Detection of a milk factor that facilitates folate uptake by intestinal cells

Infants' Folate Markers and Postnatal Growth in the First 4 Months of Life in Relation to Breastmilk and Maternal Plasma Folate

BACKGROUND

Human milk is the sole source of folate in exclusively breastfed infants. We investigated whether human milk folate or maternal plasma folate are associated with infants' folate status and postnatal growth in the first 4 months of life.

METHODS

Exclusively breastfed infants (n = 120) were recruited at age < 1 month (baseline). Blood samples were available at baseline and at the age of 4 months. Plasma and breastmilk samples were available from the mothers at 8 weeks postpartum. The concentrations of (6S)-5-methyltetrahydrofolate (5-MTHF) and different folate status markers were measured in samples of the infants and their mothers. The z-scores of weight, height, and head circumference of the infants were measured five times between baseline and 4 months.

RESULTS

Women with 5-MTHF concentrations in breastmilk <39.9 nmol/L (median) had higher plasma 5-MTHF compared to those with milk 5-MTHF concentrations >39.9 nmol/L (mean (SD) plasma 5-MTHF = 23.3 (16.5) vs. 16.6 (11.9) nmol/L; p = 0.015). At the age of 4 months, infants of women who were higher suppliers of 5-MTHF in breastmilk had higher plasma folate than those of low-supplier women (39.2 (16.1) vs. 37.4 (22.4) nmol/L; adjusted p = 0.049). The concentrations of breastmilk 5-MTHF and maternal plasma folate were not associated with infants' longitudinal anthropometric measurements between baseline and 4 months.

CONCLUSIONS

Higher 5-MTHF in breastmilk was associated with higher folate status in the infants and the depletion of folate in maternal circulation. No associations were seen between maternal or breastmilk folate and infants' anthropometrics. Adaptive mechanisms might counteract the effect of low milk folate on infant development.

The antagonism of folate receptor by dolutegravir: developmental toxicity reduction by supplemental folic acid

OBJECTIVE

Maternal folate (vitamin B9) status is the largest known modifier of neural tube defect risk, so we evaluated folate-related mechanisms of action for dolutegravir (DTG) developmental toxicity.

DESIGN

Folate receptor 1 (FOLR1) was examined as a target for DTG developmental toxicity using protein and cellular interaction studies and an animal model.

METHODS

FOLR1 competitive binding studies were used to test DTG for FOLR1 antagonism. Human placenta cell line studies were used to test interactions with DTG, folate, and cations. Zebrafish were selected as an animal model to examine DTG-induced developmental toxicity and rescue strategies.

RESULTS

FOLR1 binding studies indicate DTG is a noncompetitive FOLR1 antagonist at therapeutic concentrations. In-vitro testing indicates calcium (2 mmol/l) increases FOLR1-folate interactions and alters DTG-FOLR1-folate interactions and cytotoxicity. DTG does not inhibit downstream folate metabolism by dihydrofolate reductase. Early embryonic exposure to DTG is developmentally toxic in zebrafish, and supplemental folic acid can mitigate DTG developmental toxicity.

CONCLUSION

Folates and FOLR1 are established modifiers of risk for neural tube defects, and binding data indicates DTG is a partial antagonist of FOLR1. Supplemental folate can ameliorate increased developmental toxicity due to DTG in zebrafish. The results from these studies are expected to inform and guide future animal models and clinical studies of DTG-based antiretroviral therapy in women of childbearing age.

In vitro bioaccessibility of added folic acid in commercially available baby foods formulated with milk and milk products

Milk contains a certain amount of folate binding proteins. The binding capacity varies in acidic conditions and affects the bioavailability of folic acid. Folic acid is commonly added into baby foods to ensure adequate intake of infants. The aim of this study was to determine the bioaccessibility of added folic acid in baby foods formulated with milk and milk products under different gastric pH values by an in vitro digestive system. The bioaccessibility of folic acid ranged between 56-71 and 35-49% in infant formula samples, between 59-78 and 31-67% in cereal-based baby foods, and between 42-67 and 38-57% in follow-on baby milk at gastric pH 1.5 and pH 4, respectively. Our results demonstrate that the bioaccesibility of folic acid that is added to baby food is affected by gastric pH. Therefore, it was observed that the bioaccesibility of folic acid was lower in the higher gastric pH.

Efficacy of an MFGM-enriched complementary food in diarrhea, anemia, and micronutrient status in infants

OBJECTIVE

The aim of the present study was to evaluate the efficacy of a milkfat globule membrane (MFGM)-enriched protein fraction in a complementary food, on diarrhea, anemia, and micronutrient status.

SUBJECTS AND METHODS

A randomized, double-blind controlled design to study 550 infants, 6 to 11 months old, who received daily for 6 months a complementary food (40 g/day) with the protein source being either the MFGM protein fraction or skim milk proteins (control). Health and nutritional status of infants were examined monthly in the outpatient clinic; product intake, food patterns, and diarrhea morbidity were assessed by home visits twice per week. Hemoglobin and micronutrient status were measured at 0 and 6 months of intervention. Results are presented as the entire group and as 6 to 8 and 9 to 11 months subgroups.

