Depressed folate incorporation into milk secondary to iron deficiency in the rat

The present study was designed to determine whether reduced folate incorporation into milk can account for folate depletion of iron-deficient suckling rats. Dams were fed diets containing 2 mg/kg folate and either 8, 12 or 250 mg/kg iron throughout gestation and lactation to produce severely iron-deficient, moderately iron-depleted and iron-sufficient states in 17-d-old pups (n = 15 litters/group). On d 17 of lactation, dams were separated from litters and given intraperitoneal injections of [3',5',7,9-3H]pteroylmonoglutamic acid ([3H]PteGlu) or physiological saline. Mean [3H]PteGlu incorporation into milk of severely iron-deficient dams was 67% of that in iron-sufficient controls, while "total" and "free" milk folate activities were 54 and 61%, respectively. Values for milk [3H]PteGlu incorporation and folate activities were intermediate in moderately iron-depleted dams. Pup red blood cell folate activity was positively correlated with both free (r = 0.43, P = 0.004) and total (r = 0.37, P = 0.015) milk folate activities. Mean plasma folate activities of severely and moderately iron-deficient pups were 68 and 86% of control values, respectively. Results show that in both mild and severe iron deficiency, reduced folate secretion into milk is at least partially responsible for impaired folate status of suckling pups.     

Some nutritional effects of folate-binding protein in bovine milk on the bioavailability of folate to rats

The excretions of folate compounds into both the urine and bile were investigated in rats after the administration of pteroylglutamic acid (PteGlu) with or without the folate-binding protein (FBP) prepared from bovine milk. When the sample solution, containing either free or bound [3H]PteGlu (i.e., bound to the FBP from milk), was delivered to rats intragastrically via oral intubation, the amounts of [3H]PteGlu excreted into the feces did not change. On the other hand, the urinary excretion of 3H-labeled folate compounds, especially [3H]5-methyltetrahydrofolic acid (5-CH3-H4PteGlu), after the administration of bound [3H]PteGlu was significantly lower (P less than 0.01) than that after the administration of free [3H]PteGlu. The urinary excretion of [3H]5-CH3-H4PteGlu was directly proportional to the initial amount of free [3H]PteGlu administered. The similar effect of FBP was also observed when the biliary excretion of 3H-labeled folate compounds was investigated in situ. Furthermore, the incorporation of [3H]PteGlu into folate-requiring intestinal microorganisms was considerably reduced when it was bound to FBP. These results suggest that milk FBP has some nutritional effects on the bioavailability of folate in vivo.     

The absorption of pteroylpolyglutamate and intestinal pteroylpolygammaglutamyl hydrolase activity in zinc-deficient rats

The effect of dietary zinc deficiency on pteroylpolygammaglutamyl hydrolase (folate hydrolase) activity and on pteroylpolyglutamate absorption was studied in rats. Three groups of male Sprague-Dawley rats (zinc-deficient, restricted-fed and ad libitum-fed controls) were fed a semipurified 25% egg white protein diet. The zinc-deficient group received 0.7 mg zinc/kg diet, whereas restricted-fed and ad libitum-fed control groups received 106 mg zinc/kg diet. After 6 wk of feeding, intestinal mucosal folate hydrolase activity was determined, and the absorption of pteryl-U[14C]glutamylhexaglutamic acid [(14C]PteGlu7) and [3H]pteroylglutamic acid [(3H]PteGlu) was measured after intragastric administration. The intestinal mucosal folate hydrolase activity of zinc-deficient rats was not significantly reduced compared with two control groups. No significant differences in the absorption of [14C]PteGlu7 and [3H]PteGlu were found among the three groups. These results indicate that intestinal folate hydrolase is not zinc dependent in rats and the intestinal absorption of pteroylpolyglutamate is not reduced in zinc-deficient rats.     

