Pyridoxal kinase, pyridoxamine phosphatase

Three enzymes play an active role in the metabolism of vitamin B6 in human erythrocytes. Pyridoxal kinase uses ATP to phosphorylate pyridoxine, pyri-doxamine, and pyridoxal. Pyridoxamine oxidase oxidizes pyridoxamine-5 -phosphate and pyridoxine-5 -phosphate to pyridoxal-5 -phosphate. The phosphatase activity produces pyridoxal from pyridoxal-5 -phosphate. The assay of the three enzymes required separation of the semicarbazone derivatives of pyridoxal-5 -phosphate and pyridoxal. The mobile phase used by Ubbink and Schnell (1988) contained 2.5% acetonitrile. Detection was by fluorescence. 

Tissue uptake of vitamin Be is again by carrier-mediated diffusion of pyridoxal (and other unphosphorylated vitamers), followed by metabolic trapping by phosphorylation. Circulating pyridoxal and pyridoxamine phosphates are hydrolyzed by extracellular alkaline phosphatase. All tissues have pyridoxine kinase activity, but pyridoxine phosphate oxidase is found mainly in the liver, kidney, and brain. 

The phosphorylated vitamers are dephosphorylated by membrane-bound alkaline phosphatase in the intestinal mucosa pyridoxal, pyridoxamine, and pyridoxine are all absorbed rapidly by carrier-mediated diffusion. Intestinal mucosal cells have pyridoxine kinase and pyridoxine phosphate oxidase (see Figure 9.1), so that there is net accumulation of pyridoxal phosphate by metabolic trapping. Much of the ingested pyridoxine is released into the portal circulation as pyridoxal, after dephosphorylation at the serosal surface. 

Modeling vitamin B6 metabolism is further complicated by the fact that the activity of the kinase, oxidase, and phosphatase enzymes varies between organs and species. A very simplified diagram of vitamin B6 metabolism is shown in Fig. 2. In the intestine any phosphoiylated forms are hydrolyzed. The free vitamers are readily taken up by diffusion into the intestinal wall where significant phosphorylation (Middleton, 1979) and other metabolism (Middleton, 1985) occurs. In mice small doses (up to 14 nmol) of pyridoxine (Sakurai et aL, 1988) and pyridoxamine (Sakurai et oL, 1992) were converted almost completely to pyridoxal before being released into the portal circulation. While it is dear that the intestinal microflora produce vitamin B6,... 

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