- Email: [email protected]
Folate and vitamin B12 transport systems in the developing infant
FOLATE AND VITAMIN B12 TRANSPORT SYSTEMS IN THE DEVELOPING INFANT IRWIN H. ROSENBERG, MD,
AND JACOB
SELHUB, PHD
B vitamin transport systems in infants are not as well studied as those for amino acids and glucose. For most B vitamins, a 2-step process allows for digestion of coenzyme forms of the vitamins in food, followed by specific transport systems for the free vitamin in the intestine. Folate and vitamin B12 have specific binding proteins, which carry the vitamins in human milk and blood, and other unique binding proteins, which convey the vitamins across the placenta, intestine, and blood-brain barrier. The permeable infant intestine permits the efficient intact transfer of protein-bound folate and vitamin B12 from intestinal lumen to bloodstream. As the intestine matures, specific carriers and receptors facilitate uptake and transport of these vitamins. (J Pediatr 2006;149:S62-S63)
he human infant is highly vulnerable and highly dependent on its mother for nutritional adequacy and, for the purposes of this discussion, prevention of vitamin deficiencies. For the most part, such deficiencies are prevented by maternal delivery of vitamins to the fetus via the placenta before birth and by providing vitamins after birth as human milk or by infant formula and complementary feeding. Infant vitamin malnutrition is well described and may be life threatening, as with hemorrhagic vitamin K deficiency, infantile beri beri, infantile scurvy, and deficiencies of vitamins B6, B12, and folate. This raises important questions about the efficiency and mechanisms of transfer of nutrients from mother to fetus and the capacity of the infant to assimilate nutrients after birth. Although the topic of transplacental transfer of nutrients is rich and vital, our attention in this paper is to the development of systems in the infant for assimilation of vitamins, especially B vitamins and particularly, folic acid and vitamin B12. B vitamin transport systems in infants are not as well studied as systems for amino acids and glucose, but we do know that there are specific transport systems for the individual vitamins. Unlike groups of amino acids, none of the vitamin transport systems are shared, emphasizing the uniqueness of each vitamin despite their shared assigned nomenclature. Moreover, the digestive tract and intestine are capable of preprocessing or digesting vitamin forms in diet that are often in the complex form of coenzymes in food—for example, thiamine as thiamin pyrophosphate, niacin as nicotine adenine dinucleotide (NAD), or nicotine adenine dinucleotide phosphate (NADP), riboflavin as flavin adenine mononucleotide From Friedman School of Nutrition Science (FMN) or dinucleotide (FAD), B6 as pyridoxal phosphate, and folate as polyglutamates and Policy, Tufts University, Boston, MA. of reduced and one carbon-substituted pteroyl glutamates. In all these instances, assimDr Rosenberg was a recipient of a BristolMyers Squibb–Mead Johnson Unrestricted ilation or bioavailability of the vitamin requires a digestive step to free the vitamin from Nutrition Research Grant. Mead Johnson its coenzyme form so as to permit transport by a specific carrier into the enterocytes and sponsored the symposium and provided an across the intestine. Thus, phosphatases, nucleosidases, and polyglutamyl hydrolases honorarium for conference attendance, presentation of the paper, and submission become critical elements in vitamin bioavailability. Little is known about the timing of the of a manuscript. The authors are entirely development of the vitamin transport systems in humans and the digestive enzymes, but and exclusively responsible for its content. some evidence exists that the vitamin transport systems in the gut already are present in Presented as part of a symposium recognizing the 25th anniversary of the Bristolthe fetal intestine. These transporters are chemically distinct, capable of concentrating Myers Squibb Freedom to Discover Nutrivitamins in the intestine against high gradients by using adenosine triphosphate (ATP), tion Grants Program, held June 7-8, 2005, at the University of Cincinnati, Cincinnati, and some are sodium-dependent and linked to Na-K-ATPase. Beyond these, we will OH. emphasize unique systems, protected through evolution, which are highly relevant to the Submitted for publication Apr 14, 2006; process of meeting infant vitamin requirements. accepted Jun 1, 2006. Take the case of folic acid and vitamin B12, the last 2 vitamins identified just before Reprint requests: Irwin H. Rosenberg, MD, Jean Mayer USDA, HNRCA, Tufts Univerthe middle of the 20th century, near the end of what can be referred to as the era of