Folate absorption in women with a history of neural tube defect-affected pregnancy

BACKGROUND: The risk of neural tube defects (NTDs) is significantly reduced by supplemental folic acid. NTD risk may be associated with impaired absorption of polyglutamyl folate, the primary form of naturally occurring food folate, and of folic acid in supplements or fortified food. Stable-isotope methods provide the specificity needed to test this hypothesis. OBJECTIVE: We determined whether women who had an NTD-affected pregnancy had a reduced ability compared with control women to absorb polyglutamyl folate relative to folic acid. DESIGN: Healthy, nonpregnant women with a history of an NTD-affected pregnancy (cases; n = 11) and control women (n = 11) were administered an oral dose containing a mixture of [(2)H]pteroylpentaglutamate ([(2)H(2)]PteGlu(5); 233 nmol) and [(13)C]pteroylmonoglutamate ([(13)C(5)]PteGlu(1); 567 nmol) after a 30-d saturation protocol (2 mg unlabeled folic acid/d). Relative extents of absorption were evaluated by urinary excretion of (2)H(2)- and (13)C(5)-labeled folates 48 h postdose. RESULTS: During the first 24 h postdose, cases excreted less (f1.gif" BORDER="0"> +/- SD) [(2)H(2)]PteGlu(5) (21 +/- 12% compared with 37 +/- 19%; P = 0.01) and [(13)C(5)]PteGlu(1) (17 +/- 8% compared with 31 +/- 14%; P = 0.007) than did controls. No significant differences between cases and controls were detected in the percentage of [(2)H(2)]PteGlu(5) or [(13)C(5)]PteGlu(1) excreted during the second 24 h postdose or when the data were averaged over 48 h. However, excretion of the [(2)H(2)]folates tended to be lower in cases than in controls over the 48-h period (33 +/- 13% compared with 45 +/- 26%; P = 0.21). A similar trend (P = 0.29) for lower excretion of [(13)C(5)]folates in cases was also observed (31 +/- 16% compared with 39 +/- 17%). The ratio of urinary [(2)H(2)]folates to [(13)C(5)]folates did not differ significantly between cases and controls. CONCLUSION: These data suggest the need for a larger-scale study using stable-isotope methods to further investigate this hypothesis.     

Aging: effect on hepatic metabolism and transport of folate in the rat

Effects of aging on hepatic folate metabolism and transport were assessed in male Fisher 344 rats. Total serum and hepatic folate levels were measured. Hepatic folates were measured by high-performance liquid chromatography and by Lactobacillus casei assay. Transport of 5-methyltetrahydrofolate (5-CH3-H4PteGlu) was measured in isolated hepatocytes. Serum folate declined with aging; however, neither the total folate level nor the distribution of hepatic folate coenzymes was affected by the aging process. The level of liver folate monoglutamates was not significantly different in any group. The initial rate of uptake of 5-CH3-H4PteGlu was significantly decreased in hepatocytes from the 24-mo-old rats, as was the ability to concentrate this folate from the medium. Aged rats maintain apparently normal levels of hepatic folates despite decreased serum levels and decreased ability to take up folates, suggesting that membrane transport of folates may not be a limiting factor in hepatic folate assimilation.     

Nitrous oxide provokes changes in folylpenta- and hexaglutamates

Liver folates isolated from rats fed a control diet consisted of H4PteGlu5 (44% of total) and 5-methyl-H4PteGlu5 (19%) with smaller amounts of 10-formyl-H4PteGlu5 (13%), H4PteGlu6 (13%), 10-formyl-H4PteGlu6 (4%) and 5-methyl-H4PteGlu6 (2%). Folates from methionine-deficient rats contained more nearly equal amounts, on a molar basis, of folylpenta- and hexaglutamates, and hence these were more suitable subjects for comparing the metabolism of these compounds in vivo. Livers from methionine-deficient rats contained H4PteGlu5 (30%), 5-methyl-H4PteGlu5 (13%), 10-formyl-H4PteGlu5 (10%), H4PteGlu6 (22%), 5-methyl-H4PteGlu6 (9%) and 10-formyl-H4PteGlu6 (11%). With exposure to N2O, 5-methyl-H4PteGlu5 and 6 increased to 27% and 13%, respectively, whereas H4PteGlu5 and 6 decreased to 20% and 17%, respectively. Nitrous oxide (N2O) perturbed both the penta- and hexaglutamates; the effect was somewhat more pronounced with the pentaglutamates. The partial depletion in tissue H4PteGlun with N2O treatment helps explain the concomitant inhibition of the oxidation of [ring-14C]histidine, an event dependent on tetrahydrofolates.     

