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The Influence of Dietary Restriction on Vitamin B-6 Vitamer Distribution and on Vitamin B-6 Metabolizing Enzymes in Rats

Laboratory of Nutrition, School of Nursing, National Yang-Ming University, Taipei, TAIWAN

Address reprint requests to: Ien-Lan Wei, PhD, Laboratory of Nutrition, School of Nursing, National Yang-Ming University, Taipei 112, TAIWAN

	ABSTRACT

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Objective: The purpose of this study was to assess the effect of dietary restriction on tissue distribution of vitamin B-6 vitamers and activities of vitamin B-6 metabolizing enzymes in rats.

Methods: Male rats were subjected to a 40% dietary restriction for 10, 20 or 40 weeks. The tissue vitamin B-6 vitamer concentrations and activities of the vitamin B-6 metabolizing enzymes of the animals were determined.

Results: The plasma pyridoxal 5'-phosphate (PLP) concentrations of the diet-restricted (DR) rats were comparable to those of the control group at week ten but were significantly lower at weeks 20 and 40. These significantly lower levels of plasma PLP in DR rats might in part be related to lower hepatic pyridoxal kinase and pyridoxamine (pyridoxine) 5'-phosphate oxidase activities. The urinary 4-pyridoxic acid excretion of the DR groups responded to the reduced food intake and were lower at weeks 10 and 20. Tissue levels of PLP were not affected by dietary restriction. In contrast, greater levels of pyridoxamine 5'-phosphate were found in liver, kidney and heart of the DR animals.

Conclusion: The duration of dietary restriction influenced the distribution of vitamin B-6 vitamers. When plasma PLP is used to evaluate vitamin B-6 status, the length of dietary restriction should be considered.

Key words: dietary restriction, vitamin B-6, tissues, rats

	INTRODUCTION

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Dietary restriction has been shown to increase mean and maximum life span as well as to decrease age-related diseases in animals [1–4]. Lower incidences of chronic nephropathy [5–7] and tumorigenesis [8–10] have been reported in diet-restricted rats. Increased immune responses after exogenous stimulation have also been demonstrated in the diet-restricted rodents [3,5]. The beneficial effects of dietary restriction have been the main concern of previous studies. The nutritional status of restricted animals, however, has rarely been addressed. Studies have shown that protein synthesis and protein turnover are influenced by dietary restriction [11,12]. The coenzyme form of vitamin B-6, pyridoxal 5'-phosphate (PLP), participates in many biochemical processes, especially protein metabolism [13]. It is of interest whether dietary restriction also affects the vitamin B-6 metabolism.

Two types of dietary manipulation were observed in previous dietary restriction studies. The restricted animals in some studies [1–3,5,9] were fed a diet enriched with vitamins and minerals. The total energy intake of restricted animals was reduced compared to the ad libitum control, but the vitamins and minerals were not. Other studies [4,6–8,10] report feeding the restricted and control animals the same non-enriched diet. In these studies, the intake of calories and other nutrients were all reduced in restricted animals. The nutritional status of the restricted animals in previous studies would be different, depending upon the diet regimen used.

The present study focused on the effects of dietary restriction and varying durations of dietary restriction on vitamin B-6 metabolism. A non-enriched diet regimen was used. Male rats were subjected to a 40% diet restriction for 10, 20 or 40 weeks. The tissue vitamin B-6 vitamer concentrations and activities of the vitamin B-6 metabolizing enzymes of the animals were determined.

	MATERIALS AND METHODS

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Sprague-Dawley male rats (Experimental Animal Center of National Yang-Ming University, Taipei, Taiwan), 35 days old, weighing 100 to 150 g, were randomly divided into two groups. The rats of the control group were allowed free access to water and an AIN-76A diet (ICN Biochemicals, Cleveland, OH) [14]. The diet-restricted (DR) group consumed the same diet, but the amount was restricted to 60% of the average food intake of the control rats. Animals were housed individually in stainless steel cages and in a room maintained at constant temperature (23°C) and humidity (50%) with alternating twelve-hour periods of light and dark. The animal-care procedures followed were based on the established guidelines for Animal Care and Use of the Committees of National Science Council, Taiwan, Republic of China. The food intakes of the animals were recorded daily, and their body weights were measured once a week.