RESULTS

A total of 499 infants completed the study. Global prevalence of diarrhea was 3.84% and 4.37% in the MFGM group and control group, respectively (P < 0.05). Consumption of the MFGM protein fraction reduced episodes of bloody diarrhea (odds ratio 0.54; 95% confidence interval 0.31-0.93, P = 0.025) adjusting for anemia and potable water facilities as covariates. There were no differences between groups in anemia, serum ferritin, zinc, or folate.

CONCLUSIONS

Addition of an MFGM-enriched protein fraction to complementary food had beneficial effects on diarrhea in infants and may thus help to improve the health of vulnerable populations.

Folate and cobalamin status in relation to breastfeeding and weaning in healthy infants

BACKGROUND

Folate and cobalamin status changes markedly during infancy.

OBJECTIVE

We aimed to examine the influence of breastfeeding on folate and cobalamin status in healthy infants.

DESIGN

In a longitudinal study, we measured serum folate, cobalamin, holotranscobalamin, holohaptocorrin, methylmalonic acid, and homocysteine at birth and at ages 6, 12, and 24 mo (n = 361, 262, 244, and 224, respectively). Breastfeeding status and nutrient intake were assessed by using questionnaires and 7-d weighed-food records (at 12 mo).

RESULTS

All indexes changed significantly from birth to age 24 mo (P < 0.001). Folate was high until age 6 mo and then declined. At age 6 mo, folate was positively correlated with duration of exclusive breastfeeding (rho = 0.29; P < 0.001). Cobalamin status declined after birth in breastfed but increased in nonbreastfed infants. Thus, holotranscobalamin (pmol/L) was lower in breastfed than in nonbreastfed children at age 6 mo [geometric mean: 37 (95% CI: 33, 40) and 74 (64, 86), respectively], at 12 mo [51 (46, 56) and 76 (70, 82), respectively], and at 24 mo [65 (50, 83) and 90 (85, 97), respectively; P < 0.05 for all]. Complementary feeding did not increase (6 mo) or modestly increased (12 mo) cobalamin status in breastfed children. At 12 mo, cobalamin intake (microg/d), excluding breast milk cobalamin, was lower in breastfed than in nonbreastfed infants [geometric mean: 1.4 (1.3, 1.6) and 2.4 (2.1, 2.6), respectively; P < 0.001]. However, after adjustment for total cobalamin intake, cobalamin status (ie, holotranscobalamin) remained significantly lower in breastfed than in nonbreastfed infants [54 (49, 59) and 70 (64, 78), respectively; P < 0.001].

CONCLUSIONS

Low cobalamin status is a characteristic finding in breastfed children. Reference limits according to age and breastfeeding status should be considered in early childhood.

Effects of Dietary Calcium Intervention on Adolescent Mothers and Newborns

OBJECTIVE

To evaluate the effects of dietary calcium (Ca) intervention on adolescent pregnant mothers and their newborns.

METHODS

Seventy-two pregnant adolescent mothers were randomized into one of 3 groups: control, orange juice fortified with calcium, and dairy. The orange juice and dairy groups were required to take more than 1,200 mg Ca. Calcium tablets were added for those not able to meet required Ca. Maternal and infant weight, length, and blood pressure (BP) were recorded. Maternal dietary records were evaluated. Mother's blood was drawn for serum Ca, phosphate (P), magnesium (Mg), and vitamin 25-hydroxyvitamin D (D). Cord blood was collected for serum Ca and D. Newborn total body Ca was determined.

RESULTS

All mothers were similar in weight, height, and BP. Mothers in the orange juice plus calcium and dairy groups had higher intakes of Ca (1,472 mg and 1,771 mg) than controls (862 mg). One half of the mothers in the orange juice plus calcium group required Ca tablets. Mothers in the dairy group had higher intakes of P, D, and Mg, higher serum folate and D, and higher cord D levels. Mothers in the orange juice plus calcium group had higher serum P but lower serum folate and D. Infants (3,517+/-273 g) in the dairy group were heavier than infants in the control (3,277+/-177 g) and orange juice plus calcium (3,292+/-165 g) groups. Infants in the dairy group had higher total body calcium than control infants.

CONCLUSION

Calcium diet supplemented with dairy products during adolescent pregnancy resulted in higher maternal vitamin D and folate serum levels and higher newborn weight and bone mineralization compared with controls.

Bioavailability of folic acid from fortified pasteurised and UHT-treated milk in humans

OBJECTIVE

The aim of this study was to investigate whether milk fortified with folic acid enhances the folate status of humans and whether the presence of folate-binding proteins (FBP) in pasteurised milk affects the bioavailability of folic acid from fortified milk. In untreated and pasteurised milk, folate occurs bound to FBP, while FBP is (partly) denatured in ultra-high-temperature (UHT)-treated milk. The effect of FBP on folate bioavailability is still unclear.

DESIGN, SUBJECTS AND SETTING

Healthy, free-living subjects (n=69) aged 18-49 y participated in a 4-week double-blind, placebo-controlled dietary intervention study.

INTERVENTION

In addition to a fully controlled diet, the subjects consumed each day 500 ml of pasteurised or UHT milk, either fortified or not with 200 mug folic acid.