Differential kinetic behavior and distribution for pteroylglutamic acid and reduced folates: a revised hypothesis of the primary site of PteGlu metabolism in humans

Single (13)C(6)-labeled doses of pteroylmonoglutamic acid (PteGlu: 634 nmol; n = 14), (6S-)5-formyltetrahydrofolic acid (431-569 nmol; n = 16), or [(15)N(1-7)]-intrinsically labeled spinach (mainly 5-methyltetrahydrofolate) (588 nmol; n = 14) were fed to fasting adult volunteers. Plasma-labeled 5-methyltetrahydrofolic acid responses were monitored for 8 h. There was a slower rate of increase in plasma-labeled 5-methyltetrahydrofolic acid and longer time to peak (171 +/- 9 min; mean +/- SEM) following an oral dose of [(13)C(6)]PteGlu than either [(13)C(6)]5-formyltetrahydrofolic acid (54 +/- 10 min) or [(15)N(1-7)]spinach folate (60 +/- 13 min) suggesting saturated metabolic capacity for the biotransformation of PteGlu. Mathematical modeling generated a significantly higher mean "apparent absorption" for 5-formyltetrahydrofolic acid (38%) and spinach folate (44%) than for PteGlu (24%). The high "relative absorption" of reduced folates to PteGlu was unexpected given that PteGlu itself, from (14)C-tracer mass balance experiments, is almost completely absorbed. Although it is ubiquitously accepted that a physiological dose of PteGlu is reduced and methylated in the epithelial cells of the small intestine, and that essentially only 5-methyltetrahydrofolic acid is exported into the hepatic portal vein (HPV), as is the case for absorbed reduced 1-carbon-substituted folates, modeling indicated greater liver sequestration when PteGlu was used as the test dose, suggesting that PteGlu enters the HPV unaltered and that the liver is the primary site of initial metabolism. Because of the observed differential plasma response and the hypothesized difference in the site of initial metabolism, the historical use of PteGlu as a "reference folate" in studies of folate bioavailability is seriously questioned.     

5-Methyltetrahydrofolate is reabsorbed and metabolized in isolated perfused rat kidney.

The renal regulation of folate excretion is an important component in maintaining the body burden of folate. The tubular processes for folate disposition have been examined by a variety of methods to elucidate the mechanism by which renal folate excretion is regulated. Accordingly, the isolated perfused rat kidney technique was evaluated by investigating the clearance and metabolic patterns of 5-methyltetrahydrofolate (5-CH3-H4PteGlu). Kidneys from male Sprague-Dawley rats were perfused in vitro with [3H]5-CH3-H4PteGlu (1-2000 nmol/L). Linear regression analysis of 5-CH3-H4PteGlu excretion vs. filtered load revealed a tubular transport maximum of 7.5 pmol x min-1.g-1. A dual component system for tubular transport of 5-CH3-H4PteGlu was found: a high capacity, nonsaturable system and a low capacity, saturable system represented by the transport maximum. Furthermore, HPLC analysis of urine demonstrated renal uptake and metabolism of the labeled tracer. Tetrahydrofolate was identified as one metabolic product that indicated secretion of this compound. Additional metabolites were identified from kidney samples. Results suggest that 5-CH3-H4PteGlu undergoes net reabsorption by a dual component transport system; some of the reabsorbed 5-CH3-H4PteGlu is metabolized to other products that may be secreted in the urine.     

Histidine-imbalanced diets stimulate hepatic histidase gene expression in rats.

