Stimulation of mouse brain gamma-hydroxybutyric acid transport by pyridoxal 5'-phosphate

A synaptosomal preparation from mouse brain transported gamma-hydroxybutyric acid in a manner displaying saturation kinetics. A Kt of 48 +/- 11 microM was calculated. In the presence of pyridoxal 5'-phosphate, uptake was markedly enhanced. Stimulation of uptake was observed with increasing concentrations of pyridoxal 5'-phosphate, up to a maximum of about 3.5 mM. Increasing concentrations above this value led to a steady decrease in the stimulation of uptake, till at 7 mM pyridoxal 5'-phosphate and above there was a noticeable inhibition of uptake. The presence of increasing concentrations of gamma-hydroxybutyric acid gradually reduced the amount of stimulation, thus pyridoxal 5'-phosphate appeared to be producing its effect by reducing the Kt of the transport system rather than increasing its Vmax.