Plasma pyridoxal phosphate

Extrahepatic tissues take up both pyridoxal and pyridoxal phosphate from the plasma. Pyridoxal phosphate is hydrolyzed to pyridoxal, which can cross cell membranes, by extracellular alkaline phosphatase, which is then trapped intracellularly by phosphorylation. 

A variety of studies have shown that 10% to 20% of the population of developed countries have marginal or inadequate stams, as assessed by erythrocyte transaminase activation coefficient (Section 9.5.36) or plasma pyridoxal phosphate (Section 9.5.1 Bender, 1989b). This may be sufficient to enhance the responsiveness of target tissues to steroid hormones (Section 9.3.3), and may be important in the induction and subsequent development of hormone-dependent cancer of the breast and prostate. Vitamin Be supplementation may be a useful adjunct to other therapy in these common cancers certainly, there is evidence that poor vitamin Be nutritional stams is associated with a poor prognosis in women with breast cancer. 

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. 

RD Reynolds, CL Natta. Depressed plasma pyridoxal phosphate concentrations in adult asthmatics. Am J Clin Nutr 41 684-688, 1985. 

Homocystinuria can be treated in some cases by the administration of pyridoxine (vitamin Bs), which is a cofactor for the cystathionine synthase reaction. Some patients respond to the administration of pharmacological doses of pyridoxine (25-100 mg daily) with a reduction of plasma homocysteine and methionine. Pyridoxine responsiveness appears to be hereditary, with sibs tending to show a concordant pattern and a milder clinical syndrome. Pyridoxine sensitivity can be documented by enzyme assay in skin fibroblasts. The precise biochemical mechanism of the pyridoxine effect is not well understood but it may not reflect a mutation resulting in diminished affinity of the enzyme for cofactor, because even high concentrations of pyridoxal phosphate do not restore mutant enzyme activity to a control level. 

W2. Wachstein, M., Kellner, J. D., and Oritz, J. M., Pyridoxal phosphate in plasma and leukocytes of normal and pregnant subjects following Be load tests. Froc. Soc. Exptl. Biol. Med. 103, 350-353 (1960). 

In humans, peripheral neuropathy due to isoniazid is influenced by the acetylator phenotype (see chap. 5), being predominantly found in slow acetylators. This is probably due to the higher plasma level of isoniazid in this phenotype. In this case, therefore, acetylation is a detoxication reaction, removing the isoniazid and rendering it unreactive toward pyridoxal phosphate. 

Amino groups are tunneled to glutamate from all amino acids except lysine and threonine. The enzymes are aminotransferases, and they are reversible. The two most important of these enzymes are alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Aminotransferases require pyridoxal phosphate as a coenzyme. The presence of elevated levels of aminotransferases in the plasma can be used to diagnose liver disease. 

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. 

In subjects with hypophosphatasia, the rare genetic lack of extracellular alkaline phosphatase, plasma concentrations of pyridoxal phosphate are very much higher than normal (up to 4 /rmol per L, compared with a normal range of about 100 nmol per L), and intracellular concentrations of pyridoxal phosphate are lower than normal (Narisawa et al., 2001). 

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. 

Cystathionine /3-synthetase contains heme as well as pyridoxal phosphate, but this seems to have a regulatory rather than catalytic role the yeast enzyme does not contain heme (Jhee et al., 2000 Kabil et al., 2001). A common genetic polymorphism in human cystathionine /S-synthetase (a 68-base-pair insertion, occurring in about 12% of the general population) is associated with a lower than normal increase in plasma homocysteine after a methionine load in patients with low vitamin Be status, suggesting that the variant enzyme may have higher affinity for its cofactor than the normal form - the reverse of the position in the vitamin Bg responsive genetic diseases discussed in Section 9.4.3 (Tsaietal., 1999). 

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). 

A number of studies have measured the activation of plasma transaminases by pyridoxal phosphate added in vitro however, it is difficult to interpret the results, because plasma transaminases arise largely accidentally, as a result of cell turnover, and the amount released will depend on tissue damage. Furthermore, there is a considerable amount of pyridoxal phosphate in plasma, largely associated with serum albumin, and the extent to which plasma transaminases are saturated will depend largely on the relative affinity of albumin and the enzyme concerned for the coenzyme, rather than reflecting the availability of pyridoxal phosphate for intracellular metabolism. Studies on erythrocyte transaminase activation coefficient are easier to interpret, because the extent to which the enzymes are saturated depends mainly on the availability of pyridoxal phosphate. 

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. 

In 1998, the reference intake in the United States and Canada was reduced from the previous Recommended Daily Allowance of 2 mg per day for men and 1.6 mg per day for women (National Research Council, 1989) to 1.3 mg per day for both (Institute of Medicine, 1998). The report cites six smdies that demonstrated that this level of intake would maintain a plasma concentration of pyridoxal phosphate at least 20 nmol per L although, as shown in Table 9.5, the more generally accepted criterion of adequacy is 30 nmol per L. 

Protein Binding. In plasma, pyridoxine not significantly bound, pyridoxal phosphate almost completely bound. 

It was also observed (W6) that at termination of normal pregnancy, vitamin Be values were significantly depressed in maternal leucocytes and plasma. In blood of women in the last trimester of pregnancy, however, normal average amounts of the vitamin were found, although these individuals had a distinct abnormality in tryptophan metabolism. Amounts of pyridoxal phosphate are high in cord blood. 

Despite normal plasma pyridoxal 5-phosphate values, the urinary excretion of xanthurenic acid was found abnormally elevated in 3 epileptic children with disturbed tryptophan metabolism (HO). Administration of pyridoxine restored xanthurenuria to normal and raised plasma pyridoxal 5-phosphate levels. 

Concentrations of plasma homocysteine, plasma pyridoxal 5 -phosphate (active vitamin B6), serum folate, erythrocyte folate, and serum vitamin B12 have been measured both during fasting and after methionine in 60 epileptic patients (aged 14-18 years) and 63 sex- and age-matched controls before therapy and after 1 year of therapy with valproate or carbamazepine (33). After 1 year the patients who took valproate and carbamazepine had significantly increased plasma homocysteine concentrations compared with both baseline and control values and there was a significant fall in serum folate and plasma pyridoxal 5 -phosphate. Serum vitamin B12 and erythrocyte folate were unchanged. 

In healthy volunteers, theophylline reduced circulating pyridoxal phosphate (vitamin B6) concentrations, presumably by noncompetitive inhibition of pyridoxal kinase. Theophylline concentrations of approximately 10 gg/ml produced only partial inhibition, plasma pyridoxal kinase and pyridoxal concentrations being unaffected. The authors speculated that with theophylline overdose and greater inhibition, vitamin B6 deficiency might contribute to seizures (SEDA-14, 2). 

Manore, M. M., Vaughan, L. A., Carroll, S. S., and Leklem, J. E. (1989). Plasma pyridoxal 5 -phosphate concentration and dietary vitamin B-6 intake in free-living, low-income elderly people. Am. J. Clin. Nutr. 50, 339-345. 

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