Metabolism of vitamin B6 in rat liver mitochondria

The capacity of rat liver mitochondria to transport and metabolize B6 compounds has been studied. In B6-sufficient rats, 20% of the B6 content in liver is located in mitochondria. The ratio of pyridoxal-P to pyridoxamine-P in isolated mitochondria is increased with 2-oxoglutarate as substrate and decreased with glutamate. Isolated mitochondria contain pyridoxal-P(pyridoxamine-P) hydrolase activity in the intermembranous space. They exhibit no pyridoxal kinase activity and less than 5% of the pyridoxamine-P(pyridoxine-P) oxidase activity in cytosol and cannot synthesize pyridoxal-P or pyridoxamine-P from pyridoxine, pyridoxal, or pyridoxine-P. When isolated hepatocytes are incubated with [14C]pyridoxine, [14C]pyridoxal-P synthesized in the cytosol is taken up by mitochondria at a rate approximating linearity. Additionally, as reflected by the lower specific radioactivity of [14C]pyridoxal-P in mitochondria than that in cytosol, the extent of mixing of newly synthesized and endogenous pools of pyridoxal-P in these subcellular compartments is heterogeneous. When cytosol from hepatocytes incubated with [14C]pyridoxine is dialyzed to equilibrium, a dialyzable pool of [14C]pyridoxal-P with specific radioactivity similar to that of the added [14C]pyridoxine is identified. It appears that pyridoxal-P from this newly synthesized pool is preferentially transported into mitochondria or secreted and degraded by hepatocytes.