Choline. An Essential Nutrient for Metabolism

Transcription

1 Choline An Essential Nutrient for Metabolism

2 Choline is a low molecular weight, positively charged quaternary (2-hydroxyethyl-trimethyl) amine with unique chemical and biochemical properties. It is present in nature as a component of a number of biological molecules, and can be prepared in water-soluble salt form. Choline s chemical attributes are the root of its diverse modes of biochemical functionality, and affect its absorption, distribution, metabolism and excretion from the human body. Choline has many important functions in the healthy structure and function of the human body, being at the center of many convergent biological processes. It is integral in mediating processes of cell growth, function and repair and enabling the metabolism and mobilization of other micro-and macronutrients, including vitamin cofactors, amino acids and lipids. Intake of choline profoundly affects the availability of the nutrient to the numerous tissues and organs whose viability depend on it. Water-soluble choline salts are an effective delivery form for the nutrient in dietary supplements and fortified foods. Meeting the body s need for choline When free choline is needed in a tissue or organ, it is either biosynthesized, extracted from circulation (1;2), or scavenged from local endogenous supply in the body (3). While the body can produce choline, biosynthesis is insufficient, even under ideal conditions. The efficiency of endogenous choline production varies with age, gender, and hormonal status, and is itself influenced by diet. All cells synthesize choline via the cytidine diphosphocholine (CDP-choline) mechanism (1;4), known as the Kennedy pathway. Another mode of choline biosynthesis is via the sequential methylation of phosphatidylethanolamine, catalyzed by the enzyme phosphatidylethanolamine N-methyltransferase (PEMT) (5;6). PEMT exists in abundance primarily in the liver, presumably to satisfy the needs incurred there in phospholipid synthesis and to supplement the CDP-choline pathway in times of metabolic stress (6). As the efficiency of this enzyme is estrogen-regulated (7;8), it is not surprising that estrogen-deficient segments of the population, particularly men and post-menopausal women, are most likely to become choline deficient with inadequate dietary intake (9). Despite the abundant estrogen in circulation during gestation, pregnant women are also at increased risk for choline deficiency (10) because of the drain on choline imposed by the fetus (11;12). Choline availability to the fetus (13-15), and later, to the nursing infant, is directly affected by efficiency of PEMT activity (16) and maternal dietary habits (2;17;18). Dietary intake is especially critical to individuals with loss-of-function variations in their genes for choline metabolism, particularly PEMT (5;7). Figure 1. Choline structure acetylcholine acetyl coenzyme A Kennedy pathway phosphocholine betaine dimethylglycine diacylglycerol sphingomyelin ceramide lysophosphatidylcholine phosphatidylcholine PEMT pathway phosphatidylethanolamine methionine S-adenosyl methionine homocysteine platelet-activating factor Figure 2. Choline: An important metabolic intermediate and precursor

3 Choline: Essential for every body Choline is an irreplaceable material requirement of cells for multiple biological purposes. It is considered an essential nutrient for humans (19) because biosynthesis does not ordinarily supply enough to sustain normal organ function, even in a healthy human body. Choline deficiency results from the convergence of less-than-optimal dietary habits and enzyme impairment at the individual genetic level (20-22). It is compounded by an individual s hormonal status (7;9;23) and availability of partially compensatory B-vitamins. Deprivation of dietary choline first manifests itself in a decrease in plasma choline. Sustained deprivation will lead to an increase in plasma homocysteine, an accumulation of fat in vacuoles of liver cells (steatosis), and damage to liver cell membranes (9). As deprivation devolves into dietary deficiency, cell suicide is activated in lymphocytes (24), damage is sustained by muscle cells, and organ dysfunction begins (25). Dietary administration of choline easily prevents and reverses these symptoms of deficiency (5). In fact, current United States dietary recommendations for choline are based on the daily dosage necessary to prevent these abnormalities in most individuals, i.e. 7 mg/kg body weight/day (19). Based on data from studies of choline-deprived humans and animals, Dietary Reference Intakes (DRIs) were developed for choline nutrition in healthy individuals (19) and published by the Food and Nutrition Board of the Institute of Medicine (US) in Substance Function Structure Phosphatidylcholine Membrane phospholipid Sphingomyelin Platelet-activating factor Cell signaling Betaine Osmolyte, methyl donor Acetylcholine Neurotransmitter Table 1. Biologically important choline derivatives (from references (1-6)) 2