RESULTS

Consumption of fortified milk increased folate concentrations in serum and in red blood cells (RBC) by 6.6-7.0 nmol/l (P<0.001) and 32-36 nmol/l (P<0.01), respectively. Similarly, plasma homocysteine concentrations were lowered 0.88-0.89 micromol/l (P=0.001) in subjects who consumed fortified milk. The bioavailability of folic acid from pasteurised milk relative to that of folic acid from UHT milk was 74-94% (NS), depending on the parameter used.

CONCLUSIONS

Milk fortified to supply an additional 200 microg of folic acid/s substantially increased folate status, and decreased plasma total homocysteine concentrations in young, healthy subjects. Milk is therefore a suitable matrix for fortification to enhance the folate status in humans. No significant effect of endogenous FBP was found on the bioavailability of folic acid from milk.

Megalin binds and mediates cellular internalization of folate binding protein

Folate is an essential vitamin involved in a number of biological processes. High affinity folate binding proteins (FBPs) exist both as glycosylphosphatidylinositol-linked, membrane associated folate binding proteins and as soluble FBPs in plasma and some secretory fluids such as milk, saliva and semen. The function and significance of FBPs are unresolved, however, it has been suggested that they may facilitate folate uptake, e.g. during suckling. The present study shows that megalin, a large, multiligand endocytic receptor and member of the low-density lipoprotein-receptor family, is able to bind and mediate cellular uptake of FBP. Surface plasmon resonance analysis shows binding of bovine and human milk FBP to immobilized megalin, but not to low density lipoprotein receptor related protein. Binding of (125)I-labeled folate binding protein (FBP) to sections of kidney proximal tubule, known to express high levels of megalin, is inhibitable by excess unlabeled FBP and by receptor associated protein, a known inhibitor of binding to megalin. Immortalized rat yolk sac cells, representing an established model for studying megalin-mediated uptake, reveal (125)I-labeled FBP uptake which is inhibited by receptor associated protein and by antimegalin antibodies. Microinjection of (125)I-labeled FBP into renal tubules in vivo shows proximal tubular uptake by endocytosis. Megalin is expressed in several absorptive epithelia, including intestine and kidney proximal tubule, and thus the present findings provide a mechanism for intestinal and renal endocytic uptake of soluble FBP.

Effect of cow milk on food folate bioavailability in young women

BACKGROUND

The findings of animal and human studies suggest that a certain component in milk enhances food folate bioavailability.

OBJECTIVE

The objective was to evaluate whether cow milk enhances the bioavailability of food folate in humans.

DESIGN

Thirty-one young women were fed low-folate diets on a 4-d rotation with (n=17) or without (n=14) cow milk for 8 wk. Plasma and erythrocyte folate and plasma total homocysteine (tHcy) concentrations were monitored.

RESULTS

Plasma folate concentrations steadily declined in both the milk and the nonmilk groups; however, the declines were not significantly different between groups (P=0.09). In contrast, erythrocyte folate concentrations declined only in the nonmilk group and remained unchanged in the milk group; the difference between the 2 groups was significant (P=0.01). Plasma tHcy concentrations significantly increased in the nonmilk group and remained unchanged in the milk group; the difference between the 2 groups was significant (P=0.02).

CONCLUSION

The inclusion of cow milk in the diet enhanced the bioavailability of food folate as assessed by the changes in erythrocyte folate and plasma tHcy concentrations but not in plasma folate concentrations. The mechanisms of action by the components of cow milk remain to be investigated.

Effect of folate-binding protein on intestinal transport of folic acid and 5-methyltetrahydrofolate across Caco-2 cells

BACKGROUND

Milk products are a potential matrix for fortification with synthetic folic acid or natural 5-methyltetrahydrofolate (5-CH3-H4folate) to enhance the daily folate intake. In milk, folate occurs bound to folate-binding proteins (FBP). Our previous studies with an in vitro gastrointestinal model showed that 70% of the initial FBP content of the milk product was retained in the duodenal lumen. While folic acid remained bound to FBP after gastric passage, 5-CH3-H4folate was mainly present as free folate in the duodenal lumen.

AIM OF THE STUDY

To investigate the effect of FBP on the absorption of folic acid and 5-CH3-H4folate from the intestinal lumen.

METHODS

The transport of [3H]-folic acid and [14C]-5-CH3-H4folate across enterocytes was studied in the presence or absence of bovine FBP using monolayers of Caco-2 cells grown on semi-permeable inserts in a two-compartment model. The apparent permeability coefficients (P(app)) of folic acid and 5-CH3-H4folate were determined and compared with the permeability of reference compounds for low (mannitol) and high (caffeine) permeability.

RESULTS

The transport from the apical to the basolateral side of the Caco-2 cells was higher (P < 0.05) for folic acid (P(app) = 1.7*10(-6) cm/s) than for 5-CH3-H4folate (P(app) = 1.4*10(-6) cm/s) after 2 h incubation to 1 microM folic acid or 5-CH3-H4folate test solutions (pH 7). The permeability of folic acid and 5-CH3-H4folate across Caco-2 monolayers appeared to be higher (P < 0.05) than that of mannitol (P(app) = 0.5*10(-6) cm/s) but lower (P < 0.05) than that of caffeine (P(app) = 34*10(-6) cm/s). The addition of FBP to the medium led to a lower (P < 0.05) intestinal transport and cellular accumulation of folic acid and 5-CH3-H4folate.

