Increased uterine uptake and nuclear retention of [3H]oestradiol through the oestrous cycle and enhanced end-organ sensitivity to oestrogen stimulation in vitamin B6 deficient rats

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.

Pyridoxal phosphate inhibits pituitary cell proliferation and hormone secretion

Pyridoxal phosphate (PLP), a bioactive form of pyridoxine, dose-dependently (10-1000 microm) inhibited cell proliferation in rat pituitary MMQ and GH3 cells and in mouse AtT-20 cells. After 4 d, MMQ cell numbers were reduced by up to 81%, GH3 cell numbers were reduced by up to 64% (P < 0.05), and AtT-20 cell numbers were reduced by up to 90%. Cell proliferation rates recovered and dose-dependently reverted to control levels after PLP withdrawal. After 4 d, PLP (400 and 1000 microm) decreased [3H]thymidine incorporation by up to 71% (P < 0.05). PLP (400-1000 microm) reduced GH3 cell GH and prolactin secretion and AtT-20 cell ACTH secretion (adjusted for cell number) by approximately 70% after 2 d. The 100 microm PLP also inhibited prolactin secretion (65%, P < 0.05) in primary rat pituitary cells treated for 2 d. PLP decreased the percentage of AtT-20 and GH3 cells in S phase and increased those in G0-G1 phase. Furthermore, PLP induced AtT-20 and GH3 cell apoptosis (28 vs. 6, P < 0.05; 26 vs. 3, P < 0.05, respectively) and dose-dependently reduced content of the antiapoptosis gene Bcl-2. These results indicate that pharmacological doses of PLP inhibit pituitary cell proliferation and hormone secretion, in part mediated through PLP-induced cell-cycle arrest and apoptosis. Pyridoxine may therefore be appropriate for testing as a relatively safe drug for adjuvant treatment of hormone-secreting pituitary adenomas.

Pyridoxal 5'-phosphate related changes in retention of 1,25-dihydroxy vitamin D-receptor ligands in rat intestinal mucosa cell nuclei

After feeding rats a vitamin B-6-deficient diet, we observed a decrease in pyridoxal 5'-phosphate concentrations in intestinal mucosa cells to 32 and 48% of control in cytoplasm and cell nuclei, respectively. Correlation analysis suggested that there were two pyridoxal 5'-phosphate pools in the nuclei: a "mobile" pool (equivalent to about 5% the concentration of the cytoplasmic pyridoxal 5'-phosphate), and a "stable" pool, which was independent of cytoplasmic fluctuations of pyridoxal 5'-phosphate (about 9 pmol pyridoxal 5'-phosphate/mg DNA). Reduction in pyridoxal 5'-phosphate content in the cells of vitamin B-6-deficient animals was accompanied by a substantial increase in 1,25-dihydroxyvitamin D-receptor ligand concentration in the cell nuclei (76.6 +/- 19.7 vs 762 +/- 291 fmol/mg DNA, mean +/- SEM). The degree of 1,25-dihydroxyvitamin D accumulation in the nuclei appeared to be an exponential function of the "mobile" nuclear pyridoxal 5'-phosphate concentration. Semilogarithmic transformation of the data yielded a straight line, representing an inverse correlation between the cytoplasm-related nuclear pool of pyridoxal 5'-phosphate and the logarithm of the 1,25-dihydroxyvitamin D concentration in the nuclei (r = -0.95). These data suggest that pyridoxal 5'-phosphate may be related to 1,25-dihydroxyvitamin D retention in the nuclei, possibly through interaction of the pyridoxal 5'-phosphate with the vitamin D receptor protein in the nuclei.

