TRANSPORT AND METABOLISM OF PYRIDOXAL AND PYRIDOXAL-PHOSPHATE IN THE SMALL-INTESTINE OF THE RAT

The transport and metabolism of [3H]pyridoxal (PL) and [3H]pyridoxal-5''-phosphate (PLP) were studied by using isolated, vascularly perfused rat small intestine. Physiological levels (0.02 .mu.mol) of the 2 compounds were administered via the lumen. After a 10 min experimental period the amount of isotope removed from the lumen and the labeled compounds in the lumen, mucosa and perfusate were determined. After 10 min about 40% of the radioactivity in PL or PLP had been transported to the perfusate and tissue. Dilution of 0.02 .mu.mol [3H]PL with 20 .mu.mol of unlabeled PL or inhibition with 20 .mu.mol of 4''-deoxypyridoxine did not change the percentage of isotope absorbed; the transport of PL apparently occurs by passive diffusion. Substantial amounts of labeled PLP, pyridoxic acid and pyridoxamine phosphate were detected in the mucosa when [3H]PL was administered, but the major form released to the portal circulation was PL. After administration of [3H]PLP, the major labeled vitamer present in the lumen and transported to the perfusate was PL. Apparently hydrolysis of PLP occurs in the lumen. Dilution with 20 .mu.mol of unlabeled PLP decreased the percentage of isotope absorbed, reduced dephosphorylation in the lumen and increased the amount of labeled PLP in the mucosa and perfusate. At physiological levels PLP apparently is absorbed chiefly as PL after hydrolysis in the lumen. PLP is transported across the intestinal wall at a slower rate than PL.