CONCLUSIONS

Compared to the reference compounds, folic acid and 5-CH3-H4folate showed a moderate permeability across Caco-2 cells, which indicates that folate absorption from the intestinal lumen is not likely to be complete. The intestinal transport of folic acid and 5-CH3-H4folate was found to be dependent on the extent of binding to FBP at the luminal side of the cells.

The binding of folic acid and 5-methyltetrahydrofolate to folate-binding proteins during gastric passage differs in a dynamic in vitro gastrointestinal model

Despite its low natural folate concentration, milk is responsible for 10-15% of the daily folate intake in countries with a high dairy consumption. Milk products can be considered as a potential matrix for folate fortification, e.g., with synthetic folic acid, to enhance the daily intake of folate. In untreated milk, the natural folate, 5-methyltetrahydrofolate (5-CH3-H4folate), is bound to folate-binding proteins (FBP). In this study, the extent of binding to FBP for folic acid and 5-CH3-H4folate was investigated in a dynamic in vitro model simulating human gastric passage. Protein binding of folic acid and 5-CH3-H4folate was characterized using gel-exclusion chromatography. Before gastric passage, folic acid and 5-CH3-H4folate were bound mainly to FBP (76-79%), whereas 7% was free. Folic acid remained bound to FBP to a similar extent after gastric passage. For 5-CH3-H4folate, the FBP-bound fraction gradually decreased from 79 to 5% and the free fraction increased from 7 to 93%. Although folic acid enters the proximal part of the intestine bound to FBP, 5-CH3-H4folate appears to be present mainly as free folate in the duodenal lumen. The stability of FBP was similar in both folate/FBP mixtures, i.e., 70% of the initial FBP content was retained after gastric passage. This study indicated that FBP are partly stable during gastric passage but have different binding characteristics for folic acid and 5-CH3-H4folate in the duodenal lumen. This could result in different bioavailability from folic acid- and 5-CH3-H4folate-fortified milk products.

Bioaccessibility of Folic Acid and (6S)-5-Methyltetrahydrofolate Decreases after the Addition of Folate-Binding Protein to Yogurt as Studied in a Dynamic In Vitro Gastrointestinal Model

Milk products are only moderate sources of folate. Nevertheless, they are of interest due to their content of folate-binding proteins (FBP), which in some studies have been reported to increase folate bioavailability. The effect of FBP on folate bioavailability has been widely discussed. The aim of this study was to investigate the bioaccessibility of folic acid and (6S)-5-methyltetrahydrofolate (5-CH3-H4folate) from fortified yogurt using a dynamic in vitro gastrointestinal model (TIM). In addition, the effect of FBP on folate bioaccessibility and the stability of FBP added to yogurt during gastrointestinal passage were investigated. Folate bioaccessibility was 82% from yogurt fortified with folic acid and 5-CH3-H4folate. The addition of FBP to yogurt decreased (P < 0.05) folate bioaccessibility. The lowering effect of FBP was more pronounced in yogurt fortified with folic acid (34% folate bioaccessibility) than from yogurt fortified with 5-CH3-H4folate (57% folate bioaccessibility). After gastrointestinal passage, 17% of the FBP in yogurt fortified with 5-CH3-H4folate and 34% of the FBP in yogurt fortified with folic acid were recovered. No difference in folate bioaccessibility was found between folate-fortified yogurt and folate-fortified pasteurized milk (P = 0.10), whereas the lowering effect of FBP was (P < 0.05) greater in yogurt compared with pasteurized milk. In conclusion, based on the high bioaccessibility of folic acid and 5-CH3-H4folate, yogurt without active FBP can be considered to be an appropriate food matrix for folate fortification.

Folic acid and 5-methyltetrahydrofolate in fortified milk are bioaccessible as determined in a dynamic in vitro gastrointestinal model

Dairy products are a potential matrix for folate fortification to enhance folate consumption in the Western world. Milk folate-binding proteins (FBP) are especially interesting because they seem to be involved in folate bioavailability. In this study, folate bioaccessibility was investigated using a dynamic computer-controlled gastrointestinal model [TNO gastrointestinal model (TIM)]. We used both ultrahigh temperature (UHT)-processed milk and pasteurized milk, differing in endogenous FBP concentrations and fortified with folic acid or 5-methyltetrahydrofolate (5-CH(3)-H(4)folate). To study FBP stability during gastrointestinal passage and the effect of additional FBP on folate bioaccessibility, FBP-fortified UHT and pasteurized milk products were also tested. Folate bioaccessibility and FBP stability were measured by taking samples along the compartments of the gastrointestinal model and measuring their folate and FBP concentrations. Folate bioaccessibility from folic acid-fortified milk products without additional FBP was 58-61%. This was lower (P < 0.05) than that of the 5-CH(3)-H(4)folate-fortified milk products (71%). Addition of FBP reduced (P < 0.05) folate bioaccessibility from folic acid-fortified milk (44-51%) but not from 5-CH(3)-H(4)folate-fortified milk products (72%). The residual FBP levels in the folic acid- and 5-CH(3)-H(4)folate-fortified milk products after gastrointestinal passage were 13-16% and 0-1%, respectively, of the starting amounts subjected to TIM. In conclusion, milk seems to be a suitable carrier for folate, because both folic acid and 5-CH(3)-H(4)folate are easily released from the matrix and available for absorption. However, our results suggest that folic acid remains partly bound to FBP during passage through the small intestine, which reduces the bioaccessibility of folic acid from milk in this model.