A high protein concentration in the diet induces the gene expression of several amino acid degrading enzymes such as histidase (Hal) in rats. It is important to understand whether the amino acid pattern of the dietary protein affects the gene expression of these enzymes. The purpose of the present work was to study the effect of a histidine-imbalanced diet on the activity and mRNA concentration of rat hepatic histidase. Seven groups of six rats were fed one of the following diets: 1) 6% casein (basal), 2) 20% casein, 3) 35% casein, 4) an imbalance diet containing 6% casein plus a mixture of indispensable amino acids (IAA) equivalent to a 20% casein diet without histidine (I-20), 5) 6% casein plus a mixture of IAA equivalent to a 35% casein diet without histidine (I-35), 6) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 20% casein diet, 7) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 35% casein diet. Serum histidine concentration was inversely proportional to the protein content of the diet, and it was significantly higher in rats fed the corrected diets compared to their respective imbalanced diet groups. Hal activity increased as the protein content of the diet increased. Greater histidine imbalance resulted in lower food intake and higher Hal activity. Rats fed histidine-corrected diets had lower activity than their respective imbalanced groups. Differences in Hal activity were associated with differences in the concentration of Hal mRNA. These results indicate that rats fed a histidine-imbalanced diet exhibit reduced food intake and weight gain and increased Hal gene expression as a consequence of an increased amino acid catabolism.     

Relative bioavailability of deuterium-labeled monoglutamyl tetrahydrofolates and folic acid in human subjects.

The bioavailability of orally administered monoglutamyl folic acid and various (6S)-tetrahydrofolates was examined in humans with stable-isotope methods. Folic acid (PteGlu), tetrahydrofolate (H4folate), 5-formyl-H4folate, 10-formyl-H4folate, and 5-methyl-H4folate were prepared for oral administration in 3',5'-2H2 labeled (d2) form, and [glu-2H4]folic acid (d4-PteGlu) was prepared for intravenous injection. In each of five trials, fasting adult males (n = 7) on a folate saturation regimen (2 mg/d) were given a single oral dose of one of the d2-folates in apple juice, as well as an intravenous injection of d4-PteGlu as a control. Urine was collected for 48 h and the isotope labeling of urinary folates determined by mass spectrometry. Isotope excretion ratios of urinary folates were used as criteria of bioavailability (pooled SE = 0.10): PteGlu (1.53, least squares mean), 10-formyl-H4folate (1.02), 5-methyl-H4folate (0.99), 5-formyl-H4folate (0.1.13), and H4folate (0.71). These results indicate that differences exist in the bioavailability of monoglutamyl folates under these experimental conditions. This variation, whether due to differences in absorption or postabsorptive events, must be considered in quantitative studies of folate utilization with this type of protocol.     

Folate bioavailability in humans: effects of wheat bran and beans

The effect of wheat bran or California small white beans in the diet on absorption of monoglutamyl (PteGlu) and heptaglutamyl folic acid (PteGlu7) was studied in six men confined to a metabolic unit. Relative folate absorption was determined by measuring 24-h urinary folate excretion and serum folate levels at 0, 1, and 2 h after ingestion of a formula meal containing 1.13 mumol PteGlu or PteGlu7 (500 micrograms PteGlu equivalent). Serum data showed PteGlu absorption was more rapid than PteGlu7 absorption. Urinary excretion of PteGlu7 was 63% (50 less than or equal to mean less than or equal to 76%) of PteGlu excretion. Addition of 30 g wheat bran to the formula meal accelerated PteGlu absorption whereas PteGlu7 absorption was not significantly affected by either food. Effects of the two foods were qualitatively different. Wheat bran increased the absorption of PteGlu relative to PteGlu7 whereas beans minimized the difference between PteGlu and PteGlu7 serum areas.     

