolites is evident from a careful assay of 14C label present in the bathing media. Significant spontaneous oxidation occurs, which is slowed or reversed to a limited extent by the presence of placental tissue. Uptake of the oxidized substrate, dehydro-L-ascorbic acid, proceeds much more rapidly than uptake of ascorbic acid. At the end of a 15-min incubation, most of the substrate taken up was in the reduced form. From an additional evaluation of 14C label in the bath it is calculated that 25% of ascorbic acid formed by the tissue is released within 15 min. The cellular uptake mechanism for dehydro-L-ascorbic acid is not shared by glucose and is not dependent on the presence of Na+ but is dependent on intact cellular metabolism. The finding of avid cellular uptake and reduction of the oxidized form of ascorbic acid supports the concept that the placenta helps to clear the toxic molecule from the maternal circulation, metabolizes it, and delivers the useful reduced form to the fetus.