After 10, 20 or 40 weeks, rats were placed in individual metabolic cages in order to collect 24-hour urine specimens for determination of 4-pyridoxic acid (4-PA). Animals were deprived of food overnight and then killed by decapitation. Blood was collected in ethylenediaminetetraacetate-coated tubes. Plasma was stored at -30°C for analysis of B-6 vitamers. Liver, brain, kidney, heart and gastrocnemius muscle were immediately excised and weighed. The tissue samples for enzyme analysis were prepared following the procedure previously described [15]. In brief, the tissue homogenates, homogenized in an ice-cold sucrose solution, were centrifuged at 4°C for one hour at 100,000xg. The supernatants were saved for later determination of pyridoxal kinase (PL kinase; EC 2.7.1.35) and pyridoxamine (pyridoxine) 5'-phosphate oxidase [PMP (PNP) oxidase; EC 1.4.3.5] activities. PL kinase activity was measured according to the modified procedure of Merrill et al. [16]; each 1.2 mL assay solution contained 50 µL tissue supernatant, 20 mM potassium phosphate buffer (pH 7.0), 0.5 mM neutral ATP, 60 µM KCl, 0.2 mM ZnCl₂ and 4 µM pyridoxal. The PMP (PNP) oxidase activity was measured by following the procedure of Berg et al. [17]. The enzyme reaction was terminated by addition of 1.25 mol/L perchloric acid. After mixing and centrifugation, the PLP in the clear supernatant was determined by high-performance liquid chromatography (HPLC).

Preparation of tissue and plasma samples for HPLC determination of B-6 vitamers was based on the method of Furth-Walker et al. [18]. Urine samples were first centrifuged and then diluted with an appropriate amount of water. The HPLC system consisted of a solvent delivery system (Waters model 501, Millipore, Milford, MA), an automatic sampler (Waters 717), a data module (Waters 745B), a scanning fluorescence detector (Waters 470), comprising a 5-µL flow cell with the excitation wavelength set at 325 nm and the emission wavelength at 400 nm, and a solvent delivery pump (Waters model 501, Millipore, Milford, MA) with noise suppressor for delivery of the post-column reagent. A Waters 10-µm particle size, C18 µBondapak (3.9x300 mm) reverse-phase analytic column was used. The chromatographic conditions followed the method of Edwards et al. [19], but the flow rate was kept at 1 mL/min. Protein was determined by the method of Lowry et al. [20].

Data were analyzed by using the SPSS/PC+ statistics computer program (SPSS Inc., Chicago). All variables were evaluated by using a 2x3 factorial design of analysis of variance with the factors diet (free access, diet-restricted) and time (10, 20 or 40 weeks) [21]. The means within each of the two diet groups were compared by Scheffe’s multiple range test. The means between the two diet groups in each time period were examined using Student’s t test. A p less than 0.05 was considered statistically significant.

	RESULTS

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Body Weight, Food-Efficiency Ratio and Relative Vitamin B-6 Intake
The body weight, food efficiency ratio and relative vitamin B-6 intake of the animals are shown in Table 1. The initial body weights of the DR groups were similar to those of their respective control groups. At the end of each time period, the DR group weighed significantly less than the control group. The final body weights of DR animals were about 60% of those of the control rats. The DR groups had significantly lower food-efficiency ratios than the control groups at all time periods. In the control groups, there was a 40% decrease in the food-efficiency ratio between weeks 10 and 20 as well as between weeks 20 and 40. The DR groups had a similar reduction in food-efficiency ratios during the same time periods. The amount of vitamin B-6 intake per gram body weight (relative vitamin B-6 intake) of DR animals was 8% (p<0.05) lower than that of control rats at week ten. However, at 20 and 40 weeks, the relative vitamin B-6 intake of DR rats was comparable to that of control rats. As the animals grew older, a decrease in relative vitamin B-6 intake was observed in both the control and DR groups.

For urinary 4-PA excretion, the DR groups were significantly lower than the control groups at weeks 10 and 20. A time effect was not observed in either the control or DR groups.

Vitamin B-6 Vitamer Concentration in Tissues
The levels of PLP, PMP and PLP+PMP in liver, brain, kidney, heart and muscle are presented in Table 3. Tissue PLP concentrations were not different between the control and DR groups. Within the DR groups, the PLP levels in the liver increased (p<0.05) with increasing duration of restriction; however, the PLP concentrations in the kidney progressively decreased (p<0.05) from week 10 to 40. A parallel decrease in kidney PLP levels of control rats suggested that this reduction in kidney PLP concentration was probably related to age. This age-related decrease in kidney B-6 vitamer concentration was also observed by Bode et al. [22].

In the control groups, the kidney PMP level at week 40 was significantly lower than at week 10 and week 20. This trend in kidney PMP concentrations was also observed in DR groups. In muscle tissue, the PMP concentrations of control groups increased 47% from week 20 to 40. However, the muscle PMP levels of DR rats remained unchanged throughout the dietary restriction periods. Brain PMP concentrations were not affected by either dietary treatment or time.