Comparison of dihydrofolate reductase activities for folic acid in pigs and rats using in vivo and in vitro evaluation techniques

The activity of dihydrofolate reductase (DHFR) for folic acid (PteGlu) was evaluated in pigs by in vivo and in vitro experiments. The results were compared with those of rats. Since bile secretion of reduced folates reflects the activity of DHFR for PteGlu in the body, the bile secretion rates of reduced folates including tetrahydrofolate (H4PteGlu), 5-methyltetrahydrofolate, 5,10-methylenetetrahydrofolate, and 10-formyltetrahydrofolate were determined by using high-performance liquid chromatography with electrochemical detection, after the intravenous injection of PteGlu at 1 mg/kg body weight to pigs and rats. Although the PteGlu injection raised the total secretion rate of reduced folates. the total increased amount of reduced folates secreted into bile from 0 h to 2.5 h after PteGlu injection in pigs was about one-tenth of that in rats. The enzyme kinetics of DHFR for PteGlu was examined at the physiological condition (pH 7.4 and 3 7 degrees C). Affinity chromatography was applied to liver homogenates of pigs and rats to obtain DHFR. The final product of the enzyme reaction, H4PteGlu, was measured. The Km for pig enzyme was similar to that for rat enzyme, whereas the Vmax for the pig enzyme was less than 1/5 of that for the rat's. The comparison of the ratio of Vmax to Km between pig and rat enzymes suggests that PteGlu is a much less efficient substrate for pig liver DHFR. In short, these results from in vivo and in vitro experiments suggest that the role of DHFR for PteGlu in pigs is physiologically much less important than that in rats.     

The uptake of [3H] pteroylglutamic acid into rat liver during regeneration and its incorporation into pteroylpolyglutamates of that organ

The metabolism of injected [3H] PteGlu in hepatectomized rats was studied. Significantly higher radioactivity was found in all folate derivatives of regenerating liver. The results confirm the hypothesis according to which the decrease of endogenous polyglutamate content of regenerating liver might be ascribed to greater utilization rather than to lower synthesis of these coenzymes.     

Metabolic effects of histidine-deficient diets fed to growing rats by gastric tube

Effects of histidine deficiency on muscle carnosine and anserine levels and on activities of enzymes associated with histidine catabolism and protoporphyrin synthesis were investigated. Male Sprague-Dawley (150 g) rats were tube-fed isonitrogenous, isocaloric, defined diets containing 0%, low (0.013%) or adequate (0.45%) histidine for 8-13 days. While histidine-deficient animals maintained body weight, muscle and plasma histidine and carnosine concentrations decreased rapidly and remained low following a 3-day histidine repletion period. Hepatic histidine ammonia-lyase and histidine-pyruvate transaminase activities were decreased in histidine-deficient animals, whereas formiminotransferase activity was unchanged. Hematocrit levels and hemoglobin concentrations declined progressively during histidine depletion and the activity of erythrocyte and hepatic delta-aminolevulinic acid dehydratase also decreased relative to controls. Evidence is presented indicating that decreased histidine catabolism combined with carnosine and hemoglobin degradation can provide sufficient histidine to explain the slow onset of negative nitrogen balance associated with histidine deficiency and that impaired protoporphyrin synthesis may partially explain the anemia observed in the absence of dietary histidine.     

All-trans-retinoic acid rapidly induces glycine N-methyltransferase in a dose-dependent manner and reduces circulating methionine and homocysteine levels in rats

Glycine N-methyltransferase (GNMT) regulates the methyl group supply for S-adenosylmethionine-dependent transmethylation reactions. Retinoids have been shown to perturb methyl group metabolism by increasing the abundance and activity of GNMT, thereby leading to the loss of methyl groups. Previous studies used pharmacologic doses (30 micro mol/kg body weight) of various retinoids administered daily for a total of 10 d. Here, we examined the dose- and time-dependent relationships between all-trans-retinoic acid (ATRA) administration and induction of GNMT, as well as determining additional indices of methyl group and folate metabolism. For the dose-response study, rats were administered 0, 1, 5, 10, 15 or 30 micro mol ATRA/kg body weight for 10 d. For the time-course study, rats were given 30 micromol ATRA/kg body weight for 0, 1, 2, 4, or 8 d. A significant increase (105%) in GNMT activity was observed with doses as low as 5 micromol/kg body weight, whereas maximal induction (231%) of GNMT activity was achieved at 30 micromol/kg body weight. Induction of hepatic GNMT by ATRA was rapid, exhibiting a 31% increase after a single dose (1 d) and achieving maximal induction (95%) after 4 d. Plasma methionine and homocysteine concentrations were decreased 42 and 53%, respectively, in ATRA-treated rats compared with controls. In support of this finding, the hepatic activity of methionine synthase, the folate-dependent enzyme required for homocysteine remethylation, was elevated 40% in ATRA-treated rats. This work demonstrates that ATRA administration exerts a rapid effect on hepatic methyl group, folate and homocysteine metabolism at doses that are within the therapeutic range used by humans.     

