The body maintains a relatively large free amino acid pool in the blood, even during fasting. As a result, tissues have continuous access to individual amino acids for the synthesis of proteins and essential amino acid derivatives, such as neurotransmitters. The amino acid pool also provides the liver with amino acid substrates for gluconeogenesis and provides several other cell types with a source of fuel. The free amino acid pool is derived from dietary amino acids and the turnover of proteins in the body. During an overnight fast and during hypercatabolic states, degradation of labile protein, particularly that in skeletal muscle, is the major source of free amino acids.

The liver is the major site of amino acid metabolism in the body and the major site of urea synthesis. The liver is also the major site of amino acid degradation. Hepatocytes partially oxidize most amino acids, converting the carbon skeleton to glucose, ketone bodies, or CO₂. Because ammonia is toxic, the liver converts most of the nitrogen from amino acid degradation to urea, which is excreted in the urine. The nitrogen derived from amino acid catabolism in other tissues is transported to the liver as alanine or glutamine and converted to urea.

The branched-chain amino acids, or BCAAs (valine, isoleucine, and leucine), are oxidized principally in skeletal muscle and other tissues and not in the liver. In skeletal muscle, the carbon skeletons and some of the nitrogen are converted to glutamine, which is released into the blood. The remainder of the nitrogen is incorporated into alanine, which is taken up by the liver and converted to urea and glucose.

The formation and release of glutamine from skeletal muscle and other tissues serves several functions. In the kidney, the NH₄⁺ carried by glutamine is excreted into the urine. This process removes protons formed during fuel oxidation and helps to maintain the body’s pH, especially during metabolic acidosis. Glutamine also provides a fuel for the kidney and gut. In rapidly dividing cells (e.g., lymphocytes and macrophages), glutamine is required as a fuel, as a nitrogen donor for biosynthetic reactions, and as a substrate for protein synthesis.

During conditions of sepsis (the presence of various pathogenic organisms, or their toxins, in the blood or tissues), trauma, injury, or burns, the body enters a catabolic state characterized by a negative nitrogen balance (Fig. 40.1). Increased net protein degradation in skeletal muscle increases the availability of glutamine and other amino acids for cell division and protein synthesis in cells involved in the immune response and wound healing. In these conditions, increased release of glucocorticoids from the adrenal cortex stimulates proteolysis.

THE WAITING ROOM

The body maintains a relatively large free amino acid pool in the blood, even in the absence of an intake of dietary protein. The large free amino acid pool ensures the continuous availability of individual amino acids to tissues for the synthesis of proteins, neurotransmitters, and other nitrogen-containing compounds (Fig. 40.2). In a normal, well-fed, healthy individual, approximately 300 to 600 g of body protein is degraded per day. At the same time, roughly 100 g of protein is consumed in the diet per day, which adds additional amino acids. From this pool, tissues use amino acids for the continuous synthesis of new proteins (300 to 600 g) to replace those degraded. The continuous turnover of proteins in the body makes the complete complement of amino acids available for the synthesis of new and different proteins, such as antibodies. Protein turnover allows shifts in the quantities of different proteins produced in tissues in response to changes in physiologic state and continuously removes modified or damaged proteins. It also provides a complete pool of specific amino acids that can be used as oxidizable substrates; precursors for gluconeogenesis and for heme, creatine phosphate, purine, pyrimidine, and neurotransmitter synthesis; for ammoniagenesis to maintain blood pH levels; and for numerous other functions.

The concentration of free amino acids in the blood is not nearly as rigidly controlled as blood glucose levels. The free amino acid pool in the blood is only a small part (0.5%) of the total amino acid pool in whole-body protein. Because of the large skeletal muscle mass, approximately 80% of the body’s total protein is in skeletal muscle. Consequently, the concentration of individual amino acids in the blood is strongly affected by the rates of protein synthesis and degradation in skeletal muscle, as well as the rate of uptake and utilization of individual amino acids for metabolism in liver and other tissues. For the most part, changes in the rate of protein synthesis and degradation take place over a span of hours.

A. Interorgan Flux of Amino Acids in the Postabsorptive State

The fasting state provides an example of the interorgan flux of amino acids necessary to maintain the free amino acid pool in the blood and supply tissues with their required amino acids (Fig. 40.3). During an overnight fast, protein synthesis in the liver and other tissues continues, but at a diminished rate compared with the postprandial state. Net degradation of labile protein occurs in skeletal muscle (which contains the body’s largest protein mass) and other tissues.

1. Release of Amino Acids from Skeletal Muscle during Fasting

The efflux of amino acids from skeletal muscle supports the essential amino acid pool in the blood (see Fig. 40.3). Skeletal muscle oxidizes the BCAAs (valine, leucine, isoleucine) to produce energy and glutamine. The amino groups of the BCAAs, and of aspartate and glutamate, are transferred out of skeletal muscle in alanine and glutamine. Alanine and glutamine account for approximately 50% of the total α-amino nitrogen released by skeletal muscle.

The release of amino acids from skeletal muscle is stimulated during an overnight fast by the decrease of insulin and increase of glucocorticoid (such as cortisol) levels in the blood (see Chapters 28 and 41). Insulin promotes the uptake of amino acids and the general synthesis of proteins. The mechanisms for the stimulation of protein synthesis in human skeletal muscle are not all known, but probably include an activation of the A system for amino acid transport (a modest effect), a general effect on initiation of translation, and an inhibition of lysosomal proteolysis. The fall of blood insulin levels during an overnight fast results in net proteolysis and release of amino acids. As glucocorticoid release from the adrenal cortex increases an induction of ubiquitin synthesis and increase of ubiquitin-dependent proteolysis also occur.

