4-Pyridoxic acid

Vitamin Bg and related compoimds (Figure 10.2) were quantitatively separated by preparative TLC on silica gel H. After elution, the pyridoxic acid lactone method was employed for fluorimetric determination of the concentration of the vitamin forms involved [8]. Table 10.2 shows Revalues obtained for various forms of vitamin Bg, using several solvent systems. The solvent selected, ethyl acetate/pyridine/water (2 1 2, v/v), gave excellent separation of pyridoxamine, pyridoxic acid, and pyri-doxine together with pyridoxal. 

FIGURE 10.2 Structural formula of vitamin and related compounds. 1 — pyridoxine, 2 — pyridoxal, 3 — pyridoxamine, 4 — 4-pyridoxic acid 5 — pyridoxal-5 -phosphate. 

Solvent System (v/v) Pyridoxal Pyridoxine Pyridoxic Acid Pyridoxic Acid Lactone Pyridoxamine Pyridoxal Phosphate... 

The steroids aldosterone, cortisone, cortisol, 11-P-hydroxyandrostenedione, corticosterone, and rostenedione, 11-desoxycorticosterone, 17-hydroxy-progesterone, and progesterone have been performed on Ultrasphere ODS using methanokwater.19 Ranitidine N-[2-[[[5-[(dimethylamino)methyl]-2-furanyl]-methyl]thio]ethyl]-N1-methyl-2-nitro-l,l-ethenediamine has been separated using a p-Bondapak C18 column operated with acetoni-trile methanol water buffered with triethylamine phosphate.117 Pyridoxal-5 -phosphate and other B6 vitamers, including pyridoxamine phosphate, pyri-doxal, pyridoxine, and 4-pyridoxic acid, were separated as bisulfite adducts... 

Pyridoxine, HC1 is more stable than pyridoxamine and pyridoxal. Acidic aqueous solutions of vitamin Bg are stable definitely at room temperature and may be heated at 120° for 30 minutes without decomposition (13). 

Although, owing to the wide distribution of vitamin Bg in nature, clinical deficiency symptoms are seldom observed, there is little doubt that pyridoxine is essential in human nutrition. Pyridoxine is absorbed from the gastrointestinal tract and is converted to the active form pyri-doxal phosphate. Absorption is decreased in gastrointestinal diseases and also in subjects taking isoniazid (3). It is excreted in the urine as 4-pyridoxic acid (2). The metabolism of vitamin Bg in human beings has been investigated (56). 

Moller (57) has recognized pyridoxine, pyridoxal, pyridoxa-mine, and 4-pyridoxic acid as the excretion products of vitamin Bg. Complete balance studies have been made in pigs and in babies on all the known vitamin Bg metabolic compounds. In babies, the total output exceeded the intake. The assumption that a limited synthesis of vitamin Bg occurs seems justifiable. 

Lumeng et al. (58) have reported the plasma content of Bg vitamers and its relationship to hepatic vitamin Bg metabolism. Orally ingested pyridoxine is rapidly metabolised in liver and its products are released into the circulation in the form of pyridoxal phosphate, pyridoxal, and pyridoxic acid. 

Fujino (142) has improved a method for the determination of vitamin Bg, by utilising the fluorescence of 4-pyridoxic acid which is produced by oxidation of vitamin Bg with permanganate. This method is applied for the determination of vitamin Bg in biological fluids (86, 143-144). 

Free pyridoxal either leaves the cells or is oxidized to 4-pyridoxic acid by aldehyde dehydrogenase (which is present in all tissues) and also by hepatic and renal aldehyde oxidases. 4-Pyridoxic acid is actively secreted by the renal tubules, so measurement of the plasma concentration provides an index of renal function (Coburn et al., 2002). There is some evidence that oxidation to 4-pyridoxic acid increases with increasing age in elderly people, the plasma concentration of pyridoxal phosphate is lower, and that of 4-pyridoxic acid higher, than in younger subjects even when there is no evidence of impaired renal function (Bates et al., 1999b). Small amounts of pyridoxal and pyridox-amine are also excreted in the urine, although much of the active vitamin Be that is filtered in the glomerulus is reabsorbed in the kidney tubules. 