4 Digestion and metabolism of choline The metabolism of a meal or ingested supplement begins in the stomach with breakdown of the food matrix by acids and enzymes. The partially digested chyme is moved to the small intestine, where enterocytes bundle up fats, including phospholipids, into chylomicron lipoproteins and release them into the lymph, from where they subsequently enter the blood. Carbohydrates and proteins are broken down into their smallest substituents, namely monosaccharides and amino acids, and are directly released into the bloodstream. Likewise, oral administration of choline readily affects serum choline levels (26-28). When free choline reaches the upper small intestine, a concentration-regulated carrier mechanism transports it into circulation (29). Certain organs and tissues will absorb it in advance of any further metabolism by gut microbiota (26;30-32). It is subsequently stored as the free base or, more commonly, in a phosphorylated form such as phosphocholine (28). In this way, it can be most easily used for the variety of biological purposes for which it is necessary. When blood glucose is elevated, the pancreas increases the peptide hormone insulin, which regulates carbohydrate and fat metabolism in the body by directing glucose uptake by the liver, muscle, and adipose tissue. Choline, like glucose, enters the liver via the portal vein (32). It is used in the liver primarily for the construction of phospholipid membranes needed to package lipids and export them to adipose tissue for storage. Primarily in the liver and in the kidney, choline is converted into the osmolyte betaine in an irreversible oxidation process. In this form, it regulates influx and efflux of water in cells, which is particularly important in the kidney. Against all odds, free choline crosses the blood-brain-barrier (2;33;34) to enter the brain. The concentration of free choline available at neuron terminals in the brain helps regulate choline s conversion to the important neurotransmitter acetylcholine. While comparatively little choline meets this fate, as compared to other conversions (4), the route is important in that it is the only synthetic means to generate this important agent of cellular messaging (2;35). While the process certainly predominates in the brain, acetylcholine synthesis is significant elsewhere in the body, operating in the non-neural cholinergic mechanism present in other cells, tissues and organs, including the epithelium, endothelium, and mesothelium, mesenchyme, and immune cells (36). This pathway may be significant in mediating cellular inflammation, as will be presented later in this summary. Dietary choline is important in that it fills the nutrient pools that can attenuate the need to catabolize and recycle other choline-containing molecules, including membrane phospholipids (2;4;37;38), during periods of high metabolic activity. If choline is ingested in bound (rather than free base) form, for example as cytidine diphosphocholine (CDP-choline) (2;39) or glycerylphosphorylcholine (α-gpc choline) (40), it is quickly hydrolyzed and metabolized as free choline is, suggesting that there is no special benefit to its delivery this way. Lecithin, the naturally-occurring lipid-bound source of choline (as phosphatidylcholine), contains comparatively little of the active choline moiety by weight. Upon ingestion, lecithin is hydrolyzed to a form that is circulated in the lymph in the form of chylomicrons (41). It is transported to multiple organs and tissues, such as the brain, kidney, liver and spleen, and is progressively broken down by multiple enzymes, eventually yielding the free choline molecule (32). Nutrient metabolism by gut microbes is a current area of active biomedical research. Gut bacterial populations and metabolism are highly variable within populations, but also per individual. The activity of gut microbes is known to be influenced by diet (42) and other physiological stressors (43). Choline s biological conversion into trimethylamine (44) and derivative compounds in the gut is regarded as a competitive fate for the molecule, in that it keeps the nutrient from fulfilling its important biochemical purposes. This route is significant in the disposition of choline when it is in considerable excess. 3