T
ATP DRI FAD FBP
S62
Adenosine triphosphate Dietary reference intakes Flavin adenine dinucleotide Folate-binding protein
FMN NAD NADP
Flavin adenine mononucleotide Nicotine adenine dinucleotide Nicotine adenine dinucleotide phosphate
sity, 711 Washington St, Boston, MA 02111; E-mail: [email protected]. 0022-3476/$ - see front matter Copyright © 2006 Mosby Inc. All rights reserved. 10.1016/j.jpeds.2006.06.053
vitamin discovery in nutrition. Whether you are a history enthusiast or not, the historical record in the 1940s is highly relevant to our topic here. A quarter of a century after the discovery of the first and only vital amine (vitamin), thiamine, Lucy Wills reported in the 1930s that a form of macrocytic anemia in India could be cured by nutritional means. It would take another15 years before folic acid was isolated and chemically characterized as the substance that could reverse or cure the “tropical macrocytic anemia” identified by Wills in pregnant Indian women. Lederle Laboratories was the first to synthesize folic acid in 1945; next was Parke Davis Labs, which actually got natural folate right by isolating the polyglutamyl conjugate of folate. Initially, it was hoped that the polyglutamate was the antipernicious anemia principle, but that idea gave way in 1947 after the structure of vitamin B12 was elucidated. In this tumultuous decade of the 1940s is the basis for the confusing and complicated interaction of folic acid and B12, the basis for the upper level for folate in the dietary reference intakes (DRIs),1 and even the distinction between synthetic folic acid and natural reduced folyl polyglutamates, explaining some of the bioavailability differences. As we soon shall see, we also had the genesis of a large mistake in the recent DRIs, which assigned human milk folate to have low bioavailability in the new, and still questionable, Dietary Folate Equivalents. Some at Mead Johnson will remember how the misclassification of human milk folate bioavailability led to a conflict among infant formulas in how to meet folate requirements on the basis of assumed human milk folate bioavailability. The so-called “masking of pernicious anemia” derives from the late 1940s and early 1950s, when synthetic folic acid was used in substantial doses (milligram quantities) in a vain and possibly counter-productive effort to treat pernicious anemia. Not only are the histories of folate and vitamin B12 intertwined, but also there are important intersections of their metabolism and some techniques of transport used by both vitamins. The 2 vitamins share coenzyme functions in the reaction catalyzed by methionine synthase, in which methyltetrahydrofolate provides the methyl group for the remethylation of homocysteine in a vitamin B12 coenzymatic reaction. This cycle is essential for the myriad methylations using s-adenosylmethionine, including DNA methylation and regulation of gene expression, and for modulating blood levels of potentially toxic homocysteine. For this discussion, one commonality is in the mechanism of circulating the vitamin with specific binding proteins, transcobalamins in the case of B12 and folate-binding protein for folate. Folate in human milk, mostly methyl tetrahydro folate, is bound to and secreted with its unique binding protein, which is similar to the folate receptor alpha in the placenta and choroids plexus of the blood-brain barrier. Human milk folate is absorbed bound to protein, largely intact, through the permeable infant intestine after birth. The infant intestine remains permeable to proteins, including immuno-
Folate And Vitamin B12 Transport Systems In The Developing Infant
