A number of studies have shown that a large proportion of l-glutamate is metabolized during its transcellular journey through pig enterocytes. Indeed, in pigs, virtually all the enteral l-glutamate is metabolized by the gut during absorption (Reeds et al. 1996). In a 7-kg piglet, an increase in l-glutamate concentrations was observed in portal and arterial blood plasma when the basal milk formula, administered enterally at a rate of 510 μmol kg/h, was supplemented with a relatively high dose of monosodium glutamate, i.e., 1,250 μmol/kg/h (Janeczko et al. 2007). Similarly, in larger (60 kg) pigs, transient portal and arterial hyperglutamatemia was observed when the diet (i.e., 800 g meal containing 142 g casein) was supplemented with 10 g monosodium glutamate (Blachier et al. 1999), indicating that only very large doses of l-glutamate may exceed the intestinal capacity to catabolize this amino acid.

It has been demonstrated that dietary l-glutamate is the most important contributor among substrates for mucosal oxidative energy metabolism in pigs (Stoll et al. 1999). ATP production and utilization are very active in pig enterocytes. Indeed, although the gastrointestinal tract represents only approximately 5 % of body weight, it is responsible for around 20 % of whole-body oxygen consumption (Vaugelade et al. 1994). The pig intestinal epithelial cells have notably high energy demand (Madej et al. 2002; Wu and Knabe 1995) due to the rapid renewal of the epithelium within few days (Wiese et al. 2003), thus requiring intense anabolic metabolism. Sodium extrusion at the basolateral membranes through the activity of Na/K ATPase following nutrient absorption may also represent a major ATP-consuming process in enterocytes (Buttgereit and Brand 1995).

The metabolic steps involved in l-glutamate utilization in pig enterocytes involve firstly transamination with oxaloacetate to produce alpha-ketoglutarate and l-aspartate (Blachier et al. 2009) (Fig. 5.1). l-glutamate can also be transaminated in the presence of pyruvate to produce l-alanine and alpha-ketoglutarate. Transamination appears to be the principal route by which l-glutamate is converted to alpha-ketoglutarate in pig enterocytes, because these cells have little capacity for the conversion of l-glutamate into alpha-ketoglutarate (and ammonia) through glutamate dehydrogenase (Madej et al. 2002). l-glutamate and l-glutamine are similarly oxidized by pig enterocytes (Blachier et al. 1999). However, for l-glutamine, this amino acid has to firstly enter the mitochondria in order to be degraded to l-glutamate and ammonia by the phosphate-dependent glutaminase activity (Wu et al. 1994, 1995a; Fig. 5.2).

l-glutamate that arises within the mitochondria can be either metabolized locally or exported into the cytosol, where the amino acid is metabolized into alpha-ketoglutarate by transamination before reentering the mitochondria and the tricarboxylic acid cycle (Duée et al. 1995). When both l-glutamate and l-glutamine are simultaneously presented to isolated pig enterocytes, l-glutamate can inhibit l-glutamine utilization and oxidation by inhibiting mitochondrial glutaminase activity (Blachier et al. 1999). Presumably, the sparing effect of l-glutamate over l-glutamine is dependent on the relative concentration of both amino acids in mitochondria.

l-glutamate and/or l-glutamine can be used by pig enterocytes to produce other amino acids including l-aspartate (Blachier et al. 1999), l-alanine (Wu et al. 1995a), l-proline (Wu and Knabe 1994), l-ornithine (Blachier et al. 1992), and l-citrulline (Wu et al. 1994) (Fig. 5.1). Note that neither ornithine nor citrulline is present in proteins. l-aspartate and l-alanine are thus produced in the course of glutamate catabolism in pig enterocytes. l-aspartate, either produced endogenously from glutamate/glutamine or originating from the exogenous luminal content, can enter mitochondria and is then oxidized in the TCA cycle (Windmueller and Spaeth 1976), thus representing another oxidative fuel for enterocytes. The stepwise conversion of l-glutamate to l-ornithine is performed in mitochondria. Then, l-ornithine can serve as a precursor for citrulline production (Blachier et al. 1991b). l-citrulline is then released into the portal vein and passes through the liver without major uptake and is then presumably used for de novo synthesis of l-arginine in kidneys (Flynn and Wu 1996).

Together with l-cysteine and glycine, l-glutamate is the precursor for the synthesis of glutathione in pig enterocytes (Fig. 5.3).

Studies in pigs suggest extensive utilization of dietary cysteine by the intestine (Stoll et al. 1998). It has been reported that in fed piglets, mucosal glutathione is derived largely from the direct metabolism of enteral l-glutamate (Reeds et al. 1997). The ratio of reduced to oxidized glutathione is an important measure for both the determination of the intracellular redox status and for the cell’s capacity to control intracellular concentrations of both oxygen-reactive and nitrogen-reactive species (Chakravarthi et al. 2006; Kemp et al. 2008).

