Intestinal absorption of pyridoxal-5'-phosphate: disappearance from perfused segments of rat jejunum in vivo.

The luminal disappearance of unlabeled pyridoxal-5'-phosphate (PLP) was studied in vivo in rat jejunum utilizing a perfused segment model. The PLP was measured by a tyrosine decarboxylase assay. [14C]Dextran was used as a nonabsorbable marker to calculate net water absorption. Initial studies validated the use of [14C]dextran as a nonabsorbable marker and established proper conditions of segment perfusion and sample collection for the measurement of PLP luminal disappearance. Subsequent studies demonstrated significant (67.2%) but incomplete inhibition of PLP disappearance by 80 mM phosphate. When nonperfused PLP in 1.1 mM phosphate buffer or perfused PLP in 80 mM phosphate buffer were incubated in vitro for 15 minutes at 37 degrees in a water bath, only negligible changes in PLP concentration were noted. However, when perfused PLP in 1.1 mM phosphate buffer was similarly incubated in vitro, there was a rapid decay in PLP concentration. L-Phenylalanine (5 mM) significantly inhibited this in vitro decay. Conclusions derived from these studies are: 1) The model used is a valid means of studying in vivo luminal disappearance of PLP in the rat jejunum; 2) a major portion of the disappearance seems to involve hydrolysis by alkaline phosphatase; 3) a significant portion of this hydrolysis occurs intraluminally; and 4) a second mechanism of PLP disappearance, which is nonphosphatase-mediated, also appears operative and may represent absorption of the intact, phosphorylated vitamin.