Effect of vitamin B-6 nutrition and diabetes on vitamin B-6 metabolism

Direct and Functional Biomarkers of Vitamin B6 Status

Measures of B6 status are categorized as direct biomarkers and as functional biomarkers. Direct biomarkers measure B6 vitamers in plasma/serum, urine and erythrocytes, and among these plasma pyridoxal 5'-phosphate (PLP) is most commonly used. Functional biomarkers include erythrocyte transaminase activities and, more recently, plasma levels of metabolites involved in PLP-dependent reactions, such as the kynurenine pathway, one-carbon metabolism, transsulfuration (cystathionine), and glycine decarboxylation (serine and glycine). Vitamin B6 status is best assessed by using a combination of biomarkers because of the influence of potential confounders, such as inflammation, alkaline phosphatase activity, low serum albumin, renal function, and inorganic phosphate. Ratios between substrate-products pairs have recently been investigated as a strategy to attenuate such influence. These efforts have provided promising new markers such as the PAr index, the 3-hydroxykynurenine:xanthurenic acid ratio, and the oxoglutarate:glutamate ratio. Targeted metabolic profiling or untargeted metabolomics based on mass spectrometry allow the simultaneous quantification of a large number of metabolites, which are currently evaluated as functional biomarkers, using data reduction statistics.

Type 1 diabetes impairs vitamin B(6) metabolism at an early stage of women's adulthood

Vitamin B(6) (pyridoxine) metabolism in diabetes has never been investigated except for a few reports on plasma pyridoxal 5'-phosphate (PLP). These studies indicated that this most active (coenzyme) vitamer can be reduced. The present clinical investigation aimed to measure all vitamers in blood and urine by high performance liquid chromatography as well as important related factors, in women during active reproductive years. Thirty-two insulin-treated type 1 diabetic (T1D) patients, without renal complication, and 27 well-matched healthy controls, aged 30 to 40 years old, were recruited using rigorous criteria. Both groups had normal hemoglobin and serum albumin levels. Plasma PLP and pyridoxal (PL) did not differ significantly in the T1D group but alkaline phosphatase (ALP) activity was greater (p < 0.01). This produced a shift in plasma PLP-PL profile, as evidenced by a lower plasma PLP/PL ratio (p < 0.05). Enhanced ALP activity meant more PLP being dephosphorylated to PL (the membrane transfer form), with more ending up in erythrocytes to be rephosphorylated in its active form, as suggested by the significant positive correlation (p < 0.001) between plasma PL and erythrocyte PLP. More PL into blood circulation also means more oxidation of this vitamer to 4'-pyridoxic acid in kidneys, as confirmed by the positive correlation between plasma PL and urinary 4'-pyridoxic acid (p < 0.001). The positive correlation (p < 0.001) between ALP activity and glycosylated hemoglobin indicated a direct effect of the disease. The T1D-induced alteration in vitamin B(6) metabolism, consecutive to enhanced ALP activity, may put patients at greater risk of vitamin B(6) deficiency and diabetic complications.

Antioxidant activity of a Schiff base of pyridoxal and aminoguanidine

We recently reported that PL-AG, a Schiff base of pyridoxal and aminoguanidine, was more effective than aminoguanidine (AG), a well-known anti-diabetic-complication compound, in preventing nephropathy in diabetic mice and presented brief data indicating the antioxidant activity of the adduct. In the present study, we additionally investigated the inhibitory activity of PL-AG in comparison with that of AG against in vitro and in vivo oxidation. PL-AG was more potent than AG and reference compounds such as pyridoxal and pyridoxamine in any of the five antioxidant activities examined in vitro, i.e., hydrogen peroxide-scavenging, hydroxyl radical-scavenging, superoxide radical-scavenging, ascorbic acid-autoxidation inhibitory, and low-density lipoprotein (LDL)-oxidation inhibitory activities, the last two of which were assessed in the presence of Cu(2+). Unlike AG, PL-AG did not show the pro-oxidant activity. The inhibitory activity of PL-AG against lipid peroxidation in diabetic rats was higher than that of AG, for example, the amounts of malondialdehyde in erythrocytes (nmol/g hemoglobin; mean +/- SD) in normal, untreated diabetic, AG-treated diabetic, and PL-AG-treated diabetic rats were 3.53 +/- 0.35, 4.99 +/- 0.23, 4.65 +/- 0.45, and 4.06 +/- 0.35, respectively. A fluorescent substance different from PL-AG was found in the plasma and urine of rats treated with PL-AG. The chemical structure of this substance, i.e., oxidized PL-AG, was determined by a combination of nuclear magnetic resonance, mass, and infrared spectrometry. AG dramatically decreased the pyridoxal phosphate level in the diabetic rat liver, whereas PL-AG only moderately affected it. Our results indicate that the antioxidant activity of PL-AG is due to its chelation with transition metal ions and to scavenging of reactive oxygen species. They also suggest that PL-AG is more promising for the treatment of diabetic complications than AG.

Effect of diabetes on vitamin B6 requirement in experimental animals

AIM

In the diabetic state, energy must be supplied mainly by amino acids and fat; therefore the metabolic processes are very similar to those of animals fed a high-protein diet. Vitamin B6-dependent enzymes, which are highly involved in amino acid metabolism, are important in diabetics. We investigated vitamin B6 content, and aspartate aminotransferase and glycogen phosphorylase activities, in several tissues of streptozotocin-induced diabetic and control rats.

METHODS

The rats were fed a vitamin B6-free diet and administered an equivalent amount of pyridoxine based on body weight.

RESULTS

Vitamin B6 content in all tissues examined, except for the liver, was lower in the diabetics than in controls. Aspartate aminotransferase activity was higher in the liver of diabetics than in the controls, but not in the other tissues. Glycogen phosphorylase activity in the gastrocnemius muscle of diabetics decreased to two-thirds of the control level.

CONCLUSIONS

These data might indicate that diabetic animals should have a higher intake of vitamin B6 because a diabetic state can lead to a vitamin B6-deficiency.

Effect of pyridoxine and insulin administration on brain glutamate dehydrogenase activity and blood glucose control in streptozotocin-induced diabetic rats

Blood glucose level and kinetic parameters of glutamate dehydrogenase (GDH) were measured in the cerebellum, brain stem and cerebral cortex of control, insulin treated, pyridoxine treated, pyridoxine and insulin treated and untreated streptozotocin-diabetic rats. The combined administration of insulin and pyridoxine was found to be better in controlling the hyperglycaemia. Insulin with pyridoxine treatment brought back the increased maximal velocity of GDH during diabetes to control state. Also, there was an increase in Michaelis-Menten constant. These results suggest that pyridoxine and insulin together serve a better control for diabetes.