In vivo absorption and phosphorylation of pyridoxine . HCl in rat jejunum.

The absorption and phosphorylation of 3H-pyridoxine . HCl were studied in vivo in isolated loops of rat jejunum. Uptake was rapid and linear over the concentration range of 0.2-1 mM; tissue content of absorbed vitamin was saturable and consisted primarily of phosphorylated forms of vitamin B6. Phosphorylation was saturable with a Km of 11.6 micronM and a Vmax of 1.65 nmol/5 min/g wet tissue. Inhibition of phosphorylation changed neither the t 1/2 of 3H-pyridoxine . HCl disappearance from the lumen nor the calculated uptake after either 5-min or 30-min absorption periods. Inhibition of phosphorylation, however, significantly decreased the t 1/2 for transmural absorption and significantly increased the calculated transmural absorption after a 5-min absorption period. These studies indicate that (a) pyridoxine absorption, over the concentration range tested, is nonsaturable and consistent with passive diffusion; (b) jejunal phosphorylation of pyridoxine occurs in vivo, is saturable, and results in saturation of tissue content of the absorbed vitamin; and (c) phosphorylation has no significant effect on the uptake of pyridoxine but significantly delays the transmural absorption of a finite amount of the vitamin.