Pyridoxine vitamin pyridoxamine phosphate

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. 

Vitamin B6. Figure 1 Structure of pyridoxin, pyridoxal, pyridoxamine, and the coenzymes pyridoxal-5 -phosphate and pyridoxamine-5Y-phosphate. 

Six compounds have vitamin Bg activity (Figure 45-12) pyridoxine, pyridoxal, pyridoxamine, and their b -phosphates. The active coenzyme is pyridoxal 5 -phos-phate. Approximately 80% of the body s total vitamin Bg is present as pyridoxal phosphate in muscle, mostly associated with glycogen phosphorylase. This is not available in Bg deficiency but is released in starvation, when glycogen reserves become depleted, and is then available, especially in liver and kidney, to meet increased requirement for gluconeogenesis from amino acids. 

Vitamin Bg a family of molecules having vitamin Be activity these include pyridoxal, pyridoxine, and pyridoxamine precursors to the coenzyme pyridoxal phosphate. 

We have just noted the role that pyridoxal phosphate plays as a coenzyme (cofactor) in transamination reactions (see section 15.6). Pyridoxal 5 -phosphate (PLP) is crucial to a number of biochemical reactions. PLP, together with a number of closely related materials that are readily converted into PLP, e.g. pyridoxal, pyridoxine and pyridoxamine, are collectively known as vitamin Bg, which is essential for good health. 

This enzyme [EC 2.7.1.35] (also known as pyridoxine kinase, pyridoxamine kinase, and vitamin kinase) catalyzes the reaction of ATP with pyridoxal to produce ADP and pyridoxal 5 -phosphate. Pyridoxine, pyridoxamine, and various other derivatives can also act as substrates. 

The six principal B6 vitamers are widely distributed in foods (102,103). They include pyridoxine (PN), pyridoxal (PL), pyridoxamine (PM), and their 5 -phosphate esters, pyridoxine phosphate (PNP), pyridoxal phosphate (PLP), and pyridoxamine phosphate (PMP) (Fig. 5). The predominate B6 vitamer in animal-based foods is PLP, whereas plant products generally contain PN and PM or their phosphorylated forms. Conjugated vitamers in the form of PN-glycosides have also been isolated from plant-based foods. Pyridoxal is readily converted to PM during cooking and food processing. Total vitamin B6 is the sum of the six principal vitamers inclusion of the conjugated forms depends on the extraction procedure. 

Fig. 4. Structures of (a) pyridoxine (vitamin Bg), (b) pyridoxal phosphate and (c) pyridoxamine phosphate.

All three forms of vitamin B6 [pyridoxal, pyridoxine, and pyridoxamine] are phosphorylated by a single kinase that uses ATP as the phosphate donor. This assay describes the use of pyridoxamine as the substrate. 

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.

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. 

Vitamin B Three substances are classed under the term pyridoxine or adermine pyridoxol, pyridoxal and pyridoxamine. Pyridoxine was isolated by various study groups in 1938. Its structure was described by Folkers and Kuhn in 1939. Pyridoxal and pyridoxamine were discovered by Snell in 1942. Pyridoxal phosphate and pyridoxamine phosphate are biologically active substances. Intestinal absorption of Bg is dose-dependent and not limited. In alcoholism, a deficiency of vitamin Bg is encountered in 20—30% of cases, whereas the respective percentage is 50—70% in alcoholic cirrhosis. Vitamin Bg is an important coenzyme for transaminases, which transfer amino groups from amino adds to keto acids. In this way, biochemical pathways between the dtiic acid cycle and carbohydrate and amino acid metabolisms are created. (104)... 

TLC of vitamin Be compounds, on various layers in different solvents, was studied. The Rf values of pyri-doxine, pyridoxal, pyridoxamine, pyridoxal ethyl acetate, 4-pyridoxic acid, 4-pyridoxic acid lactone, pyridoxine phosphate, pyridoxal phosphate, and pyridoxamine phosphate were 0.62, 0.68, 0.12, 0.54, 0.91, 0.91, 0.95, 0.95, and 0.86, respectively, by TLC on silica gel HF254 with... 

Pyridoxine (pyridoxol), pyridoxamine, and pyridoxal are the three natural forms of vitamin Bg. They are converted to pyridoxal phosphate, which is required for the synthesis, catabohsm, and interconversion of amino acids. 

