Intestinal absorption of pyridoxal 5'-phosphate at physiological levels in rats

The intestinal absorption of pyridoxal 5'-phosphate (PLP) at physiological levels (10(-7) -10(-6) M) was studied in comparison with that of pyridoxal (PL) in rat, using in vitro everted sac and an intestinal preparation that permitted continuous in situ collection of mesenteric venous blood. After PLP administration (10(-6) -10(-3) M) in situ, larger amounts of PLP were found in the mesenteric venous plasma than after PL administration at the same dose. The amount of PLP found in the mesenteric venous plasma was dependent on its dose at lower concentrations up to 10(-4) M but became independent at higher concentrations. After PL administration at various doses, the amount of PL found in the mesenteric venous blood increased linearly with the dose. When various concentrations of PLP were added to the mucosal side, under the in vitro condition with protection from alkaline phosphate hydrolysis, PLP was detected in the serosal side and the extent of PLP transport was dependent on the initial concentration of PLP in the mucosal side. When various concentrations of PL were added to the mucosal side, the extent of PL transport was independent of the initial concentration of PL in the mucosal side. In rat pretreated with actinomycin D, PLP transport in vitro was inhibited but not that of PL. N2-induced anoxia and pyridoxamine 5'-phosphate and anion transport inhibitor (4,4'-diisothiocyanostilben-2,2'-disulfonic acid disodium salt) showed no effect on PLP transport. These results suggest that PLP can be absorbed in the phosphorylated form and imply the presence of a saturable process for direct absorption of PLP itself and a diffusive process for PL absorption. In addition, the result of the in vivo neonatal experiment suggests that the neonatal intestine also can transport PLP in phosphorylated form.