Nutritional and physiologic significance of human milk proteins

Human milk contains a wide variety of proteins that contribute to its unique qualities. Many of these proteins are digested and provide a well-balanced source of amino acids to rapidly growing infants. Some proteins, such as bile salt-stimulated lipase, amylase, beta-casein, lactoferrin, haptocorrin, and alpha1-antitrypsin, assist in the digestion and utilization of micronutrients and macronutrients from the milk. Several proteins with antimicrobial activity, such as immunoglobulins, kappa-casein, lysozyme, lactoferrin, haptocorrin, alpha-lactalbumin, and lactoperoxidase, are relatively resistant against proteolysis in the gastrointestinal tract and may, in intact or partially digested form, contribute to the defense of breastfed infants against pathogenic bacteria and viruses. Prebiotic activity, such as the promotion of the growth of beneficial bacteria such as Lactobacilli and Bifidobacteria, may also be provided by human milk proteins. This type of activity can limit the growth of several pathogens by decreasing intestinal pH. Some proteins and peptides have immunomodulatory activities (eg, cytokines and lactoferrin), whereas others (eg, insulin-like growth factor, epidermal growth factor, and lactoferrin) are likely to be involved in the development of the intestinal mucosa and other organs of newborns. In combination, breast-milk proteins assist in providing adequate nutrition to breastfed infants while simultaneously aiding in the defense against infection and facilitating optimal development of important physiologic functions in newborns.

Dietary interactions influence the effects of bovine folate-binding protein on the bioavailability of tetrahydrofolates in rats

The newborns of mammals have a high folate demand, yet obtain adequate folate nutrition solely from their mothers' milk despite its low folate content. Milk folate is entirely bound by an excess of folate-binding protein (FBP), prompting speculation that FBP may affect the bioavailability of the limited folate supply. Previous research has shown that FBP-bound folic acid is more gradually absorbed, thereby reducing the peak plasma folate concentration and preventing loss into the urine. Natural folates are reduced derivatives of folic acid, with milk predominantly containing 5-methyltetrahydrofolate, yet little research has been carried out to determine the role of FBP in the bioavailability of reduced folates. We studied the effect of FBP on folate nutrition of rats in both single-dose and 4-wk feeding experiments. The effect of FBP was influenced by the presence of other milk components. FBP increased bioavailability of dietary folate when it was consumed with other whey proteins or with soluble casein. However, in the presence of acid-precipitated casein or a whey preparation enriched in lipids, bioavailability was decreased. These results highlight the difficulties of extrapolating from experimental results obtained using purified diets alone and of studying interactions among dietary components. They suggest that the addition of FBP-rich foods to folate-rich foods could enhance the bioavailability of natural folates, but that the outcome of such a combination would depend on interactions with other components of the diet.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

[Dietary folates--a timely review. Stability, physiological significance, bioavailability, analytical determination methods, effect of food handling]

Because of the unequal and, in some instances, low stability of different folate vitamers against extreme conditions the analytical determination of folate and the estimation of folate losses in food processing and preparation cause considerable difficulties. HPLC allows determination of the native folate derivative patterns. As the bioavailability of folates is influenced by a variety of factors and different methods were employed for assessing bioavailability there is a considerable inconsistency in the results of these studies. Folates labeled with radioactive or stable isotopes provide new approaches to metabolic and bioavailability studies.

Immunoreactive folate-binding proteins from human saliva. Isolation and comparison of two distinct species

Human saliva contains a single 72,000-M(r) species which specifically reacted with rabbit anti-[human placental folate receptor (PFR)] serum on SDS/PAGE and Western blots. Although a specific radioimmunoassay for human PFR and related folate-binding proteins (FBPs) identified 55 ng of cross-reacting material (CRM) per mg of crude salivary proteins, only a minor fraction (1.6 ng) specifically bound radiolabelled folate. The major fraction of CRM did not contain bound endogenous folate and did not bind radiolabelled folates. On the basis of folate binding, salivary CRM species to PFR were designated as either functional (f-FBP) or non-functional (nf-FBP) species respectively. nf-FBPs and f-FBPs were isolated by different purification schemes. Both purified f-FBPs and nf-FBPs migrated as a single apparent 72,000-M(r) species on SDS/PAGE, but on Sephacryl S-200 gel filtration and sucrose-density-gradient ultracentrifugation they were eluted/sedimented with 40,000-M(r) markers. Each microgram of purified f-FBP and nf-FBP was measured in the radioimmunoassay for PFR as being equivalent to 18 ng and 24 ng of CRM respectively, indicating low epitope-relatedness to PFR. The Kd of f-FBPs was 50 pM and 0.94 mol of folate was bound/mol of protein. f-FBPs exhibited an unusual dependence on Triton X-100 for optimal ligand binding, despite the fact that Triton X-100 micelle binding was not demonstrated. The relative order of affinity of f-FBPs for pteroylglutamate greater than methotrexate greater than 5-formyltetrahydrofolate greater than 5-methyltetrahydrofolate was also distinct from that of purified PFR. Whereas amino acid and carbohydrate analysis revealed that nf-FBP (M(r) 51,400) and f-FBP (M(r) 39,200) were distinct glycoproteins with 8 and 13% carbohydrate respectively, isoelectric focusing and immunological studies suggested some structural identity. The presence of f-FBP and nf-FBP in normal saliva raises new questions about their possible role in vivo.