Uptake and Metabolism. The vitamin Bg family consists of pyridoxine, pyridoxal, pyridoxamine, pyridoxine phosphate, pyridoxal phosphate (PLP), and pyridoxamine phosphate (Fig. 8.33). The commercial form is pyridoxine. Pyridoxal phosphate is the coenzyme form. It and pyridoxamine phosphate are from animal tissues. Pyridoxine is from plant tissues. All phosphorylated forms are hydrolyzed in the intestinal tract by phosphatases before being absorbed passively. Conversion to the phosphorylated forms occurs in the liver. Notice that niacin (NAD) and riboflavin (FMN, FAD) are required for interconversion among the vitamin Bq family. The phosphorylated forms are transported to the cells where needed. The major excretory product is 4-pyr-idoxic acid. 

Bi) is converted to thiamine pyrophosphate simply by the addition of pyrophosphate. It is involved in aldehyde group transfer. Niacin (nicotinic acid) is esterified to adenine dinucleotide and its two phosphates to form nicotinamide adenine dinucleotide. Pyridoxine (vitamin B ) is converted to either pyridoxal phosphate or pyridoxamine phosphate before complexing with enzymes. Riboflavin becomes flavin mononucleotide by obtaining one phosphate (riboflavin 5 -phosphate). If it complexes with adenine dinucleotide via a pyrophosphate ester linkage, it becomes flavin adenine dinucleotide. 

Pytidoaal (vitamin B ) Pyrldoxal phosphate Amino group and decarboxylation (i) Pyridoxine (ii) Pyridoxamine or pyridoxal fiii) Pyridoxal phosphate PtnlcUlium spent mycelium Yeast Rice polishings Cere Wheat seeds Maize seeds Cornsteep liquor Cottonseed flour... 

Pyridoxine (Vitamin Bg) IX), pyridoxal (A), and pyridoxamine XI), which are normally available in the diet, must be converted in vivo to the active co-enzyme pyridoxal phosphate XII) as shown in Figure 4.1. Inhibition of pyridoxal phosphate production in vivo results in reduced activity of pyridoxal-dependent enzymes. The extent of the inhibition may vary from one enzyme system to another, depending on the affinities of the apo-cnzymes for the co-enzyme. 

Pyridoxamine phosphate see Transamination. Pyridoxine see Vitamins (vitamin B ). 

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 1.5 Bs vitamers. All six forms have vitamin Bs activity hence they are called Bs vitamers. Two of the phosphorylated forms, pyridoxal phosphate and pyridoxamine phosphate, act as coenzymes. P5ridoxine phosphate is considered to be the intermediate on the way from pyridoxine to p5ridoxal phosphate.

Pyridoxal, pyridoxamine and pyridoxine are interconvertible in animal tissues, and all are converted to the functional forms, pyridoxal phosphate and pyridoxamine phosphate. Excessive amounts of the vitamin over and above tissue requirements are excreted, in part unchanged and in part after oxidation to 4-pyridoxic acid. Several soil micro-organisms, chiefly pseudomonads, have been isolated that grow on various forms of vitamin Be as a sole source of carbon, nitrogen and energy. Compounds listed in pathways A and B, Fig. 5, have been isolated as intermediates formed during oxidation of the vitamin to COa and water by two different species of these organ-isms - . 

The simultaneous determination of the B vitamins, thiamine, riboflavin, pyridoxal, pyridoxine, and pyridoxamine in a pharmaceutical product using CZE was described by Huopalahti and Sunell (90). Hydrochlorid acid was used for the extraction of the vitamins from the multivitamin-multimineral tablet. The applied potential was 6.0 kV, and a 75-p.m fused-silica capillary tubing was used. The electrolyte used was a 20-mM sodium phosphate buffer pH 9.0. A clear separation of standards as well as of the pharmaceutical sample is shown in Figure 14. This method appears to be a fast and simple technique for the simultaneous determination of water-soluble vitamins in pharmaceutical products, where the... 

Chemistry— Vitamin B-6 is found in foods in three forms which are readily interconvertible—pyridoxine, pyridoxal, and pyridoxamine. Also vitamin B-6 is found in physiological systems in the forms of pyridoxal phosphate and pyridoxamine phosphate. The structural formulas of the three naturally occurring free forms of these compounds are given in Fig. V-30. 

Aminotransferases contain pyridoxal phosphate, a derivative of the vitamin pyridoxine (B ), as prosthetic group. During the reaction, outlined in Figure 16.3a, the prosthetic group is converted into pyridoxamine phosphate (Figure 16.3b). Pyridoxal... 

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