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.

Moderate folate depletion increases plasma homocysteine and decreases lymphocyte DNA methylation in postmenopausal women.

To determine the human folate requirement on the basis of changes in biochemical pathways, we studied the effect of controlled folate intakes on plasma homocysteine and lymphocyte DNA methylation and deoxynucleotide content in healthy postmenopausal women. Eight women (49-63 y of age) were housed in a metabolic unit and fed a low folate diet containing 56 microg/d of folate for 91 d. Folate intake was varied by supplementing 55-460 microg/d of folic acid (pteroylglutamic acid) to the diet to provide total folate intake periods of 5 wk at 56 microg/d, 4 wk at 111 microg/d and 3 wk at 286-516 microg/d. A subclinical folate deficiency with decreased plasma folate was created during the first two periods. This resulted in significantly elevated plasma homocysteine and urinary malondialdehyde, and lymphocyte DNA hypomethylation. The folate depletion also resulted in an increased ratio of dUTP/dTTP in mitogen-stimulated lymphocyte DNA and decreased lymphocyte NAD, changes suggesting misincorporation of uracil into DNA and increased DNA repair activity. The DNA hypomethylation was reversed with 286-516 microg/d of folate repletion, whereas the elevated homocysteine decreased with 516 but not 286 microg/d of folate. The results indicate that marginal folate deficiency may alter DNA composition and that the current RDA of 180 microg/d may not be sufficient to maintain low plasma homocysteine concentrations of some postmenopausal women.

Tissue B-6 vitamer concentrations in rats fed excess vitamin B-6.

Diets containing 1, 10, 100, 175 or 250 times the NRC recommended level of pyridoxine HCl (7 mg/kg) were fed to rats (218 g, 12 per group) to evaluate the effects on tissue B-6 vitamer concentrations. After 10 wk, food intake and body weights did not differ among groups. Overt toxicity was not observed. Tissues were taken from five rats of each group after overnight food deprivation (unfed rats); the remaining seven rats in each group were allowed access to food (fed rats). In plasma of unfed rats, 4-pyridoxic acid and pyridoxal concentrations increased significantly (P < 0.05) with increasing dietary pyridoxine; pyridoxal phosphate was not affected by dietary pyridoxine. Concentrations of pyridoxal phosphate and pyridoxal increased significantly with increasing dietary pyridoxine in erythrocytes of unfed rats. Excretion of urinary B-6 vitamers and 4-pyridoxic acid in a 24-h period increased with dietary pyridoxine in fed rats. As dietary pyridoxine was increased, kidney pyridoxal concentrations increased significantly in fed rats only. Dietary pyridoxine did not affect vitamer concentration in muscle and liver of either unfed or fed rats, or in brain of unfed rats. Muscle glycogen phosphorylase, which contains pyridoxal phosphate, was not affected by dietary pyridoxine. There was a marginally significant (P = 0.058) increase in erythrocyte alanine, but not in aspartate, aminotransferase activity with increasing dietary pyridoxine. Plasma concentration of pyridoxal phosphate, which is used as a measure of vitamin B-6 status, did not reflect intake of pyridoxine in this study.(ABSTRACT TRUNCATED AT 250 WORDS)

Overt vitamin B-6 deficiency affects rat pancreatic digestive enzyme and glutathione reductase activities.

Previous reports suggest that vitamin B-6 deficiency contributes to pancreatic insufficiency. However, the susceptibility of pancreatic function to marginal vitamin B-6 intake has not been defined. The present study examines digestive enzyme activity and steady-state mRNA levels, as well as antioxidant enzyme status from rats fed different vitamin B-6 diets. Groups (n = 12) of adult female Long-Evans rats were assigned to five dietary groups and fed their respective diets for 6 wk. Control and food-restricted rats were fed the control diet (7 mg pyridoxine/kg diet) freely, or food intake was restricted to the lowest intake of the experimental groups. The experimental groups were fed purified diets containing 0 (deficient), 0.25 or 1 (marginal) mg pyridoxine/kg diet. Plasma amylase and pancreatic amylase, trypsin and chymotrypsin activities were significantly lower in deficient rats compared with rats fed the control diet. Lower enzyme activities were accompanied by 83 and 55% lower amylase and trypsinogen mRNA levels compared with levels in rats fed the control diet. Other than low glutathione reductase in deficient rats, pancreatic antioxidant enzyme activity was similar in all dietary groups. These data suggest that the exocrine pancreas is impaired by vitamin B-6 deficiency, but marginal pyridoxine intake maintains function.

Insensitivity of the tryptophan-load test to marginal vitamin B-6 intake in rats.

The tryptophan-load test for vitamin B-6 nutritional status was administered to adult female Long-Evans rats fed graded levels of pyridoxine hydrochloride (PN.HCl) in two experiments, and its sensitivity to marginal vitamin B-6 intake was evaluated. In Experiment 1, rats were 4-h meal-fed an AIN-76A (20% casein) diet devoid of PN.HCl for 3 wk, then repleted (n = 12) for 6 wk with 4-h pair-fed meals of either 0.25, 0.5, 1.0 or 7.0 (control) mg PN.HCl/kg diet. In Experiment 2, rats (n = 16) were pair-fed for 10 wk either 0.0, 0.5, 1.0 or 7.0 (control) mg PN.HCl/kg diet, with 24-h access to food. Vitamin B-6 nutritional status was assessed at the end of each experiment. Except in rats fed 0 mg PN.HCl/kg diet, mean body weights were not significantly different among diet groups of either experiment. Plasma pyridoxal phosphate (PLP), pyridoxal and total vitamin B-6 concentrations, determined by HPLC, were very sensitive to gradations in dietary PN.HCl concentrations (P less than 0.05). Red blood cell endogenous and PLP-stimulated alanine and aspartate aminotransferase activity did not statistically differentiate all levels of dietary vitamin B-6, although the calculated activity coefficient for each enzyme (stimulated/endogenous activity) did. Urinary xanthurenic acid excretion following a tryptophan load [24.5 mumol (5 mg) L-tryptophan/100 g body weight, injected intraperitoneally] was significantly (P less than 0.05) elevated compared with controls only in the group fed 0 mg PN/HCl/kg diet. At the tryptophan dose used here, the tryptophan-load test was not useful in detecting marginal vitamin B-6 intake in rats.