The concentrations of PLP+PMP in liver, brain and muscle were similar in the control and DR animals. In the kidney and heart, the PLP+PMP concentrations were greater in DR animals than in controls.

Vitamin B-6 Metabolizing Enzymes
Activities of PL kinase and PMP(PNP) oxidase of control and DR rats were determined in liver, brain, kidney, heart and muscle. Dietary restriction had significant effect only on liver PL kinase and PMP(PNP) oxidase activities (Table 4). DR groups exhibited significantly lower PL kinase and PMP(PNP) oxidase activities than control groups at all time periods. Hepatic PL kinase activity increased 30% in control rats between 20 and 40 weeks, whereas the hepatic PL kinase activity of DR groups remained unchanged during all time periods. Hepatic PMP(PNP) oxidase activities were low at week 20 for both the control and DR groups.

	DISCUSSION

Plasma PLP is considered to be an indicator of vitamin B-6 status [13,23]. The levels of plasma PLP are influenced by dietary intake of vitamin B-6 [24,25] as well as protein [26,27]. With an increase in dietary vitamin B-6 intake, plasma PLP concentration increases, whereas an increase in protein intake decreases plasma PLP concentration. In the present study, the DR groups consumed 40% less food than the control groups. The absolute intake of vitamin B-6 as well as protein was 40% lower in DR rats than in the control animals. The plasma PLP levels of the DR animals, however, were not in concert with their dietary vitamin B-6 or protein intake, indicating that the reduction in total food intake also influenced plasma PLP levels. This change in plasma PLP levels in response to the reduction in food intake was not immediate. The plasma PLP levels of the DR animals did not differ from those of the control group at week ten. When the duration of dietary restriction extended to 20 and 40 weeks, the plasma PLP levels of DR rats were lowered. It appeared that the changes in plasma PLP concentrations in response to reduced food intake depended upon the length of dietary restriction.

Lumeng et al. [28] reported that levels of plasma PLP result from the dynamic equilibrium between hepatic synthesis and tissue extraction and/or degradation. The DR animals in the present study were able to maintain comparable levels of plasma PLP for 10 weeks probably through a reduction in body turnover of vitamin B-6. The major metabolite of vitamin B-6 is 4-PA. The urinary 4-PA excretion of DR animals at week 10 was significantly reduced to 38% of the control levels. Such reduction in urinary 4-PA excretion allowed the body to minimize the changes in the body’s overall pool of vitamin B-6. However, the DR animals’ effort to maintain plasma PLP levels was effective for only ten weeks.

The liver is the primary source of plasma PLP [28]. The hepatic enzymes PL kinase and PMP(PNP) oxidase are responsible for the formation of PLP [13]. The significantly lower levels of plasma PLP found at weeks 20 and 40 in the DR animals by the present study might partly result from significantly lower hepatic PL kinase and PMP(PNP) oxidase activities. The low hepatic PL kinase activity of the DR groups could be the result of the combined effects of reduced food and vitamin B-6 intake. Different responses of hepatic PL kinase activity to changes in vitamin B-6 intake have been reported. Some studies have shown that the activities of hepatic PL kinase were lower in vitamin B-6 deficient animals [29,30]. Others found no difference in liver PL kinase activity during vitamin B-6 deficiency [17,31]. Berg et al. [17] suggested that the differences between the reported responses of PL kinase activity to vitamin B-6 deficiency might be due to the differences in strains of rats. Results of the present study, however, demonstrated that other factors, such as food intake of the animals and duration of the diet restriction might also affect the activity of PL kinase.

The liver PMP(PNP) oxidase activity is not sensitive to changes in vitamin B-6 intake [17,29]. Consequently, it appeared that the reduced hepatic PMP(PNP) oxidase activity of the DR animals in the present study was not related to the lower intake of vitamin B-6. The enzyme PMP(PNP) oxidase is a flavoprotein [32]. Prompt responses of liver PMP(PNP) oxidase to changes in dietary riboflavin have been reported [33,34]. Hence, the low intake of riboflavin, a result of reduced food intake by the DR animals, might be responsible for the lower hepatic PMP(PNP) oxidase activity found in the present study.

