INTESTINAL ABSOPPTION OF VITAMIN B₆

I. PYRIDOXOL UPTAKE BY RAT INTESTINAL TISSUE

Abstract

The uptake of ³H-pyridoxol by everted intestinal rings of the rat was studied mainly at concentrations of 0.1-4μM of the vitamin in the medium. Incubation of the tissue at 37°C for 60 min in Krebs-Ringer bicarbonate medium containing glucose and labelled pyridoxol resulted in intracellular accumulation of the labelled compounds. The total intracellular concentration of the compounds after the incubation was always higher than the extracellular concentration, and this accumu-lation was especially appreciable at relatively low external concentra-tions of the ³H-vitamin. The uptake under the same conditions was partially saturated with increasing concentration of ³H-pyridoxol in the medium, whereas the uptake during the initial 5 min of incubation increased in a linear manner with an increase of external concentration. The uptake was markedly depressed by lowering the temperature of incubation or addition of 4-deoxypyridoxol. The inhibition by 4-deoxypyridoxol affected the time course of uptake mainly in its later period, and also depressed both the linear and saturable components contained in the relation of uptake to the external concentration of the ³H-vitamin. However, a considerable uptake occurred even at the maximum inhibition with 4-deoxypyridoxol. Examination of the intracellular forms of the ³H-vitamin taken up revealed that the major components were three B₆ phosphates. This phosphorylating tendency was particularly extreme when the tissue was incubated under conditions allowing marked accumulation of the ³H-vitamin. Of these phosphate esters, ³H-pyridoxamine 5'-phosphate appeared most highly responsible for the accumulation. 4-Deoxypyridoxol remark-ably diminished the ³H-vitamin B₆ phosphates with a consequent de-crease of total intracellular ³H-vitamin B₆ compounds, although it permitted free ³H-pyridoxol increase. From these results it is con-cluded that pyridoxol enters the intracellular space by simple diffusion, is accumulated by metabolic conversion to the phosphate forms of the vitamin, and the converion is inhibited by 4-deoxypyridoxol.