Amino Acids in Nutrition and Health Amino acids in systems function and health.

Amino Acids in Nutrition and Health Amino acids in systems function and health. Abstract Dietary protein digestion is an efficient process resulting in the absorption of amino acids by epithelial cells, mainly in the jejunum. Some amino acids are extensively metabolized in enterocytes supporting their high energy demand and/or production of bioactive metabolites such as glutathione or nitric oxide. In contrast, other amino acids are mainly used as building blocks for the intense protein synthesis associated with the rapid epithelium renewal and mucin production. Several amino acids have been shown to support the intestinal barrier function and the intestinal endocrine function. In addition, amino acids are metabolized by the gut microbiota that use them for their own protein synthesis and in catabolic pathways releasing in the intestinal lumen numerous metabolites such as ammonia, hydrogen sulfide, branched-chain amino acids, polyamines, phenolic and indolic compounds. Some of them (e.g. hydrogen sulfide) disrupts epithelial energy metabolism and may participate in mucosal inflammation when present in excess, while others (e.g. indole derivatives) prevent gut barrier dysfunction or regulate enteroendocrine functions. Lastly, some recent data suggest that dietary amino acids might regulate the composition of the gut microbiota, but the relevance for the intestinal health remains to be determined. In summary, amino acid utilization by epithelial cells or by intestinal bacteria appears to play a pivotal regulator role for intestinal homeostasis. Thus, adequate dietary supply of amino acids represents a key determinant of gut health and functions. Keywords Amino acids · Intestinal epithelial cells · Intracellular metabolism · Microbiota · Bacterial metabolites · Intestinal barrier 1.1 Introduction The quantities of dietary proteins ingested every day by Humans, whatever their animal or plant origin, are vastly different according to food availability and cultural dietary habits. In Western Europe and United States for instance, protein consumption averages approximately 1.5-fold the recommended daily amount (Rand et al. M. Beaumont GenPhySE, Université de Toulouse, INRA, INPT, ENVT, Toulouse, France e-mail: F. Blachier (*) Université Paris-Saclay, AgroParisTech, INRAE, UMR PNCA, Paris, France e-mail: 1 2 2003; Dubuisson et al. 2010; Pasiakos et al. 2015). In sharp contrast, for instance in Southern Ethiopia, the prevalence of inadequate dietary protein intake represents as much as 94% in women (Asayehu et al. 2017). Protein digestion in the mammalian digestive tract is globally a very efficient process, being generally higher than 90% (Bos et al. 2005; Tomé 2012); even if some dietary proteins, like for instance proteins in rapeseed, are digested with lower efficiency (Bos et al. 2007). The amino acids and oligopeptides that are released from dietary and endogenous proteins in the lumen of the small intestine are absorbed mainly in the proximal jejunum through the enterocytes by a variety of transporters present in the brush-border and baso-lateral membranes of enterocytes (Bröer 2008; Mailliard et al. 1995). The intestinal epithelium can be viewed as a selective barrier towards luminal compounds in a context of a renewal of the intestinal epithelium that is complete within a few days (Potten and Allen 1977; Potten 1997) through mitosis of pluripotent stem cells and differentiation in different phenotypes with specialized functions (Lin 2003; Barker et al. 2008; Moore and Lemischka 2006). In this chapter, we will present how some amino acids are metabolized by the intestinal epithelial cells during their transcellular journey from the lumen to the bloodstream. The consequences of these processes for enterocyte functionality will be presented. Then, the regulatory roles of amino acids in intestinal homeostasis will be described with a focus on the gut barrier and endocrine functions. We will also give an overview on the ways by which the intestinal microbiota metabolizes amino acids; and how such metabolic capacity is linked to functional implications in both the small and large intestine. Then, we will examine how dietary amino acids have an impact on the intestinal microbiota composition. The aim of the authors is not to cover in an exhaustive way the different topics presented in this chapter, but rather to give some representative examples illustrating how amino acid and their derived compounds may have an impact on intestinal physiology. 1.2 Amino Acid Metabolism by the Intestinal Cells and Functional Implications From experimental works performed in animal models, mostly rodents and pigs, and from more limited clinical studies with human volunteers, it appears clearly that a significant part of several dispensable and indispensable amino acids present in the small intestine content are metabolized during their journey from the luminal side of the intestinal epithelium to the portal bloodstream (Baracos 2004). The in vitro studies of amino acid metabolism in the small and large intestine epithelial cells generally used isolated living absorptive enterocytes (Blachier et al. 1993) and colonocytes (Cherbuy et al. 1995) for determining their metabolic capacities towards the different amino acids and their metabolites produced within the luminal content. This in vitro design allows to document the metabolic capacity of intestinal cells towards amino acids, but not to fully extrapolate to the in vivo situation when numerous substrates are present at the same time in the luminal content. A major in vivo experimental design used to estimate the apparent amino acid intestinal absorption and metabolism consists of measuring the amino acid concentrations at different time after a meal in both the arterial and portal blood, as well as measuring continuously the blood flow in the portal vein (Rérat et al. 1988). These experiments help to determine if a given amino acid is globally degraded (e.g. glutamine and glutamate) or produced (e.g. aspartate and alanine) in the intestinal mucosa (Blachier et al. 1999). The limitation of such experiments resides in the fact that the portal vein does not exclusively drain amino acids from the intestine, but also from several other visceral tissues. The utilization of alimentary proteins labelled with stable isotope allows for following more precisely the metabolic fate of amino acids during their intestinal absorption (Morens et al. 2003). Utilization of amino acids in intestinal epithelial cells supports not only protein and nucleotide synthesis, but also the synthesis of various compounds with important biological functions like for instance the tripepM. Beaumont and F. Blachier 3 tide glutathione (Reeds et al. 1997). It is worth noting that in the enterocytes from the small intestine, the amino acids can be supplied from both the luminal route (notably in the postprandial period), but also from the baso-lateral (blood) side (Windmuller and Spaeth 1975); while for the colonocytes, the amino acid supply is believed to be from the blood side exclusively (Darragh et al. 1994), even if this latter point remains somewhat controversial as some amino acid transporters have been identified on the luminal side of colonocytes (van der Wielen et al. 2017). 