In addition to lower hepatic-enzyme activity, significantly greater levels of PMP in the kidney, liver and heart might also have contributed to the reduction in plasma PLP levels at weeks 20 and 40 of the DR animals in the present study. The mechanisms responsible for these greater levels of PMP in the tissues of DR animals could not be determined by the present study. However, there are two possibilities. First, the equilibrium between the PLP and PMP might be shifted toward formation of PMP in the liver, kidney and heart of DR animals. Lui et al. [35] demonstrated that when isolated rat-liver mitochondria were incubated with glutamate, the PLP level was reduced and resulted in an increase in PMP concentration. In the same study, when 2-oxoglutarate was used as substrate, a reduction in PMP concentration and an elevation in PLP levels took place. During food deficit, the uptake of alanine and glutamine by the splanchnic exceeds the uptake of other amino acids [36]. The changes in substrates could cause a shift in transamination reactions and favor the formation of PMP. Second, the lower hepatic PMP(PNP) oxidase activity of the DR animals might also contribute to the elevation in hepatic PMP concentrations. The PMP(PNP) oxidase catalyzes the reaction during the conversion of PMP and PNP to PLP. Besides this function, Kazarinoff and McCormick [37] suggested that hepatic PMP(PNP) oxidase might be responsible for maintaining the concentration of PLP when transamination substrates favor the formation of PMP. The low hepatic PMP(PNP) oxidase activity associated with the greater levels of liver PMP in the DR animals of the present study supports the views of Kazarinoff and McCormick [37]. Accumulation of liver PMP has been suggested to cause the inhibition of tumor growth during vitamin B-6 deficiency [38]. A more complete understanding of the functional effects of the greater levels of PMP in the liver, kidney and heart of the DR animals requires further study.

A previous study [23] showed that the plasma PLP reflected the tissue content of PLP in animals fed with graded levels of vitamin B-6. However, in the present study, plasma PLP did not reflect the tissue PLP levels of 20- and 40-week diet-restricted animals. Regulatory control mechanisms for PLP concentrations have been observed in liver. The concentration of liver PLP is regulated through the mechanisms of protein binding and degradation [39]. Although the regulation of vitamin B-6 metabolism in brain, kidney and heart is less known, control mechanisms for regulation PLP concentration might be responsible for the comparable levels of PLP in tissues of DR animals.

Muscle contains the largest pool of vitamin B-6 in the body [13]. Most of the vitamin B-6 in muscle is in the form of PLP associated with glycogen phosphorylase [13]. Muscle PLP is resistant to depletion [40,41]. Only starvation causes the release of muscle PLP [40]. In the present study, the muscle PLP levels of DR animals were found similar to those of control rats, indicating that 40% dietary restriction exerted no effect on muscle PLP. In contrast to muscle PLP, PMP levels of DR animals were significantly lower than those of control rats at week 40. The trend with respect to muscle PMP over the 40-week experimental period was different in the control and DR groups. The muscle PMP levels of control groups increased from week 10 to week 40, whereas the muscle PMP levels of the DR groups remained unchanged during the same period. The differences in muscle PMP levels between the control and DR animals seemed to reflect that the DR animals did not accumulate muscle PMP with time, but rather that muscle PMP was released. This was supported by the evidence that muscle PMP+PLP concentrations did not vary between the control and DR groups.

Coburn [42] suggested that the nutrient requirements of diet-restricted animals are reduced compared to non-diet-restricted animals because restricted animals are smaller. In the present study, the amount of vitamin B-6 intake per gram of body weight (relative vitamin B-6 intake) of DR animals was 8% (p<0.05) lower than that of control rats at week ten. The plasma and tissue levels of vitamin B-6 were comparable between the control and DR animals at week ten. When the duration of dietary restriction increased to 20 and 40 weeks, the relative vitamin B-6 intake was not different between the control and DR groups. The PLP+PMP concentrations in the tissues of DR animals were similar to or greater than those of control rats. These results supported the proposition of Coburn [42] and indicated that the vitamin B-6 status of DR animals was not compromised by dietary restriction. However, plasma PLP concentrations of DR animals at weeks 20 and 40 did not reflect the tissue levels of vitamin B-6 and were significantly lower. This observation raises the question of the appropriateness of using plasma PLP as a vitamin B-6 status indicator during extended periods of dietary restriction.

In conclusion, in the present study, the bodies of DR rats maintained normal levels of vitamin B-6 in plasma and tissues by reducing urinary 4-PA excretion during 10-week dietary restriction. When the duration of dietary restriction extended to 20 and 40 weeks, the distribution of tissue vitamin B-6 vitamers changed. The concentrations of tissue PLP in DR animals were maintained at weeks 20 and 40, whereas the plasma PLP levels were lowered. These results suggest that plasma PLP should not be used as a vitamin B-6 status indicator during long periods of dietary restriction. Greater levels of PMP were observed in the liver, kidney and heart of DR animals. The functional effect of this elevated PMP resulting from dietary restriction remains to be studied.

	ACKNOWLEDGMENTS

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This work was supported by the National Science Council of Taiwan, R.O.C., under grant number NSC83-0412-B010-55.

Received August 1, 1998. Accepted October 1, 1998.

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