Effect of folic acid supplementation on small-for-gestational-age infants born at term

The effect of 250 micrograms folic acid (FA)/day or placebo given to 21 small-for-gestational-age infants born at term was studied during the first 3 months of life. The design of the study was double blind with random allocation. No randomization was performed in respect of breast-feeding or formula-feeding with a folate content of 60-70 micrograms/l. No significant differences were observed in haemoglobin concentration, haematocrit, weight and length between FA-supplemented and non-supplemented infants. A negative correlation was demonstrated between gestational age and erythrocyte folate (E-folate) concentration at 1 week. E-folate content was higher when no supplement was given in breast-fed than in formula-fed infants.

Anti-idiotypic antibody probes of neuronal nicotinic receptors

Monoclonal anti-idiotypic antibodies specific for the combining site on a monoclonal antinicotine were used in immunocytochemistry to localize nicotine binding sites on rat brain cortical sections and in immunoaffinity chromatography to isolate receptor from solubilized brain tissue. The receptor, which consists of two subunits with Mr values of 43 and 50 kDa, was eluted from the antiidiotype column with either pH3 citrate buffer or 25 mM (-)-nicotine, but was not present in eluates from immobilized anti-Electrophorus acetylcholine receptor or anti-methotrexate. The anti-idiotypes specifically inhibited [3H]nicotine binding to rat brain homogenate and (-)-nicotine inhibited anti-idiotype binding to brain sections based on abrogation of immunofluorescence staining. These results are consistent with the operational definition of the anti-idiotypes as the internal image of nicotine, and demonstrate their value as immunochemical probes of nicotinic receptors.

Folate transport in enterocytes and brush-border-membrane vesicles isolated from the small intestine of the neonatal goat

1. The uptake of 5-methyltetrahydrofolate (MTHF) and folic acid (pteroylmonoglutamate) by enterocytes and brush-border-membrane vesicles prepared from the small intestine of the 6-d-old male goat was determined using a rapid-filtration assay. 2. Both MTHF and folic acid were taken up by membrane vesicles at 25 degrees and enterocytes at 37 degrees by a pH-dependent mechanism with maximum uptake when the pH of the incubation medium was 5.0. 3. Experiments in which the osmotic pressure of the medium was raised in successive increments with the non-absorbable sugar cellobiose indicated that transport rather than membrane binding was the main component of uptake. 4. Experiments at pH 5.0 showed that uptake of MTHF and folic acid was saturable and that the characteristics of folate transport were similar in both tissue preparations: (a) transport rates of both MTHF and folic acid were constant during the first 1-2 min for a given folate concentration, then declined to reach a steady-state in 10-30 min; (b) initial velocities of transport of MTHF and folic acid increased in proportion to their concentrations up to 7-10 microM, but the rate of increase slowed thereafter until saturation was reached at 20-25 microM (Km for brush-border-membrane vesicles 40.8 and 62.9 microM, Km for enterocytes 50.9 and 55.2 microM for MTHF and folic acid respectively). Values of Vmax for membrane vesicles (pmol/mg protein per min) were 46.5 MTHF or 40.3 folic acid; enterocytes Vmax (pmol/10(7) cells per min) 15.9 MTHF or 30.6 folic acid; (c) uptake of MTHF and folic acid by brush-border-membrane vesicles and enterocytes measured under steady-state conditions approached saturation at 50 microM for each analogue (Km for membrane vesicles 58.1 and 55.2 microM, Km for enterocytes 43.1 and 49.4 microM for MTHF and folic acid respectively; Vmax for membrane vesicles 49.7 and 77.8 pmol/mg protein, for enterocytes 52.8 and 54.7 pmol/10(7) cells for MTHF and folic acid respectively). 5. It was concluded that transport of MTHF and folic acid was by a similar pH-dependent mechanism both in brush-border-membrane vesicles and in intact enterocytes. At pH 5.0, transport involved a saturable carrier-mediated process located in the brush-border membrane.

