High doses of vitamin B6 in the rat are associated with inhibition of hepatic tryptophan metabolism and increased uptake of tryptophan into the brain

Pellagra and alcoholism: a biochemical perspective

Historical and clinical aspects of pellagra and its relationship to alcoholism are reviewed from a biochemical perspective. Pellagra is caused by deficiency of niacin (nicotinic acid) and/or its tryptophan (Trp) precursor and is compounded by B vitamin deficiencies. Existence on maize or sorghum diets and loss of or failure to isolate niacin from them led to pellagra incidence in India, South Africa, Southern Europe in the 18th century and the USA following the civil war. Pellagra is also induced by drugs inhibiting the conversion of Trp to niacin and by conditions of gastrointestinal dysfunction. Skin photosensitivity in pellagra may be due to decreased synthesis of the Trp metabolite picolinic acid → zinc deficiency → decreased skin levels of the histidine metabolite urocanic acid and possibly also increased levels of the haem precursor 5-aminolaevulinic acid (5-ALA) and photo-reactive porphyrins. Depression in pellagra may be due to a serotonin deficiency caused by decreased Trp availability to the brain. Anxiety and other neurological disturbances may be caused by 5-ALA and the Trp metabolite kynurenic acid. Pellagra symptoms are resolved by niacin, but aggravated mainly by vitamin B6. Alcohol dependence can induce or aggravate pellagra by inducing malnutrition, gastrointestinal disturbances and B vitamin deficiencies, inhibiting the conversion of Trp to niacin and promoting the accumulation of 5-ALA and porphyrins. Alcoholic pellagra encephalopathy should be managed with niacin, other B vitamins and adequate protein nutrition. Future studies should explore the potential role of 5-ALA and also KA in the skin and neurological disturbances in pellagra.

Vitamin B6: Beyond Adequacy

There is good agreement concerning average requirements and reference intakes for vitamin B6 but less agreement over safe upper levels from supplements. High-dose supplements cause sensory nerve damage. Supplements of vitamin B6 have been advocated for treatment of the premenstrual syndrome, with little evidence of efficacy. There are plausible mechanisms for an antidepressant action and protection against steroid hormone—dependent cancers but no evidence from clinical trials. Pyridoxamine reduces the glycation of proteins and so could be beneficial in preventing the adverse effects of poor glycemic control in diabetes. There are plausible mechanisms for an antihypertensive action but only suggestive evidence from small intervention trials. There is no evidence that supplements of vitamin B6 have any beneficial effect in hyperhomocysteinemia. There is neither a plausible mechanism nor any evidence from controlled trials for any effect of supplements of vitamin B6 in preventing a decline in cognitive function with aging, amelioration of dementia or autism, or improvement of the carpal tunnel syndrome.

Non-nutritional uses of vitamin B6

Vitamin B6is a water-soluble vitamin, and is readily metabolized and excreted, so it has generally been assumed to have negligible toxicity, although at very high levels of intake it can cause peripheral nerve damage. Nutritional deficiency disease is extremely rare, although a significant proportion of the population shows biochemical evidence of inadequate status, despite apparently adequate levels of intake. The vitamin has been used to treat a wide variety of conditions, which may or may not be related to inadequate intake. In some conditions use of vitamin B6supplements has been purely empirical; in other conditions there is a reasonable physiological or metabolic mechanism to explain why supplements of the vitamin many times greater than average requirements may have therapeutic uses. However, even in such conditions there is little evidence of efficacy from properly conducted controlled trials.

Dietary excess of vitamin B-6 affects the concentrations of amino acids in the caudate nucleus and serum and the binding properties of serotonin receptors in the brain cortex of rats

Vitamin B-6 is a cofactor in many reactions of nitrogen metabolism. Deficiency alters tissue amino acid concentrations but effects of excess vitamin B-6 have not been well described. We fed female rats (218 g, 7 per group) 1 (control), 10, 100, 175 or 250x) the National Research Council recommended level of pyridoxine HCl (7 mg/kg) for 10 wk and measured serum amino acids, amino acids and neurotransmitters in brain regions and the binding properties of serotonin receptors in the cerebral cortex using a ketanserin binding assay. Rats were decapitated, and unheparinized blood was obtained. In the caudate nucleus, concentrations of glutamate, threonine, taurine, methionine, gamma-amino-butyric acid and the sum of the essential amino acids in groups 10X and 100X were approximately 130 to 180% of control levels (P < 0.05); groups 1X, 175X and 250X were not different. A similar pattern was seen in the serum for serine, glycine, aspartate and ornithine; the latter two amino acids increased to over 200% of control in group 100X. In the ketanserin binding assay, both the antagonist binding affinity and the maximal number of binding sites were higher for group 100X than for 1X, 175X and 250X, and were higher for 10X than for 1X. Norepinephrine in the raphe nucleus followed a similar biphasic pattern. Excess dietary pyridoxine affected brain and serum concentrations of some amino acids and binding properties of cortical serotonin receptors in a biphasic pattern over the range of concentrations fed in this study.