High-performance liquid chromatographic determination of 3-hydroxykynurenine with fluorimetric detection; comparison of preovulatory phase and postovulatory phase urinary excretion

A high-performance liquid chromatographic assay for 3-hydroxykynurenine in human urine is described. A fluorescent derivative of 3-hydroxykynurenine was prepared, based on the reaction of the compound with p-toluenesulphonyl chloride in a basic medium. The analytical method for the measurement of the fluorescent compound employed a Tosoh ODS 80 column eluted with 10 mM potassium dihydrogenphosphate (pH 4.5) and acetonitrile (3:2, v/v) and detection at an excitation wavelength of 375 nm (10 nm bandpass) and an emission wavelength of 455 nm (10 nm bandpass). The column temperature was maintained at 25 degrees C. The detection limit was 3 pmol (673 pg) at a signal-to-noise ratio of 5:1. The fluorescent derivative of 3-hydroxykynurenine was eluted at ca. 12.5 min. The technique was applied to the analysis of human urine. The total analysis time was ca. 15 min.

Tryptophan metabolism in vitamin B6-deficient mice

Vitamin B6 deficiency was induced in mice by maintenance for 4 weeks on a vitamin B6-free diet. Tryptophan metabolism was assessed by determining the urinary excretion of tryptophan metabolites, the metabolism of [14C]tryptophan in vivo and the formation of tryptophan and niacin metabolites by isolated hepatocytes. The vitamin B6-deficient animals excreted more xanthurenic acid and 3-hydroxykynurenine, and less of the niacin metabolites N1-methyl nicotinamide and methyl-2-pyridone-4-carboxamide, than did control animals maintained on the same diet supplemented with 5 mg vitamin B6/kg. After intraperitoneal injection of [14C]tryptophan, vitamin B6-deficient mice showed lower liberation of 14CO2 from [methylene-14C]tryptophan and [U-14C]tryptophan than did controls, indicating impairment of kynureninase (EC 3.7.1.3) activity. There was no difference between the two groups of animals in the metabolism of [ring-2-14C]tryptophan. Hepatocytes isolated from the vitamin B6-deficient animals formed more 3-hydroxykynurenine and xanthurenic acid than did cells from control animals, but also formed more NADP and free niacin.

Effects of vitamin B6 deficiency and repletion on the uptake of steroid hormones into uterus slices and isolated liver cells of rats

1. In vitro, pyridoxal phosphate extracts steroid-hormone receptors from tight nuclear binding (Cidlowski & Thanassi, 1981); in vitamin B6-deficient rats there is increased and prolonged nuclear accumulation of oestradiol in the uterus and testosterone in the prostate, associated with enhanced biological responsiveness of these target tissues to steroid hormone action (Symes et al. 1984; Bowden et al. 1986). 2. Slices of uterus from vitamin B6-deficient rats accumulated more [3H]oestradiol than did tissue from repleted animals. Acute repletion with vitamin B6 (0.5-1 h before killing) further increased the uptake of the steroid. 3. Isolated hepatocytes from vitamin B6-deficient rats accumulated more [3H]dexamethasone than did cells from repleted animals. Pre-incubation of the hepatocytes with pyridoxal phosphate resulted in a further increase in the uptake of the steroid. 4. The results suggest that in addition to the putative role of pyridoxal phosphate in releasing steroid-hormone-receptor complexes from tight nuclear binding (Cidlowski & Thanassi, 1981), vitamin B6 deficiency may also increase the concentration of steroid-hormone receptors or enzymes and other steroid-binding proteins in target tissues.

Effect of zinc and/or pyridoxine deficiency upon oestrogen retention and oestrogen receptor distribution in the rat uterus

A deficiency of vitamin B6 has been reported to enhance oestrogen responsiveness of the uterus in rats whereas zinc deficiency provokes a syndrome suggestive of a diminution in oestrogen sensitivity. In this study [3H]oestrogen uptake by the uterus was increased in rats deficient in either nutrient and the differences were additive in the dually deficient animals. The total number of oestrogen receptors per g tissue was unaffected by either nutrient but the proportion of the receptors recovered from the nuclear fraction increased from about 6 to 74% when both nutrients were withheld. The results are consistent with the hypothesis that both zinc and pyridoxal phosphate play important metabolic roles in end-organ responsiveness to oestrogen.