Influence of goat's-milk folate-binding protein on transport of 5-methyltetrahydrofolate in neonatal-goat small intestinal brush-border-membrane vesicles

1. The influence of goat's-milk folate-binding protein (FBP) on the uptake of 5-methyltetrahydrofolate (MTHF) by brush-border-membrane vesicles prepared from the small intestine of the 6-d-old goat was investigated using a rapid-filtration assay. 2. Uptake of MTHF by the membrane vesicles was strongly enhanced by FBP within the pH range 4.5-6.5, with an optimum at pH 5-5.5. 3. Both the initial rate of MTHF uptake and uptake of MTHF at equilibrium were markedly increased in the presence of FBP. 4. Uptake of MTHF by brush-border-membrane vesicles was maximal when the molar ratio FBP:MTHF was 1.0-2.5. 5. The relation between pH and 125I-labelled FBP binding to the membranes was similar to that for uptake of MTHF, with an optimum at pH 5. 6. In experiments in which the osmotic pressure of the incubation medium was progressively increased with cellobiose, 125I-labelled FBP was found to be taken up primarily by binding to the brush-border-membrane surface. 7. Uptake of 125I-labelled FBP was time-dependent and saturable, with a Km of 0.39 (SE 0.07) microM and Vmax of 6.73 (SE 0.92) micrograms/mg protein. 8. Experiments in which various milk proteins (cow FBP, goat FBP, alpha-lactalbumin, beta-lactoglobulin, bovine serum albumin and lactoferrin) were allowed to compete in turn with 125I-labelled FBP for uptake by brush-border-membrane vesicles indicated that high-affinity binding was probably specific to FBP, although lactoferrin reduced uptake possibly by non-specific coating of the mucosal surface. 9. It was concluded that a folate transport mechanism mediated by the FBP in milk exists at the intestinal brush border of neonatal goats. It is suggested that this may reinforce the developing endogenous transport system.

Influence on immunoreactive folate-binding proteins of extracellular folate concentration in cultured human cells

The influence of extracellular folate concentration on cellular levels of the folate transport protein and its soluble product was studied directly in cultured human nasopharyngeal carcinoma (KB) cells. As determined by radioimmunoassay, levels of the folate transport protein and the soluble folate-binding protein were 58 +/- 17 (mean +/- SD) and 5 +/- 2 pmol/mg cell protein, respectively, in KB cells maintained in standard medium (containing 2,300 nM folic acid). These levels significantly increased to 182 +/- 34 and 26 +/- 6 pmol/mg cell protein, respectively, in KB cells serially passaged in low folate medium (containing 2-10 nM 5-methyltetrahydrofolate). Increases in folate-binding protein levels occurred more rapidly in KB cells serially passaged in very low folate medium containing less than 2 nM folate and were prevented by the addition of 100 nM 5-methyltetrahydrofolate or 0.1-1 microM 5-formyltetrahydrofolate to this medium. When KB cells which had been passaged in low folate medium were passaged back into either standard medium or low folate medium supplemented with reduced folates, the levels of both folate-binding proteins fell linearly towards the levels in KB cells continuously maintained in standard medium. The folate transport protein was identified in and underwent similar changes in human and mouse mammary tumor cells. These studies indicate that the folate transport system is probably regulated by the extracellular folate concentration through changes in intracellular metabolite levels.

Effect of human milk folate binding protein on folate intestinal transport

The present study examined the effect of human milk folate binding protein (FBP) on the intestinal transport of 5-methyltetrahydrofolate (5-CH3H4PteGlu). This was performed by examining the transport of radiolabeled 5-CH3H4PteGlu bound to FBP using everted sacs of rat intestine. In the jejunum at pH 6, transport of 27 nM bound 5-CH3H4PteGlu was linear with time for 30 min of incubation. Transport of 13 nM bound 5-CH3H4PteGlu was higher in the jejunum than in the ileum at both pH 6 (2.1 +/- 0.3 and 0.36 +/- 0.03 pmol/g wet wt/25 min, respectively) and pH 8 (1.9 +/- 0.3 and 0.32 +/- 0.02 pmol/g wet wt/25 min, respectively). In the jejunum, transport of 13 nM bound 5-CH3H4PteGlu at pH 6 was less than transport of an equimolar concentration of free 5-CH3H4PteGlu (2.1 +/- 0.3 and 5.1 +/- 0.5 pmol/g wet wt/25 min, respectively) but was similar at pH 8 (1.9 +/- 0.3 and 2.47 +/- 0.3 pmol/g wet wt/25 min, respectively). In the ileum transport of bound and free 5-CH3H4PteGlu was similar at pH 6 (0.36 +/- 0.03) and 0.41 +/- 0.06 pmol/g wet wt/25 min, respectively) and pH 8 (0.32 +/- 0.02 and 0.43 +/- 0.1 pmol/g wet wt/25 min, respectively). The transport process of bound 5-CH3H4PteGlu in the jejunum was energy, temperature, and Na+ dependent, but not pH dependent, and was competitively inhibited by sulfasalazine. Ninety-two percent of the transport substrate that appeared in the serosal compartment following incubation with bound 5-CH3H4PteGlu was found to be free (unbound) 5-CH3H4PteGlu. These results show that human milk FBP decreases the rate of transport of 5-CH3H4PteGlu in the jejunum and suggest that FBP-bound 5-CH3H4PteGlu may utilize the same transport system as free 5-CH3H4PteGlu. The results also suggest a role for human milk FBP in regulating the nutritional bioavailability of folate.