1.2.1 Glutamate, Glutamine, Arginine and Related Amino Acid Metabolism in Intestinal Absorptive Cells Glutamate and glutamine are extensively metabolized in enterocytes (Darcy-Vrillon et al. 1994) and colonocytes (Darcy-Vrillon et al. 1993). Glutamate can be used in enterocytes for protein synthesis or can be extensively metabolized in other pathways including those involved in enterocyte ATP production (Blachier et al. 2009) (Fig. 1.1). Indeed, glutamine and glutamate are among the most important contributors for energy metabolism in mammalian enterocytes (Ashy et al. 1988) and colonocytes (Ardawi and Newsholme 1985). ATP production and utilization are intense in enterocytes. This corresponds to the fact that although the gastrointestinal tract represents approximately 5% of the body weight, it is responsible for around 20% of whole body oxygen consumption (Vaugelade et al. 1994; Yen et al. 1989). The intestinal epithelium presents a high energy demand (Watford et al. 1979) due to the rapid renewal of the epithelium, thus requiring intense anabolic metabolism. In addition, sodium extrusion through the Na/K ATPase activity following nutrient and electrolyte absorption is likely to represent a major ATP-consuming Fig. 1.1 Glutamate metabolism in intestinal absorptive cells Glutamate is metabolized to alpha ketoglutarate (alpha KG) by transamination with pyruvate (PYR) and oxaltoacetate (OAA). Alpha KG then enter the TCA cycle. Glutamate is also a precursor for the stepwise production of citrulline and proline 1 Amino Acids in Intestinal Physiology and Health 4 process in enterocytes and colonocytes (Buttgereit and Brand 1995). The metabolic steps involved in glutamate utilization in enterocytes involve transamination with oxaloacetate to produce alphaketoglutarate and aspartate (Fig. 1.1). Incidentally aspartate, in addition to glutamine and glutamate, represent a major fuel for the absorptive enterocytes (Windmueller and Spaeth 1976). Glutamate can also be transaminated in the presence of pyruvate to produce alanine and alphaketoglutarate, these latter compounds entering the tricarboxylic cycle in the mitochondria. In contrast, for glutamine oxidation, an initial conversion of glutamine into glutamate and ammonia by the phosphate-dependent glutaminase activity has to proceed in the mitochondria of enterocytes (Pinkus and Windmueller 1977, Duée et al. 1995) (Fig. 1.2). Glutamate, together with cysteine and glycine, are the amino acid precursors for the synthesis of glutathione in mammalian cells including intestinal epithelial cells (Coloso and Stipanuk 1989); and inhibition of mucosal glutathione synthesis is associated with alteration of intestinal functions that can be prevented by giving glutathione monoester orally (Martensson et al. 1990). In addition to their capacity to synthesize glutathione, human enterocytes take up extracellular glutathione (Iantomasi et al. 1997). Glutathione in intestinal mucosa appears to derive largely from the metabolism of enteral glutamate (Reeds et al. 1997). The ratio of reduced to oxidized glutathione is an important parameter for fixing the intracellular redox status and controlling the intracellular concentrations of both oxygen-reactive and nitrogen-reactive species (Chakravarthi et al. 2006; Kemp et al. 2008). Glutamate and glutamine allow the net production of of proline (Wu et al. 1994a), ornithine (Henslee and Jones 1982), and citrulline (Wu et al. 1994b) (Fig. 1.1). Although neither ornithine nor citrulline are present in proteins, they represent important compounds for inter-organ metabolism. Ornithine that is mainly produced Fig. 1.2 Glutamine metabolism in intestinal absorptive cells Glutamine is converted to glutamate and ammonia. Then glutamate is converted to alpha ketoglutarate (alpha KG) that enter the TCA cycle M. Beaumont and F. Blachier 5 together with urea from arginine by the arginase activity in enterocytes (Mouillé et al. 2004), can be exported in the portal vein and be used in the liver as an intermediate in the urea cycle (Lund and Wiggins 1986). A part of ornithine released from the amino precursors is converted to citrulline in enterocytes (Blachier et al. 1991) (Fig. 1.3). Then, citrulline is released in the portal vein, and passes through the liver without major uptake, and is then used for de novo synthesis of arginine in kidneys (Cynober 1994; Dhanakoti et al. 1990). In addition, a minor part of ornithine released from arginine and glutamine can be used by enterocytes and colonocytes for the stepwise production of the polyamines putrescine, spermidine and spermine (Fig. 1.3). These amino acidderived compounds are necessary for intestinal epithelial cells mitosis (Ray et al. 2001). However, except in the neonatal period, the endogenous production of polyamines by enterocytes and colonocytes appears barely detectable in mammals (Blachier et al. 1992; Mouillé et al. 2004), and, since the polyamine circulating concentration are below micromolar concentrations (Bartos et al. 1977), the enterocyte and colonocyte polyamine content depends almost exclusively on the polyamines in the luminal contents (Kumagai and Johnson 1988; Osborne and Seidel 1990), either from dietary or microbiota origin (detailed below) (Bardocz 1993; Blachier et al. 1991) (Fig. 1.3). Arginine, apart from being a precursor of ornithine, is also a precursor of nitric oxide (NO) and citrulline in both enterocytes and colonocytes (Blachier et al. 2011, 1991, 1993; M’Rabet-Touil et al. 1993) (Fig. 1.3).