Mechanism of pyridoxine uptake by isolated rat liver cells

Uptake of [3H]pyridoxine by isolated rat hepatocytes was detected at substrate concentrations as low as 0.5 microM. At this concentration, an initial uptake rate was 1.87 +/- 0.17 pmol/10(6) cells X min at 37 degrees C. Both the initial and a subsequent much slower pyridoxine accumulation were strongly inhibited by low temperature as well as by 10 mM ethionine, which competes for available ATP, or 1 microM carbonylcyanide-p-trifluoromethoxyphenylhydrazone, which inhibits oxidative phosphorylation and the supply of ATP. The uptake process is apparently insensitive to 1 mM ouabain and is Na+ independent. The initial uptake rate was saturable at higher concentrations of [3H]pyridoxine with an apparent Km of 28 +/- 8 microM and Vmax of 106 +/- 27 pmol/10(6) cells X min. The Km value corresponds to that reported for pyridoxine as a substrate of pyridoxal kinase. Moreover, the transport process was inhibited by structural analogs of [3H]pyridoxine even at concentrations equimolar to the normal substrate. The established order of inhibitory effectiveness, 4'-deoxypyridoxine greater than unlabeled pyridoxine greater than 5'-deoxypyridoxine, is in agreement with known properties of the kinase. It is concluded that pyridoxine uptake probably occurs by diffusion, simple or facilitated, followed by metabolic trapping due to pyridoxal kinase-catalyzed phosphorylation. Pyridoxine deficiency had no significant effect on uptake of this B6 vitamer by hepatocytes.