5 Role of choline in one-carbon metabolic balance Regulation of homocysteine metabolism Choline supports metabolic balance by its contribution to the so-called cellular methyl pool. In this way, it influences the structure, organization, repair, and transcription of genes, as well as the production and function of the proteins encoded by those genes (45). Methyl groups [CH 3 -] are tags that are transferred between biological macromolecules. They function as epigenetic markers, which are functional tags that can activate or suppress the expression of genes and the activity of catalytic proteins. Studies of the effect of prenatal choline availability on DNA methylation show its profound and lifelong importance; the changes in DNA that the placement of a methyl group brings about are heritable and retained over generations (13;46-48). Once choline is oxidized in the body, it can donate its methyl groups. Methylation of homocysteine, a cellular metabolite which circulates in the blood of all individuals, converts it to methionine. This essential amino acid is needed to repair and build proteins and to serve as a precursor for other substances, such as creatine and S-adenosylmethionine (SAdoMet), the archetypal biological methyl donor (49;50). (Choline s remaining two methyl groups are released in subsequent oxidation steps and diverted to other metabolic processes (51).) Choline insufficiency in humans, even those with adequate methionine, results in an accumulation of unmetabolized homocysteine (52). A lack of choline creates an imbalance of SAdoMet and its immediate precursor (53). Such conditions thwart the optimal activity of the enzymes that assist in methyl group transfer from one macromolecule to another. Increasing choline intake has been shown to reduce plasma homocysteine in healthy and homocysteinemic subjects (54;55). Homocysteine is currently debated by the biomedical research community as a biomarker, by-product, risk-factor or active agent (56) of cellular biochemical change. Its alleged link to numerous health conditions (45) is attributed to cytotoxic and vascular effects (54;55;57;58). Its mechanism of action appears to be related to its oxidative potential and reactivity of its thiol group (51;59). In cell culture studies, unabated homocysteine has been shown to have direct and secondary oxidative effects (45), generating reactive oxygen species (ROS) that trigger degradative chain reactions in cells and tissues (e.g. vascular endothelium), and overwhelming the innate protective mechanisms conferred by antioxidant enzymes and nitric oxide. By its reaction with the thiol groups of amino acid side chains, homocysteine can affect the structure and function of proteins and enzymes. These mechanisms (59) are thought to collectively contribute to fibrosis of the liver (60), cardiovascular occlusion (61;62) and the age-related cognitive decline (63). Dysfunctional homocysteine metabolism has also been associated with negative outcomes in pregnancy, such as low birth weight, pre-eclampsia, placental abruption and recurrent pregnancy loss (64-67). STRUCTURE-FUNCTION CLAIM Choline may help reduce levels of plasma homocysteine. DNA RNA Protein or Enzyme Catalysis or other event CH 3 H 3 C N OH CH 3 Figure 3. One-carbon transfer is an important regulatory mechanism 4

6 Synergy with B-vitamins Choline works in synergy with B-vitamins in homocysteine metabolism because of their shared roles in methylation and amino acid synthesis (52;68). Changing availability of B-vitamins shifts the dynamics of reactions requiring free choline (69). For instance, adequacy of folate nutrition has been demonstrated to be inextricably linked to choline need (23;70-74). When folate is deficient in the diet, choline is decreased in the liver (72). Likewise, when choline is deficient in the diet, folate is found to be depleted in the liver (75). The activity of the choline-mediated pathway that turns over cellular homocysteine spares the complementary nutrient folate for its other very important use in DNA synthesis (74). Vitamin B 2 (riboflavin) is a key component of the cofactor flavin adenine dinucleotide (FAD) for methylene tetrahydrofolate reductase (MTHFR), the enzyme which catalyzes the first steps of the biosynthesis of sulfur-containing amino acids, including that of methionine via homocysteine (76). Vitamin B 12 (cobalamin) is a cofactor for methionine synthase, the enzyme which directly catalyzes the actual conversion (77). Vitamin B 6 is involved in an enzymatic reaction that offers a complementary route of disposal for excess homocysteine. The dietary choline requirement is apparently greater in individuals with certain variations in genes involved in B-vitamin metabolism and one-carbon transfer, particularly the gene encoding the enzyme MTHFR (9;73). It has been observed that drugs that disrupt the metabolic balance of choline and B- vitamins in the body tend to increase homocysteine levels (78), suggesting another important opportunity for dietary intervention. The interrelationship between choline and B-vitamins is highly significant and must be considered in assessing nutrient adequacy. SAMe (SAdoMet) for biological methyla on reac ons amino acid available for protein synthesis dimethylglycine methionine Vitamin B12 betaine homocysteine Vitamin B6 tetrahydrofolate 5-methyl-tetrahydrofolate cysteine Folate dihydrofolate 5,10-methylene tetrahydrofolate Vitamin B2 deoxy-pyrimidine base monomer available for DNA synthesis and repair Figure 4. Choline and the methionine cycle 5