globulins for weeks or months postnatally, after which the intestine matures and develops more tight junctions between cells. There are several features of human milk folate that clearly are distinctive from folate in foods: 1) milk folate is principally in methylated forms; 2) it is quantitatively bound to high affinity folate-binding protein (FBP); 3) its concentration in milk is approximately 10-fold greater than in maternal circulation; and 4) the transfer of human milk folate into the gut of the infant is not preceded by earlier digestion in the stomach or intestinal lumen. The acid pH in the stomach, which may be deleterious to folate stability in the adult,2 is not well developed in the infant stomach.3 The FBP-folate complex remains intact as it passes through the stomach. Although the transport of unbound folic acid in the mature intestine is a carrier-mediated process using the reduced folate carrier, which already is present in fetal tissues, the absorption of milk folate occurs by a different mechanism, exploiting the permeability of the infant intestine to proteinbound folate. For vitamin B12, binding proteins are essential for transfer in human milk, blood, and across epithelia as well. Maternal B12, bound to its circulating protein, transcolbalamin II, is taken up by receptors on the maternal placenta, and the vitamin is released into the fetal circulation. In the fetus, the free vitamin binds to transcolbalamin II produced by the fetal liver. Almost all the vitamin B12 in human milk is bound to another transcolbalamin I, or haptocorrin, which is stable to proteolytic enzymes of the gastrointestinal tract, and thus the complex may be absorbed intact by human infants. The vitamin B12 content of colostrum appears to be slightly richer than that of mature milk, reflecting, in part, the higher concentration of binding protein. In the first postnatal weeks, vitamin B12 is absorbed efficiently by the permeable infant intestine as intact B12 haptocorrin complex. As with folate, the vitamin is released in the intestinal cell with the intracellular digestion of the protein. Unbound B12 in infant formula, by contrast, will use the developing system in which association with gastric intrinsic factor will be followed by subsequent uptake of the B12intrinsic factor complex after recognition by receptors in ileal cells.
REFERENCES 1. Food and Nutrition Board, Institute of Medicine. Folate. In: Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. Washington, DC: National Academy Press; 1998, p. 196 –305. 2. Seyoun E, Selhub J. Properties of food folates determined by stability and susceptibility to intestinal pteroylpolyglutamate hydrolase action. J Nutr 1998;128:1956-60. 3. Nagita, A, Amemoto K, Yoden A, Aoki S, Sakaguchi M, Ashida K, et al. Diurnal variation in intragastric pH in children with and without peptic ulcers. Pediatr Res 1996;40:497-507.
S63
AND JACOB
SELHUB, PHD
B vitamin transport systems in infants are not as well studied as those for amino acids and glucose. For most B vitamins, a 2-step process allows for digestion of coenzyme forms of the vitamins in food, followed by specific transport systems for the free vitamin in the intestine. Folate and vitamin B12 have specific binding proteins, which carry the vitamins in human milk and blood, and other unique binding proteins, which convey the vitamins across the placenta, intestine, and blood-brain barrier. The permeable infant intestine permits the efficient intact transfer of protein-bound folate and vitamin B12 from intestinal lumen to bloodstream. As the intestine matures, specific carriers and receptors facilitate uptake and transport of these vitamins. (J Pediatr 2006;149:S62-S63)
he human infant is highly vulnerable and highly dependent on its mother for nutritional adequacy and, for the purposes of this discussion, prevention of vitamin deficiencies. For the most part, such deficiencies are prevented by maternal delivery of vitamins to the fetus via the placenta before birth and by providing vitamins after birth as human milk or by infant formula and complementary feeding. Infant vitamin malnutrition is well described and may be life threatening, as with hemorrhagic vitamin K deficiency, infantile beri beri, infantile scurvy, and deficiencies of vitamins B6, B12, and folate. This raises important questions about the efficiency and mechanisms of transfer of nutrients from mother to fetus and the capacity of the infant to assimilate nutrients after birth. Although the topic of transplacental transfer of nutrients is rich and vital, our attention in this paper is to the development of systems in the infant for assimilation of vitamins, especially B vitamins and particularly, folic acid and vitamin B12. B vitamin transport systems in infants are not as well studied as systems for amino acids and glucose, but we do know that there are specific transport systems for the individual vitamins. Unlike groups of amino acids, none of the vitamin transport systems are shared, emphasizing the uniqueness of each vitamin