Pyridoxal 5'-phosphate and pyridoxamine 5'-phosphate concentrations in blood and tissues of mice fed ethanol-containing liquid diets

Prevalence and mechanisms of hyperhomocysteinemia in chronic alcoholics

BACKGROUND

Homocysteine (Hcy) is formed as an intermediary in methionine metabolism. Impairment of Hcy remethylation or transulfuration leads to hyperhomocysteinemia, which is considered as a risk factor for atherosclerotic vascular disease and stroke in chronic alcoholics. The aim of the study was to investigate the prevalence of hyperhomocysteinemia in chronic alcoholics and the influence of alcohol consumption, vitamin deficiencies and liver damage on the plasma levels of Hcy.

METHODS

228 chronic alcoholic patients consecutively admitted for detoxication, classified according to clinical and biochemical data in normal liver (n = 117), and in mild to moderate liver disease (n = 111), and 49 healthy controls were studied. Blood levels of Hcy, vitamin B6, vitamin B12 and folate were measured.

RESULTS

Plasma Hcy was significantly higher in chronic alcoholics than in controls (9.66 +/- 8.1 vs. 6.93 +/- 2.33 mumol/liter, p < 0.025). Furthermore, plasma Hcy levels were significantly higher in chronic alcoholics with liver injury (12.17 +/- 10.14 mumol/liter) than in those with normal liver and in controls (p < 0.001). The prevalence of hyperhomocysteinemia was also significantly higher in alcoholics with liver damage than in those with normal liver and in controls (29.7%, 5.1%, and 2%, respectively, p < 0.001). Serum folate values were lower in chronic alcoholics than in controls (4.7 +/- 2.6 vs. 7.6 +/- 2.4 nmol/liter, p < 0.001). The lowest values of folate were found in alcoholics with liver disease, especially in those with hyperhomocysteinemia, with a negative correlation between the two parameters.

CONCLUSIONS

Moderate hyperhomocysteinemia is common in chronic alcoholics, mainly in those with liver damage, suggesting that, although folate deficiencies may have a contributory role, liver impairment, through changes in methionine metabolism, is the most important mechanism for the elevated plasma Hcy found in these patients.

Response of kynurenine pathway enzymes to pregnancy and dietary level of vitamin B-6

The kynurenine pathway of tryptophan metabolism produces several neuroactive metabolites including 3-hydroxykynurenine, kynurenic acid, and quinolinic acid. This pathway is sensitive to reductions in vitamin B-6 availability because two key enzymes, kynurenine aminotransferase (KAT) and kynureninase (KYNase), require pyridoxal 5'-phosphate. During pregnancy abnormal concentrations of kynurenine metabolites are also found. We measured the effects of pregnancy and vitamin B-6 availability on KAT and KYNase in liver. DBA/2Ibg and A/Ibg mice were fed diets containing 0.25, 0.5, 2.0, 3.6, or 7.0 mg/kg pyridoxine-HCl (PN-HCl) for 4 weeks. Mitochondrial KAT and cytosolic KYNase were measured in control mice and pregnant mice on gestational days 16-18. The response of the two inbred strains was similar throughout. There were no marked alterations in KAT activity as a function of diet or pregnancy. In contrast, KYNase activities were significantly reduced by dietary restriction of vitamin B-6, and pregnant mice had significantly lower activity than nonpregnant controls for all but the highest dietary level of PN-HCl. These data show that pregnancy has a more pronounced effect on KYNase activity than vitamin B-6 restriction, and that the effects of pregnancy and diet are additive. The alteration in the kynurenine pathway in pregnancy is due to a reduction in KYNase activity, which is resistant to alleviation by vitamin B-6 supplementation.