Metabolism of l-glutamate and l-glutamine in enterocytes has been studied in growing piglets. Enterocytes isolated from piglet small intestine at birth display a high capacity for l-glutamine oxidation (Darcy-Vrillon et al. 1994), indicating that this amino acid can be used as a fuel at this stage of development when the growth of the intestine mucosa is very intense (Widdowson et al. 1976; Klein and McKenzie 1983a, b). Furthermore, enterocytes isolated from newborn pigs are able to convert l-glutamine into l-citrulline and then l-arginine (Blachier et al. 1993; Wu et al. 1994). This metabolic capacity of enterocytes may reflect the high content in sow’s milk proteins of glutamate and glutamine and the relatively low content of arginine (Wu and Knabe 1994). In suckling piglets, it has been observed that, soon after birth, the net enterocyte’s capacity to convert l-glutamine to l-citrulline is severely decreased compared with piglets at birth (Wu et al. 1995b). In contrast, l-glutamine still represents a major oxidative energy substrate for enterocytes in these animals. Indeed, in suckling piglet enterocytes, l-glutamine is approximately eight times more rapidly oxidized than in enterocytes isolated from weaned pigs (Darcy-Vrillon et al. 1994). In addition, conversion of enteral glutamate to proline in neonatal piglet gastrointestinal tract has been measured by Murphy (1996). Some interactions have been reported between glutamine and carbohydrate metabolism in piglet enterocytes. For instance, in these cells, the metabolism of d-glucose in the pentose cycle which allows NADPH synthesis appears to increase the net conversion of l-glutamine into l-proline (Wu 1996).

Colonic epithelial cells are also able to metabolize glutamate and glutamine. An important characteristic of the colonic epithelium, as said above, is that except for a short period after birth, there is little or no transfer of amino acids from the lumen to the portal blood (Darragh et al. 1994). Then, amino acids (including l-glutamate and l-glutamine) must be taken from arterial blood. Pig colonocytes can use l-glutamine as oxidative substrates (Darcy-Vrillon et al. 1994). Like in enterocytes, l-glutamine is firstly converted into l-glutamate and ammonia by the mitochondrial glutaminase, and then into alpha-ketoglutarate, mainly by transamination, followed by entry into the tricarboxylic cycle.

Colonocytes can also use luminal organic acids generated from the microbial activity, including short-chain fatty acids, as oxidative substrates (Darcy-Vrillon et al. 1993). Dietary substrates for short-chain fatty acid production are mainly dietary fibers, resistant starch, and some amino acids including l-glutamate (Bindelle et al. 2011; Htoo et al. 2007). Regarding amino acid metabolism by the microbiota, although alimentary protein digestion followed by amino acid and oligopeptide absorption by the small intestine is efficient in pigs (Stein et al. 1990), substantial amounts of nitrogenous compounds of both exogenous (alimentary) and endogenous origin may enter the pig large intestine through the ileocaecal junction. The first event in the large intestine protein degradation is hydrolysis of proteins and polypeptides by microbiota and residual pancreatic proteases and peptidases, which results in peptide and amino acid release, followed by the production of numerous bacterial metabolites. In the large intestinal lumen, l-glutamate released from proteins and peptides is the precursor for acetate and butyrate production (Blachier et al. 2007), but the relative contributions of l-glutamate and alimentary indigestible polysaccharide to acetate and butyrate production have not been determined. Glutamine synthetase, in terms of protein expression and catalytic activity, has been measured in pig colonocytes (Eklou-Lawson et al. 2009). Because the l-glutamine-degrading enzyme glutaminase is also highly expressed in colonocytes, this raises the open question of the physiologic meaning of the expression within the same cells of l-glutamine-synthesizing and -degrading enzymes. Because ammonia at concentrations that can be found in the pig colonic lumen inhibits short-chain fatty acid oxidation in colonocytes (Darcy-Vrillon et al. 1996), it has been proposed that cytosolic glutamine synthetase activity, which converts l-glutamate and ammonia into l-glutamine, may represent a way to reduce the intracellular concentration of ammonia during its transfer from the lumen to the bloodstream (Eklou-Lawson et al. 2009), thus avoiding metabolic disturbances.

Utilization of l-glutamate in colonocytes is not restricted to energy metabolism. Indeed, l-glutamine metabolism in pig isolated colonocytes leads to a net production of l-aspartate, l-alanine, and lactate (Darcy-Vrillon et al. 1993). However, unlike what is observed in enterocytes, pig colonocytes produce more l-aspartate than l-alanine from l-glutamine.