7 Fat metabolism & liver health The manufacture of very low density lipoproteins occurs in the liver as a mechanism to transport non-polar lipids away from the organ within the aqueous environment of circulating blood (79-81). VLDLs are effectively a cellular packaging system designed to transport fat to adipose tissue for storage or to muscles for immediate use (43). VLDLs are composed of an inner hydrophobic core of lipids, primarily triglycerides and cholesterol esters, and an outer layer of amphiphilic molecules, mostly phospholipids, cholesterol, and the membrane protein apolipoprotein B. As a precursor of the phospholipid phosphatidylcholine, free choline is an important component of the structure of VLDLs. The choline that is used by the liver for this purpose is synthesized by a liver-specific pathway (PEMT, discussed earlier), by a partially compensatory general pathway common to all cells (CDP-choline/Kennedy), and the necessary balance must be obtained from the diet (1;6). The net effect of dietary choline deficiency is to decrease triglyceride export from the liver in VLDLs, resulting in excess fat deposition (steatosis) in that organ (82). Certain individuals are, by nature, more prone to this outcome by virtue of their inherent biosynthetic ability. As discussed previously, this is hormonally and genetically dictated to a significant extent, and may also be influenced by metabolism by gut bacteria (42). However, the outcome of fatty liver appears not to be related to a simple reduction in phosphatidylcholine synthesis, but rather an apparent effect on the size, shape and viability (ease of enzymatic breakdown) of the VLDL particles (83). Nor is the derangement of lipid metabolism limited to the liver; choline deprivation affects cellular structural integrity and lipid metabolism in other tissues, as well. Most notably, muscle cell membranes are catabolized and cellular suicide is activated in the absence of choline (25). Triglycerides accumulate in choline deficient muscle cells, not because of accelerated lipogenesis from glucose, but rather from the increased assembly of the molecules from the pool of components (fatty acids, diacylglycerol) scavenged from broken down cell membranes (84). The effect of choline deficiency in humans has been most unmistakably illustrated in individuals on controlled diets, such total parenteral nutrition (TPN) (85). Plasma free choline is observed to decrease significantly with administration of TPN that is inadequate in choline, but sufficient in B-vitamins and methionine (86;87), suggesting a unique importance of choline for which these complementary nutrients cannot compensate. The degree of bioavailability of the choline source (free versus bound) also appears to be important (60) in maintaining normal choline status as measured in serum and whole blood. Longer-term deficiency, in TPN patients as well as in otherwise healthy individuals (88-90), is associated with the clinical onset of liver dysfunction, as indicated by abnormally high measurable serum transaminase activity. Alanine- and aspartate aminotransferases are enzymes which are released from liver cells when their membrane integrity is compromised (88). These metrics are considered to be a general surrogate indicator of hepatocyte damage and steatosis (accumulation of excess fat), which cannot ordinarily be easily clincally observed (19) without invasive biopsy and/or diagnostic imaging. The etiology of liver dysfunction (91) is thought to be related to the rupture of cell membranes caused by steatosis, as well as the induction of apoptosis (controlled cell death) (92) and compensatory uncontrolled cell division in liver tissue (19;93;94). Hepatotoxicity may arise from infiltration of the liver by fatty acids, some of which are oxidized and recycled, collaterally generating reactive oxygen species (ROS) which may interact with macromolecules (e.g. DNA) or membranes (43;95). Though deficiency in choline and deficiency in lipotropes (Vitamin B 12, folate, methionine) both overtly result in fatty liver, the two states are clinically distinguishable from one another. Lipotrope deficiency is associated with wasting and with steatosis which escalates to fibrosis of the liver. Sustained absence of choline in the diet will, remarkably, induce hepatocellular necrosis, and eventually hepatocarcinoma, even in the absence of exogenous carcinogens (91;96). It appears that the absence of the choline moiety itself, rather than a lack of contributed methyl groups, is significant in this regard (97). Triacylglycerol Phosphatidylcholine Apoprotein Figure 5. Very low density lipoprotein VLDL STRUCTURE-FUNCTION CLAIM Choline may promote healthy liver function. 6

