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Nutrient Metabolism

Uptake, Hydrolysis, and Metabolism of Pyridoxine-5'-ß-D-Glucoside in Caco-2 Cells^1,2

Food Science and Human Nutrition Department, University of Florida, Gainesville, FL

³To whom correspondence should be addressed. E-mail: jfgy{at}ufl.edu.

An important dietary source of vitamin B-6, pyridoxine-5'-ß-D-glucoside (PNG), exhibits only partial bioavailability, which is limited by the extent of enzymatic cleavage of the ß-glucosidic bond to release metabolically available pyridoxine (PN). This laboratory showed that the intestinal hydrolysis of PNG is catalyzed by cytosolic PNG hydrolase (PNGH) and brush border lactase-phlorizin hydrolase (LPH). LPH-catalyzed PNG hydrolysis in vitro is competitively inhibited by lactose. In the present study, the uptake and hydrolysis of PNG were examined in Caco-2 human colon carcinoma cells, which express a functional LPH but exhibit no PNGH activity. PNG uptake at 37°C was linear over 5–500 µmol/L PNG. Uptake was not significantly reduced when Na⁺ was substituted with K⁺, Li⁺, or Tris in the medium. Increasing PNG concentration in the medium did not change intracellular concentrations of PN, pyridoxamine (PM), pyridoxamine 5'-phosphate (PMP), or pyridoxal 5'-phosphate (PLP); however, intracellular pyridoxal (PL) concentration increased. Intracellular PNG concentration was not significantly reduced in the presence of lactose, but the concentration of PL declined in proportion to extracellular lactose (P = 0.01). These results indicate that PNG can be absorbed intact in a Na⁺-independent process and is taken up by passive diffusion. The presence of lactose in this in vitro model of intestinal uptake reduced the enzymatic hydrolysis of PNG by lactase.

KEY WORDS: • pyridoxine-5'-ß-D-glucoside (PNG) • Caco-2 cells • bioavailability • lactase-phlorizin hydrolase