Effect of DHA supplementation on digestible starch utilization by rainbow trout

Rainbow trout has a limited ability to utilize digestible carbohydrates efficiently. Trout feeds generally contain high levels of DHA, a fatty acid known to inhibit a number of glycolytic and lipogenic enzymes in animals. A study was conducted to determine whether carbohydrate utilization by rainbow trout might be affected by dietary DHA level. Two low-carbohydrate (<4 % digestible carbohydrate) basal diets were formulated to contain 1 (adequate) or 4 (excess) g/100 g DHA diet respectively. The two basal diets were diluted with increasing levels of digestible starch (0 %, 10 %, 20 % and 30 %, respectively) to produce eight diets. These diets were fed to fish for 12 weeks at 15 degrees C according to a pair-fed protocol that consisted of feeding the same amount of basal diet but different amounts of starch. Live weight, N and lipid gains, hepatic glycogen and plasma glucose values significantly increased, whereas feed efficiency (gain:feed) significantly decreased, with increasing starch intake (P<0.05). The retention efficiency of N (N gain/digestible N intake) improved with starch supplementation but was not affected by DHA level (P>0.05). Starch increased the activity of glucokinase, pyruvate kinase, glucose 6-phosphate dehydrogenase and fatty acid synthase (P<0.05) but did not affect hexokinase and malic enzyme activity. DHA had no effect on growth but increased plasma glucose and reduced carcass lipid and liver glycogen contents (P<0.05). Glycolytic and lipogenic enzymes were not affected by DHA level, except for pyruvate kinase, which was reduced by increasing DHA level. These results suggest only a marginal effect of dietary DHA on the ability of fish to utilize carbohydrate.     

Folate transport in ileal brush border-membrane vesicles following extensive resection of proximal and middle small intestine in the rat

Uptake of folic acid (PteGlu) was examined in remnant ileum of rats after resection of 65% of the small intestine with the brush border-membrane vesicle technique. The results were compared to that of sham-operated rats. In both rat groups transport of PteGlu was linear for approximately 40 s of incubation and was similar in the presence of a Na+ and a K+ gradient (out greater than in). In resected rats transport of PteGlu was inhibited by the structural analogues 5-methyltetrahydrofolate (5-CH3H4PteGlu) and methotrexate (MTX), by sulfasalazine, and by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and was saturable as a function of concentration (apparent Kt = 18.3 microM). In the ileum of sham-operated rats, on the other hand, transport of PteGlu was not affected by 5-CH3H4PteGlu, MTX, sulfasalazine, or DIDS and was linear with concentration. These results suggest that the PteGlu transport system is induced in remnant ileum of the rat after extensive intestinal resection.     

Folate absorption in alcoholic pigs: in vitro hydrolysis and transport at the intestinal brush border membrane

We used the miniature pig to evaluate the effect of ethanol ingestion on the hydrolysis of pteroylpolyglutamate and on the uptake of pteroylmonoglutamate (PteGlu) by the intestinal brush border membrane, processes that are required for folate absorption. After feeding ethanol or sucrose at 60% of calories for 11 mo, the uptake of PteGlu by jejunal brush-border-membrane vesicles was similar in both groups of animals. Jejunal brush border pteroylpolyglutamate hydrolase was decreased by one-half in the ethanol-fed group. Jejunal brush-border-membrane fluidity, measured by fluorescence polarization, was similar in both groups. Acute exposure of the jejunal vesicles to ethanol increased membrane fluidity and decreased hydrolase activity but had no effect on PteGlu transport. Inhibition of jejunal folate hydrolase by chronic exposure to ethanol may be an early effect in the pathogenesis of folate malabsorption and deficiency in chronic alcoholism.     

