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.
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