Tryptophan availability and the susceptibility to stress in multiple sclerosis: a hypothesis

In his seminal description of the clinical manifestations of multiple sclerosis (MS) in 1868 Charcot suggested that psychological stress is an important factor in the pathogenesis of the disease. MS patients often relate that mental stress exacerbates their symptoms and even provokes attacks of their disease. Moreover, a subgroup of MS patients experiences exacerbation of symptoms following a period of mental stress rather than occurring at times when stress is maximal, i.e., attacks are reported to occur on the rebound from stress. Psychological stress may also contribute to the onset and long term clinical deterioration of the disease. Mental stress is associated with activation of the hypothalamic-pituitary adrenal (HPA) axis resulting in increased release of ACTH and cortisol secretion. Stress-induced activation of the HPA axis is associated with an increased metabolism of cerebral serotonin (5-HT) the synthesis of which is dependent upon the availability of its precursor tryptophan. Thus, increased tryptophan availability may partly underlie the general response to stress. Plasma and CSF tryptophan levels are diminished in chronic MS patients and it is suggested that plasma tryptophan levels are even lower in patients who are stressed. Attenuated increase in the availability of systemic tryptophan in response to stress is thought to underlie the susceptibility of MS patients to stress. This hypothesis is supported by the findings in chronic MS patients of a significantly diminished adrenal cortisol reactivity to insulin-induced hypoglycemia which is considered a stress response mediated through the 5-HT system. Consequently, since patients with MS exhibit an abnormal response to stress it follows that increased tryptophan availability through dietary supplementation would diminish their vulnerability to psychological stress. Additionally, if sustained or repeated stress contributes to progression and clinical deterioration of the disease then chronic tryptophan supplementation also may halt its progression.

Tryptophan metabolism and vitamin B6 nutritional status in patients with schistosomiasis mansoni and in infected mice

Patients infected with Schistosoma mansoni showed an abnormal response to a test dose of tryptophan, with little increase in the urinary excretion of kynurenine, hydroxykynurenine, xanthurenic and kynurenic acids, N1-methyl nicotinamide, methyl pyridone carboxamide, 5-hydroxytryptamine or 5-hydroxyindoleacetic acid. In contrast to previous reports, this is different from the pattern of tryptophan metabolism seen in vitamin B6 deficiency. Furthermore, the patients' plasma concentrations of pyridoxal phosphate were within the reference range, and supplementation for 5 days with 20 mg vitamin B6/day did not affect tryptophan metabolism. Treatment with a single dose of Praziquantel resulted in a substantial restoration of normal tryptophan metabolism. In mice infected with S. mansoni there was a similar impairment of tryptophan metabolism, as shown by considerably reduced formation of 14CO2 after the administration of a tracer dose of [14C]tryptophan. Again, the administration of vitamin B6 supplements did not correct tryptophan metabolism in the mice. Treatment with Praziquantel resulted in substantial restoration of the production of 14CO2 from [14C]tryptophan. There was no evidence of vitamin B6 deficiency (as determined by erythrocyte aspartate aminotransferase activation coefficient) associated with infection in the mice, although there was a redistribution of pyridoxal phosphate between tissues, with a reduction in the concentration of liver, spleen and kidney, and an increase in skeletal muscle.

Fenfluramine-induced behavior changes in rats prefed serotonin-altering amounts of tryptophan and pyridoxine

It has been well established that elevated dietary tryptophan (TRP) levels can increase brain serotonin concentrations, thereby influencing serotonergic transmission. We previously examined interaction between dietary substrate (TRP: 0.15 and 0.6%) and the cofactor precursor (pyridoxine HCl: 3 and 3,000 mg/kg) on brain serotonin metabolism, observing significant increases in serotonin concentrations from such dietary interaction. The present experiments were designed to explore possible behavioral consequences of the substrate-cofactor interaction. After the IP injection of fenfluramine (FA: at 5, 10, 15, and 20 mg/kg), serotonin-mediated behavior traits and the appearance of flushing were observed in rats fed experimental diets as stated above. With a 5 mg/kg dose of FA, a differential dietary effect was most visible. However, at higher FA levels (15 and 20 mg/kg), such dietary effects were no longer discernible. The appearance of flushing was also dependent on dietary TRP intake and the dosage of FA. These results indicate a clear substrate-cofactor interaction on certain serotonin-mediated behavior traits in the rat.

Neurotoxicity of pyridoxine analogs is related to coenzyme structure

In very high doses, pyridoxine is neurotoxic in humans and other animals. Using morphometry and a model system of dorsal root ganglion neurons in culture, we found that several analogs of pyridoxine were neurotoxic in vitro. Those that may be converted into active coenzymes--pyridoxal, pyridoxine, and pyridoxamine--were almost equal in toxicity. Pyridoxic acid, which is not active, was nontoxic. Pyridoxamine 5-phosphate, which cannot enter cells, also was nontoxic. Several hypotheses that link coenzyme function to toxic effect are described.