8 Role in mediating inflammation Recent research suggests that choline availability may be important in mediation of cellular inflammation, a line of defense against invasion and infection. A recent epidemiological study of adult men and women suggests that levels of typical cellular markers of inflammation in blood (e.g. C-reactive protein, interleukin-6, tumor necrosis factor-α) are decreased in individuals with the greatest exposure to choline in their diets, versus those individuals with low choline diets (98). The possible regulatory mechanism of action of choline in cellular inflammatory processes is currently the subject of research. Choline s role in reduction of circulating homocysteine is likely a partial contributor to this effect. By its metabolism of homocysteine, choline contributes to the generation of SAdoMet. This has many potential implications, as SAdoMet influences a variety of cellular processes involved in immune response (45;98). SAdoMet suppresses the production of several inflammatory first responders, activates an additional, complementary route of disposal for cellular homocysteine (cystathionine β-synthase, with Vitamin B 6 as a cofactor), and promotes the production of the antioxidant glutathione and the necessary activity of many significant methyltransferase enzymes (45;98;99). Choline s use by cells as a precursor of several key phospholipids may be important in mediating inflammatory response, as well. By the mechanism of base exchange, phosphatidylcholine derived from choline can subsequently be interconverted to phosphtidylethanolamine (PE), phosphatidylserine (PS), or phosphatidylinositol (PI) (100), which are involved in cell signaling and membrane structure and transport (101). The non-neural cholinergic system may also be significant in choline s putative anti-inflammatory effect (102). Acetylcholine, synthesized from free choline and acetyl coenzyme A, is the primary neurotransmitter of the vagus nerve, which runs most of the length of the human body and connects the central nervous system with distal organs and tissues. Signaling by the vagus nerve involves release of acetylcholine and uptake of the neurotransmitter by macrophages and other immune cells possessing acetylcholine receptors (103;104). The receipt of the neurotransmitter message inhibits the synthesis and release of cytokines and other pro-inflammatory agents (105). The process operates in tissues such as the airway and lungs, intestines, pancreas, and adipose tissue, which all respond to the magnitude of these signals (106;107), with a variety of clinical repercussions. It has been hypothesized that functional breakdown of the this mechanism contributes to pathogenesis by loss of control over the body s inflammatory response (36;108). In the case of obese adipose tissue, for instance, rampant inflammation has been shown to be related to pre-diabetic insulin resistance (107). Ensuring the ready availability of acetylcholine is likely to be important in efficient operation of the non-neural cholinergic system, via availability of functional transporters and receptors and a well-stocked supply of the precursor molecule choline. Cellular stress Brain Cell Acetylcholine receptors Cell Release of pro-inflammatory molecules Figure 6. Choline s putative role in attenuating inflammation 7

9 Conclusion Choline is currently underappreciated for its central role in the promotion of human health. It is a material requirement of cells for membrane structure and metabolic control, and its presence is integral in maintaining a balance of key biochemicals needed for cell growth, function and repair. Choline is significant in the metabolism and mobilization of other key micro- and macronutrients, including vitamin cofactors, amino acids and lipids. The participation of choline in one-carbon transfer reactions underlies its cellular regulatory epigenetic function. It is also the mechanism by which it reduces the amount of homocysteine circulating in the blood, ostensibly heading off oxidative processes and other undesirable biochemical interactions that could otherwise compromise cognitive function and prenatal and cardiovascular health. Dietary intake of choline is known to affect the availability of the nutrient to the tissues and organs where it is utilized. Choline intake is absolutely necessary, as the body does not produce enough, even under ideal conditions. Choline chloride and choline bitartrate are highly soluble, stable, bioavailable forms that can be incorporated into foods, beverages, and supplements to easily deliver important lifelong nutritional benefits to consumers. This article is for informational purposes only and is not meant to be construed as authoritative legal or medical advice. Balchem makes no representations as to its accuracy and assumes no liability or responsibility for the content of this article. Choline supplements are not intended to diagnose, treat, cure or prevent any disease Balchem Corporation. All rights reserved. 8

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