despite their shared assigned nomenclature. Moreover, the digestive tract and intestine are capable of preprocessing or digesting vitamin forms in diet that are often in the complex form of coenzymes in food—for example, thiamine as thiamin pyrophosphate, niacin as nicotine adenine dinucleotide (NAD), or nicotine adenine dinucleotide phosphate (NADP), riboflavin as flavin adenine mononucleotide From Friedman School of Nutrition Science (FMN) or dinucleotide (FAD), B6 as pyridoxal phosphate, and folate as polyglutamates and Policy, Tufts University, Boston, MA. of reduced and one carbon-substituted pteroyl glutamates. In all these instances, assimDr Rosenberg was a recipient of a BristolMyers Squibb–Mead Johnson Unrestricted ilation or bioavailability of the vitamin requires a digestive step to free the vitamin from Nutrition Research Grant. Mead Johnson its coenzyme form so as to permit transport by a specific carrier into the enterocytes and sponsored the symposium and provided an across the intestine. Thus, phosphatases, nucleosidases, and polyglutamyl hydrolases honorarium for conference attendance, presentation of the paper, and submission become critical elements in vitamin bioavailability. Little is known about the timing of the of a manuscript. The authors are entirely development of the vitamin transport systems in humans and the digestive enzymes, but and exclusively responsible for its content. some evidence exists that the vitamin transport systems in the gut already are present in Presented as part of a symposium recognizing the 25th anniversary of the Bristolthe fetal intestine. These transporters are chemically distinct, capable of concentrating Myers Squibb Freedom to Discover Nutrivitamins in the intestine against high gradients by using adenosine triphosphate (ATP), tion Grants Program, held June 7-8, 2005, at the University of Cincinnati, Cincinnati, and some are sodium-dependent and linked to Na-K-ATPase. Beyond these, we will OH. emphasize unique systems, protected through evolution, which are highly relevant to the Submitted for publication Apr 14, 2006; process of meeting infant vitamin requirements. accepted Jun 1, 2006. Take the case of folic acid and vitamin B12, the last 2 vitamins identified just before Reprint requests: Irwin H. Rosenberg, MD, Jean Mayer USDA, HNRCA, Tufts Univerthe middle of the 20th century, near the end of what can be referred to as the era of
T
ATP DRI FAD FBP
S62
Adenosine triphosphate Dietary reference intakes Flavin adenine dinucleotide Folate-binding protein
FMN NAD NADP
Flavin adenine mononucleotide Nicotine adenine dinucleotide Nicotine adenine dinucleotide phosphate
sity, 711 Washington St, Boston, MA 02111; E-mail: [email protected]. 0022-3476/$ - see front matter Copyright © 2006 Mosby Inc. All rights reserved. 10.1016/j.jpeds.2006.06.053
vitamin discovery in nutrition. Whether you are a history enthusiast or not, the historical record in the 1940s is highly relevant to our topic here. A quarter of a century after the discovery of the first and only vital amine (vitamin), thiamine, Lucy Wills reported in the 1930s that a form of macrocytic anemia in India could be cured by nutritional means. It would take another15 years before folic acid was isolated and chemically characterized as the substance that could reverse or cure the “tropical macrocytic anemia” identified by Wills in pregnant Indian women. Lederle Laboratories was the first to synthesize folic acid in 1945; next was Parke Davis Labs, which actually got natural folate right by isolating the polyglutamyl conjugate of folate. Initially, it was hoped that the polyglutamate was the antipernicious anemia principle, but that idea gave way in 1947 after the structure of vitamin B12 was elucidated. In this tumultuous decade of the 1940s is the basis for the confusing and complicated interaction of folic acid and B12, the basis for the upper level for folate in the dietary reference intakes (DRIs),1 and even the distinction between synthetic folic acid and natural reduced folyl polyglutamates, explaining some of the bioavailability differences. As we soon shall see, we also had the genesis of a large mistake in the recent DRIs, which assigned human milk folate to have low bioavailability in the new, and still questionable, Dietary Folate Equivalents. Some at Mead Johnson will remember how the misclassification of human milk folate bioavailability led to a conflict among infant formulas in how to meet folate