Transport and metabolism of vitamin B6 in rabbit brain and choroid plexus.

In uitro, isolated choroid plexuses, the anatomical location of the system that transports vitamin I~,$ (BJ from blood into cerebrospinal fluid, and brain slices concentrated :‘H from media containing [3H]pyridoxine. Within these tissues, most of the intracellular [:‘H]pyridoxine was converted to and retained as the phosphorylated [“HIB,; vitamers. However, isolated choroid plexuses and brain slices were unable to concentrate “H from media containing [“H,“2P]pyridoxine-P when dephosphorylation of the [3H,Y2P]pyridoxine-P in the media was prevented. In uiuo, 2, h after the intraventricular injection of 0.8 nmol of [3H,J’P]pyridoxine-P, most of the recovered [“HIB,, in brain and cerebrospinal fluid, although phosphorylated, was not labeled with 32P. Based on this and other evidence, it is concluded that the nonphosphorylated B,, vitamers are the principal forms transported across brain and choroid plexus cell membranes. The mechanism by which brain slices and isolated choroid plexuses concentrate extracellular [“Hlpyridoxine depends on pyridoxal kinase, the enzyme that phosphorylates all three nonphosphorylated B,; vitamers, since (a) there was an excellent correlation between phosphorylation and accumulation, (h) the concentration of 4’-deoxypyridoxine to inhibit the pyridoxal kinase activity from each tissue was the same as the concentration to inhibit accumulation of [“HI& by intact tissue, and Cc) pyridoxine itself was not concentrated. These and previous results show that the regulation of accumulation of nonphosphorylated B,; vitamers is one mechanism by which the intracellular vitamin B,; levels in brain and choroid plexus are homeostatically maintained.