Single oral doses of 13C forms of pteroylmonoglutamic acid and 5-formyltetrahydrofolic acid elicit differences in short-term kinetics of labelled and unlabelled folates in plasma: potential problems in interpretation of folate bioavailability studies

Single (13)C6-labelled doses of pteroylmonoglutamic acid (PteGlu; 634 nmol) or 5-formyltetrahydrofolic acid (431-569 nmol) were given to fasted adult volunteers, and the rise in total and (13)C-labelled plasma 5-methyltetrahydrofolic acid metabolite monitored over 8 h by HPLC and liquid chromatography-MS. The dose-adjusted area under the curve (AUC) for total (labelled plus unlabelled) plasma 5-methyltetrahydrofolic acid following a 5-formyltetrahydrofolic acid test dose was 155 % that obtained following a PteGlu test dose. Surprisingly, an average 60 and 40 % of the total plasma 5-methyltetrahydrofolic acid response to [(13)C6]PteGlu and [(13)C6]5-formyltetrahydrofolic acid, respectively, was unlabelled; an observation never before reported. Short-term kinetics of plasma [(13)C6]5-methyltetrahydrofolic acid showed a slower initial rate of increase in plasma concentration and longer time to peak following an oral dose of [(13)C6]PteGlu compared with that for an oral dose of [(13)C6]5-formyltetrahydrofolic acid, while the [(13)C6]5-methyltetrahydrofolic acid AUC for [(13)C6]5-formyltetrahydrofolic acid was 221 % that for [(13)C6]PteGlu. These data indicate that PteGlu and 5-formyltetrahydrofolic acid, which are thought to be well absorbed (about 90 %) at physiological doses, exhibit dramatically different rates and patterns of plasma response. A limitation in the rate of reduction of PteGlu before methylation could result in slower mucosal transfer of [(13)C6]5-methyltetrahydrofolic acid derived from [(13)C6]PteGlu into the plasma. This, when coupled with an observed similar plasma clearance rate for [(13)C6]5-methyltetrahydrofolic acid metabolite derived from either folate test dose, would yield a comparatively smaller AUC. These findings suggest potential problems in interpretation of absorption studies using unlabelled or labelled folates where the rate of increase, the maximum increase, or the AUC, of plasma folate is employed for test foods (mainly reduced folates) v. a 'reference dose' of PteGlu.     

Effect of dietary ascorbic acid, cholesterol and PCB on cholesterol and bile acid metabolism in a rat mutant unable to synthesize ascorbic acid

The effect of acute or chronic ascorbic acid deficiency on the activity of hepatic cholesterol 7 alpha-hydroxylase and fecal excretion of bile acids was investigated in ODS-od/od (OD) rats (a rat mutant unable to synthesize ascorbic acid) fed a purified basal diet or purified diets containing either cholesterol (2%) or polychlorinated biphenyl (PCB) (200 mg/kg). In OD rats, the dietary requirement of ascorbic acid to maintain normal growth and normal levels of cholesterol in serum and liver is about 300 mg of ascorbic acid/kg diet. In OD rats fed the basal diet, acute or chronic ascorbic acid deficiency did not affect the activity of hepatic cholesterol 7 alpha-hydroxylase and fecal excretion of bile acids. However, in OD rats fed diets containing either cholesterol or PCB, acute ascorbic acid deficiency caused a higher level of serum cholesterol, a lower activity of hepatic cholesterol 7 alpha-hydroxylase and a lower excretion of fecal bile acids than in OD rats fed a basal diet containing an adequate level of ascrobic acid. It is concluded that acute ascorbic acid deficiency causes a hypercholesterolemia due to the depression of bile acid synthesis in OD rats fed a purified diet with cholesterol or PCB.