Plasma binding of vitamin B6 compounds

PYRIDOXINE, the major dietary form of vitamin B₆, is rapidly converted in the body to pyridoxal phosphate (pyridoxal-P), the coenzyme form. Pyridoxine has been shown to be phosphorylated in liver and brain homogenates¹ and in red cells², and is subsequently oxidised to pyridoxal-P. In the red cell pyridoxal-P is dephosphorylated, and pyridoxal is the form that is then released into plasma². But the endogenous form of B₆ found in plasma is mainly pyridoxal-P. This paper reports our findings on the ability of plasma proteins to bind these two circulating forms-pyridoxal and pyridoxal-P, and their dietary precursor, pyridoxine. We found that there were marked differences in the extent to which each B₆ compound was bound. These findings can be interpreted to explain part of the mechanism which regulates the passage of B₆ compounds into and out of the red cell.