requirements on the basis of assumed human milk folate bioavailability. The so-called “masking of pernicious anemia” derives from the late 1940s and early 1950s, when synthetic folic acid was used in substantial doses (milligram quantities) in a vain and possibly counter-productive effort to treat pernicious anemia. Not only are the histories of folate and vitamin B12 intertwined, but also there are important intersections of their metabolism and some techniques of transport used by both vitamins. The 2 vitamins share coenzyme functions in the reaction catalyzed by methionine synthase, in which methyltetrahydrofolate provides the methyl group for the remethylation of homocysteine in a vitamin B12 coenzymatic reaction. This cycle is essential for the myriad methylations using s-adenosylmethionine, including DNA methylation and regulation of gene expression, and for modulating blood levels of potentially toxic homocysteine. For this discussion, one commonality is in the mechanism of circulating the vitamin with specific binding proteins, transcobalamins in the case of B12 and folate-binding protein for folate. Folate in human milk, mostly methyl tetrahydro folate, is bound to and secreted with its unique binding protein, which is similar to the folate receptor alpha in the placenta and choroids plexus of the blood-brain barrier. Human milk folate is absorbed bound to protein, largely intact, through the permeable infant intestine after birth. The infant intestine remains permeable to proteins, including immuno-
Folate And Vitamin B12 Transport Systems In The Developing Infant
globulins for weeks or months postnatally, after which the intestine matures and develops more tight junctions between cells. There are several features of human milk folate that clearly are distinctive from folate in foods: 1) milk folate is principally in methylated forms; 2) it is quantitatively bound to high affinity folate-binding protein (FBP); 3) its concentration in milk is approximately 10-fold greater than in maternal circulation; and 4) the transfer of human milk folate into the gut of the infant is not preceded by earlier digestion in the stomach or intestinal lumen. The acid pH in the stomach, which may be deleterious to folate stability in the adult,2 is not well developed in the infant stomach.3 The FBP-folate complex remains intact as it passes through the stomach. Although the transport of unbound folic acid in the mature intestine is a carrier-mediated process using the reduced folate carrier, which already is present in fetal tissues, the absorption of milk folate occurs by a different mechanism, exploiting the permeability of the infant intestine to proteinbound folate. For vitamin B12, binding proteins are essential for transfer in human milk, blood, and across epithelia as well. Maternal B12, bound to its circulating protein, transcolbalamin II, is taken up by receptors on the maternal placenta, and the vitamin is released into the fetal circulation. In the fetus, the free vitamin binds to transcolbalamin II produced by the fetal liver. Almost all the vitamin B12 in human milk is bound to another transcolbalamin I, or haptocorrin, which is stable to proteolytic enzymes of the gastrointestinal tract, and thus the complex may be absorbed intact by human infants. The vitamin B12 content of colostrum appears to be slightly richer than that of mature milk, reflecting, in part, the higher concentration of binding protein. In the first postnatal weeks, vitamin B12 is absorbed efficiently by the permeable infant intestine as intact B12 haptocorrin complex. As with folate, the vitamin is released in the intestinal cell with the intracellular digestion of the protein. Unbound B12 in infant formula, by contrast, will use the developing system in which association with gastric intrinsic factor will be followed by subsequent uptake of the B12intrinsic factor complex after recognition by receptors in ileal cells.
REFERENCES 1. Food and Nutrition Board, Institute of Medicine. Folate. In: Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. Washington, DC: National Academy Press; 1998, p. 196 –305. 2. Seyoun E, Selhub J. Properties of food folates determined by stability and susceptibility to intestinal pteroylpolyglutamate hydrolase action. J Nutr 1998;128:1956-60. 3. Nagita, A, Amemoto K, Yoden A, Aoki S, Sakaguchi M, Ashida K, et al. Diurnal variation in intragastric pH in children with and without peptic ulcers. Pediatr Res 1996;40:497-507.
S63