Vitamin pyridoxine phosphate oxidase

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. 

Pyridoxine phosphate oxidase is a flavoprotein, and activation of the erythrocyte apoenzyme hy riboflavin 5 -phosphate in vitro can he used as an index of riboflavin nutritional status (Section 7.4.3). However, even in riboflavin deficiency, there is sufficient residual activity of pyridoxine phosphate oxidase to permit normal metabolism of vitamin Bg (Lakshmi and Bamji, 1974). Pyridoxine phosphate oxidase is inhibited by its product, pyridoxal phosphate, which binds a specific lysine residue in the enzyme. In the brain, the K, of pyridoxal phosphate is of the order of 2 / mol per L - the same as the brain concentration of free and loosely bound pyridoxal phosphate, suggesting that this inhibition may be a physiologically important mechanism in the control of tissue pyridoxal phosphate (Choi et al., 1987). 

Vitamin Bg and Related Compounds.—Analogues of pyridoxol and pyridoxal phosphates in which the 5 -methylene (35) or the 5 -phosphate group (36) have been modified have been used to study the substrate specificity of pyridoxine phosphate oxidase. The methylene analogues acted as substrates whereas (36) or the 2-cyanoethyl ester of pyridoxol... 

Studies based on the use of an antivitamin, deoxypyri-doxine, have established that the daily requirement of the vitamin ranges between 1 and 2 mg in the human adult. A normal diet has been reported to provide 1-1.5 mg daily of the vitamin. Food appears to be the only source of the vitamin because most of the vitamin produced by the bacterial flora of the intestine is excreted in the feces, possibly after oxidation to 4-pyridoxic acid. The ingested vitamin is rapidly and completely absorbed, but the exact site of the absorption is not known. Although both pyridoxine and pyridoxamine can be excreted as such and are therefore normal constituents of human urine, part of the vitamin is oxidized to the 4-pyridoxic acid before excretion in the urine. Mammalian tissues contain at least two enzymes capable of oxidizing pyridoxine. Both enzymes seem to be flavoproteins. One attacks pyridoxine, the other attacks pyridoxine phosphate. The pyridoxine phosphate oxidase of liver has been purified 65 times. Although the enzyme was shown to act on pyridoxamine phosphate, pyridoxamine phosphate was oxidized only when the pH of the incubation mixture was raised to 10. Pyridoxine phosphate oxidase has no effect on pyridoxamine phosphate at physiological pH. 

Figure 9.1. Interconversion of the vitamin Be vitamers. Pyridoxal kinase, EC 2.7.1.38 pyridoxine oxidase, EC 1.1.1.65 pyridoxamine phosphate oxidase, EC 1.4.3.5 and pyridoxal oxidase, EC 1.1.3.12. Relative molecular masses (Mr) pyridoxine, 168.3 (hydrochloride, 205.6) pyridoxal, 167.2 pyridoxamine, 168.3 (dihydrochloride, 241.1) pyridoxal phosphate, 247.1 pyridoxamine phosphate, 248.2 and 4-pyridoxlc acid, 183.2.

DB McCormick, AH Merril. Pyridoxamine (pyridoxine) 5 -phosphate oxidase. In GP Tryfiates, ed. Vitamin B metabolism in growth. Westport, CT Food Nutrition Press, 1980, pp 1-26. 

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. 

The role of erythrocytes in vitamin B6 metabolism remains uncertain. Mouse and human erythrot es have higher oxidase activity and, therefore, convert pyridoxine to pyridoxal phosphate appreciably faster than erythrocytes from rat, hamster, and rabbit (Fonda, 1988). Anemic rats showed increased urinary loss of label administered as pyridoxal, suggesting that uptake by erythrocytes may conserve pyridoxal (Ink and Henderson, 1984). 

The term vitamin Bg refers to a group of naturally occurring pyridine derivatives represented by pyridoxine (pyridoxol, PN), pyridoxal (PL), and pyridoxamine (PM), and their phosphorylated derivatives. They are collectively referred to as vitamin Bg vitamers. The natural free forms of the vitamers could be converted to the key coenzymatic form, pyridoxal phosphate (PLP), by the action of two enzymes, a kinase and an oxidase. There are more than 140 PLP-dependent enzymatic reactions, and they are distributed in all organisms. These enzymes comprise diverse groups such as the oxidoreductases, transferases, hydrolases, lyases, and isomerases. About... 

There are several metabolic interrelationships between riboflavin and vitamin Bg. The conversion of pyridoxine or pyridoxamine phosphates to pyri-doxal phosphate is catalyzed by a flavoenzyme (pyri-doxaminephosphate oxidase EC 1.4.3.5), so that a deficiency of riboflavin may, at certain key sites, result in a secondary deficiency in Bg-dependent pathways. More evidence is needed to clarify the extent and importance of these interactions. 

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