The role of vitamin B12 and folate in carcinogenesis

The roles of vitamin B12 and folate in carcinogenesis are largely extensions of and linked to their roles in normal metabolism, particularly 1-carbon unit metabolism. A possible key area may be hypomethylation to "switch on" genes and methylation to "switch them off." Some vitamin analogues may act as antivitamins in these reactions, as may some vitamin-binding proteins. Others may act as specific delivery proteins. Using appropriate radioactive substrates and suspensions of vitamin-dependent normal and malignant cells, it may be possible to work out their positive and negative control of DNA synthesis.

Normal vitamin requirements in neonates and infants

Information about vitamin requirements by neonates and infants has been derived from studies of the composition of breast milk, from feed-response trials, from the occurrence of overt deficiency in infants fed damaged milk formulae, and by extrapolation from experimental deficiency studies on adult humans and on animals. Our knowledge is far from complete, however, and dietary recommendations have been formulated for only about half the known vitamins in the UK. In the near future, studies with stable isotope-labelled vitamins should help to define pool sizes and turnover rates that are associated with particular intakes and thus give firmer evidence about requirements.

Plasma and red cell folate values and folate requirements in formula-fed premature infants

Plasma and red cell folate concentrations (lactobacillus casei activity) and other pertinent blood values have been studied during the 1st year of life in 41 premature infants (mean gestational age 31.6, range 26-35 weeks). They were formula-fed, 48.5 nmol (21 micrograms) folate per 1, from 1 month of age. The infants were divided into two groups according to their birth weights (BW): group A, BW less than or equal to 1750 g and group B, BW greater than 1750 g, respectively. One-half of the infants in each group received an extra 113.5 nmol (50 micrograms) folic acid daily. The premature infants were compared with 35 breast-fed term infants considered to have an optimal folate status. The infants not receiving folic acid supplementation had low plasma and red cell folate concentrations during the first months of life, while those receiving supplementation had values comparable to the breast-fed infants. No significant differences in the gain in weight and increase in length were observed when the folic acid supplemented infants in group A were compared with the non-supplemented infants. However, in the case of group B a significant increase in length and a somewhat greater weight gain were observed for infants with folic acid supplementation in comparison with those not given extra folate. No significant differences were observed between the haemoglobin, RBC and VPRC values in the folic acid supplemented and non-supplemented infants. It is estimated that the optimal folate intake during the first months of life in formula-fed premature infants is about 150 nmol (65 micrograms) per day. This amount is higher than previously recommended. The infants from all groups had a folate intake similar to, or above, the minimal daily requirement needed for erythropoiesis.

Neonatal role of milk folate-binding protein: studies on the course of digestion of goat's milk folate binder in the 6-d-old kid

Groups of kids were reared from birth to 5 d on goat's milk. On the 6th day five of the kids received by bottle a morning feed of goat's milk with [3H]folic acid added to saturate the folate-binding proteins (FBP) (Expt 1); three kids received raw goat's milk containing only the endogenous folate and hence a large surplus folate-binding capacity (FBC) (Expt 2). The contents of the stomach, duodenum, jejunum and ileum were recovered by washing out 1.5 h after feeding (Expt 1) or at 0.5, 1 and 3.5 h after feeding (Expt 2). Recovery of [3H]folic acid 1.5 h after feeding (Expt 1) in all segments was 58.4%, mainly in a soluble form, most of this being in the stomach (37.0%) and ileum (14.3%). No surplus FBC was found in any gut segment. Sephadex G-75 chromatography of the soluble fractions of the contents of the various gut segments showed that [3H]folic acid remained bound to FBP throughout the stomach and small intestine. The bound [3H]folic acid exhibited a molecular weight of 81 000 in stomach contents, similar to that in the milk feed, presumably representing an aggregated form of the FBP, whereas in the intestinal contents its molecular weight was 39 000 indicating dissociation to monomer due to dilution in the recovery process. In Expt 2, the total recovery of free FBP in all four gut segments was 67, 54 and 23% respectively at 0.5, 1 and 3.5 h after the milk feed, and the distribution of FBP along the gut at 1 h was similar to that of [3H]folic acid-labelled FBP at 1.5 h in Expt 1.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma and red cell folate values and folate requirements in formula-fed term infants

Plasma and red cell folate concentrations (L. casei activity) and other pertinent blood values have been studied during the first year of life in 64 term infants. After weaning and until 6 months of age, 33 infants were given milk formula I (88 nmole [39 micrograms] folate per liter) and 31 infants MF II (178 nmole [78 micrograms] folate per liter). These infants were compared with 35 breast-fed term infants, considered to have an optimal folate status. The mean folate concentration in the human milk consumed at 3 months of age was 124 nmole (55 micrograms) per liter. The infants fed MF I had low plasma and red cell folate concentrations during the first months of life. The infants fed MF II had folate values almost comparable to those of the breast-fed infants. The weight gain in the infants fed MF II was significantly higher than observed in the infants fed MF I during the first 6 months of life. No significant differences related to folate deficiency were observed between the hemoglobin, RBC, and VPRC values in the formula-fed infants and those recorded in the breast-fed infants. It is estimated that the optimal folate intake during the first months of life in formula-fed infants is about 170 nmole (75 micrograms) per day. This amount is higher than previously recommended. Infants from all groups had folate intake similar to, or above, the minimal daily requirement needed for the erythropoiesis.