Muscle pyridoxal phosphate is released into the circulation (as pyridoxal) in starvation as muscle glycogen reserves are exhausted and there is less requirement for glycogen phosphorylase activity. Under these conditions, it is potentially available for redistribution to other tissues, especially the liver and kidneys, to meet the increased requirement for gluconeogenesis from amino acids (Black et al., 1978). However, during both starvation and prolonged bed rest, there is a considerable increase in urinary excretion of 4-pyridoxic acid, suggesting that much of the vitamin Be released as a result of depletion of muscle glycogen and atrophy of muscle is not redistributed, but rather is ca-tabolized and excreted (Cobum et al., 1995). 

As shown in Table 9.5, there are a number of indices of vitamin Be status available plasma concentrations of the vitamin, urinary excretion of 4-pyridoxic acid, activation of erythrocyte aminotransferases by pyridoxal phosphate added in vitro, and the ability to metabolize test doses of tryptophan and methionine. None is wholly satisfactory and where more than one index has been used in population studies, there is poor agreement between the different methods (Bender, 1989b Bates et al., 1999a). 

Urine 4-pyridoxic acid >3.0 xmol/24 h >1.3 mmol/mol creatinine... 

About half of the normal dietary intake of vitamin Be is excreted as 4-pyridoxic acid (see Figure 9.1). Urinary excretion of 4-pyridoxic acid will largely reflect recent intake of the vitamin rather than underlying nutritional status. More importantly, renal clearance of 4-pyridoxic acid is a marker of renal function, irrespective of vitamin Be status (Bates et al., 1999a Coburn et al., 2002). 

Early studies of vitamin Be requirements used the development of abnormalities of tryptophan or methionine metabolism during depletion, and normalization during repletion with graded intakes of the vitamin. Although tryptophan and methionine load tests are unreliable as indices of vitamin Be status in epidemiological studies (Section 9.5.4 and Section 9.5.5), under the controlled conditions of depletion/repletion studies they do give a useful indication of the state of vitamin Be nutrition. More recent studies have used more sensitive indices of status, including the plasma concentration of pyridoxal phosphate, urinary excretion of 4-pyridoxic acid, and erythrocyte transaminase activation coefficient. 

Bates CJ, Pentieva KD, Prentice A, Mansoor MA, and Finch S (1999b) Plasma pyridoxal phosphate and pyridoxic acid and their relationship to plasma homocysteine in a representative sample of British men and women aged 65 years and over. British Journal of Nutrition 81,191-201. 

Cohurn SP, Reynolds RD, Mahuren fD, Schaltenhrand WE, Wang Y, Ericson KL, Whyte MP, Zuhovic YM, Ziegler PI, Costill DL, Fink WJ, Pearson DR, Pauly TA, Thampy KG, and Wortsman J (2002) Elevated plasma 4-pyridoxic acid in renal insufficiency. American Joumai ofCiinicai Nutrition 73, 57-64. 

Cohurn SP, Thampy KG, Lane HW, Conn PS, Ziegler PJ, Costill DL, Mahuren JD, Fink WJ, Pearson DR, Schaltenhrand WE, et al. (1995) Pyridoxic acid excretion during low vitamin B-6 intake, total fasting, and hed rest. American Joumai ofCiinicai Nutrition 62,979-83. 

McChrisley B, Thye FW, McNair HM, and Driskell JA (1988) Plasma B6 vitamer and 4-pyridoxic acid concentrations of men fed controlled diets. Journal of Chromatography 428, 35-42. 

Quantification. High Pressure Liquid Chromatography. In plasma or urine pyridoxine, pyridoxal and 4-pyridoxic acid, sensitivity 300 ng/ml for plasma, 500 ng/ml for urine, UV detection—W. J. O Reilly eta/., 7. Chromat., 1980,183, Biomed. Appl, 9, 492-498. 

Disposition in the Body. Absorbed from the gastro-intestinal tract and converted to the active form, pyridoxal phosphate. Excreted in the urine mainly as 4-pyridoxic acid. 

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-pyridoxic acid, 183.2.

Free pyridoxal either leaves the cells or is oxidized to 4-pyridoxic acid by aldehyde dehydrogenase (which is present in all tissues) and also by hepatic and renal aldehyde oxideises. 4-Pyridoxic acid is actively secreted by the renal tubules, so meeisurement of the pleisma concentration provides an index of rened function (Coburn et ed., 2002). There is some evidence that oxidation to... 

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