Transport measurements across Caco-2 monolayers of different organic and inorganic selenium - Influence of sulfur compounds

The transport and uptake of the most common Se compounds, selenate (Seo(4)(2-)), selenite (Seo(3)(2-)), selenomethionine, and selenocystine, were investigated using confluent monolayers of Caco-2 cells, a human carcinoma cell line. Comparative measurements were performed in the absorptive (apical to basolateral side) and exsorptive (basolateral to apical side) directions. Apparent permeability coefficients (P-app), calculated from transport experiments in the absorptive direction, showed increasing values in the following rank order: about 1 x 10(-6) cm/s less than or equal to mannitol less than or equal to SeO32- selenocystine < selenomethionine < SeO42- less than or equal to about 16 x 10(-6) cm/s. The ratios of the P-app measured in the absorptive versus exsorptive directions indicated that only the organic forms presented a net polarized transport (P-app ratio >> 1), suggesting the presence of a transcellular pathway. No significant excretion was observed. The transport of selenomethionine was inhibited by its sulfur analog, methionine, suggesting a common transport mechanism. In contrast, an inhibition of the transport of selenocystine by cysteine was not observed. From the two substrates tested, sulfate and thiosulfate, only thiosulfate inhibited the transport of SeO42-. This effect was also observed for SeO32- (i.e., was unspecific), which questioned the assertion of a common transport for sulfate and SeO42- and may confirm the paracellular pathway of SeO42- suggested by the P-app ratio of about 1. The addition of glutathione (GSH) in large excess had no consequence on the passage of SeO32- but strongly increased the uptake (about fourfold). The liquid chromatography - mass spectrometry (LC-MS) data show ed that, in the ionic condition of incubation medium, GSH promptly reduced SeO32- (less than or equal to 2 min) in its elemental form Se-0, which carrot ascribe to selenodiglutathione a direct role in the effect of GSH.