2. Amino Acid Metabolism in Liver during Fasting

The major site of alanine uptake is the liver, which disposes of the amino nitrogen by incorporating it into urea (see Fig. 40.3). The liver also extracts free amino acids, α-keto acids, and some glutamine from the blood. Alanine and other amino acids are oxidized and their carbon skeletons converted principally to glucose. Glucagon and glucocorticoids stimulate the uptake of amino acids into liver and increase gluconeogenesis and ureagenesis (Fig. 40.4). Alanine transport into the liver, in particular, is enhanced by glucagon. The induction of the synthesis of gluconeogenic enzymes by glucagon and glucocorticoids during the overnight fast correlates with an induction of many of the enzymes of amino acid degradation (e.g., tyrosine aminotransferase) and an induction of urea cycle enzymes (see Chapter 36). Urea synthesis also increases because of the increased supply of NH₄⁺ from amino acid degradation in the liver.

3. Metabolism of Amino Acids in Other Tissues during Fasting

Glucose, produced by the liver, is used for energy by the brain and other glucose-dependent tissues, such as erythrocytes. The muscle, under conditions of exercise, when the adenosine monophosphate (AMP)-activated protein kinase is active, also oxidizes some of this glucose to pyruvate, which is used for the carbon skeleton of alanine (the glucose–alanine cycle; see Chapter 36).

Glutamine is generated in skeletal muscle from the oxidation of BCAAs and by the lungs and brain for the removal of NH₄⁺ formed from amino acid catabolism or entering from the blood. The kidney, the gut, and cells with rapid turnover rates, such as those of the immune system, are the major sites of glutamine uptake (see Fig. 40.3). Glutamine serves as a fuel for these tissues, as a nitrogen donor for purine synthesis, and as a substrate for ammoniagenesis in the kidney. Much of the unused nitrogen from glutamine is transferred to pyruvate to form alanine in these tissues. Alanine then carries the unused nitrogen back to the liver.

The brain is glucose dependent, but, like many cells in the body, can use BCAAs for energy. The BCAAs also provide a source of nitrogen for neurotransmitter synthesis during fasting. Other amino acids released from skeletal muscle protein degradation also serve as precursors of neurotransmitters.

B. Principles Governing Amino Acid Flux between Tissues

The pattern of interorgan flux of amino acids is strongly affected by conditions that change the supply of fuels (e.g., the overnight fast, a mixed meal, a high-protein meal) and by conditions that increase the demand for amino acids (metabolic acidosis, surgical stress, traumatic injury, burns, wound healing, and sepsis). The flux of amino acid carbon and nitrogen in these different conditions is dictated by several considerations:

1. Ammonia (NH₃) is toxic. Consequently, it is transported between tissues as alanine or glutamine. Alanine is the principal carrier of amino acid nitrogen from other tissues back to the liver, where the nitrogen is converted to urea and subsequently excreted into the urine by the kidneys. The amount of urea synthesized is proportional to the amount of amino acid carbon that is being oxidized as a fuel.
   

   The differences in amino acid metabolism between tissues are dictated by the types and amounts of different enzyme and transport proteins present in each tissue and the ability of each tissue to respond to different regulatory messages (hormones and neural signals).

   
   
2. The pool of glutamine in the blood serves several essential metabolic functions (Table 40.1). It provides ammonia for excretion of protons in the urine as NH₄⁺. It serves as a fuel for the gut, the kidney, and the cells of the immune system. Glutamine is also required by the cells of the immune system and other rapidly dividing cells in which its amide group serves as the source of nitrogen for biosynthetic reactions. In the brain, the formation of glutamine from glutamate and NH₄⁺ provides a means of removing ammonia and of transporting glutamate between different cell types within the brain. The utilization of the blood glutamine pool is prioritized. During metabolic acidosis, the kidney becomes the predominant site of glutamine uptake, at the expense of glutamine utilization in other tissues. Conversely, during sepsis, in the absence of acidosis, cells involved in the immune response (macrophages, hepatocytes) become the preferential sites of glutamine uptake.
   

   TABLE 40.1 Functions of Glutamine

   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   Protein synthesis

   Ammoniagenesis for proton excretion

   Nitrogen donor for synthesis of:

   Purines

   Pyrimidines

   NAD+

   Amino sugars

   Asparagine

   Other compounds

   Glutamate donor for synthesis of:

   Glutathione

   γ-Aminobutyric acid (GABA)

   Ornithine

   Arginine

   Proline

   Other compounds

   
   
   
   

   Lactic acid is produced from glucose and amino acid metabolism. The ketone bodies (acetoacetate and β-hydroxybutyrate) produced during fatty acid oxidation are also acids. Many α-keto acids, formed from transamination reactions, are also found in the blood.

   
   
3. The BCAAs (valine, leucine, and isoleucine) form a significant portion of the composition of the average protein and can be converted to tricarboxylic acid (TCA) cycle intermediates and used as fuels by almost all tissues. Valine and isoleucine are also the major precursors of glutamine. Except for the BCAAs and alanine, aspartate, and glutamate, the catabolism of amino acids occurs principally in the liver.
   

   The ability to convert four-carbon intermediates of the TCA cycle to pyruvate is required for oxidation of both BCAAs and glutamine. This sequence of reactions requires phosphoenolpyruvate (PEP) carboxykinase or decarboxylating malate dehydrogenase (malic enzyme). Most tissues have one, or both, of these enzymes.

   
   
4. Amino acids are major gluconeogenic substrates, and most of the energy obtained from their oxidation is derived from the oxidation of the glucose formed from their carbon skeletons. A much smaller percentage of amino acid carbon is converted to acetyl coenzyme A (acetyl CoA) or to ketone bodies and oxidized. The use of amino acids for glucose synthesis for the brain and other glucose-requiring tissues is subject to the hormonal regulatory mechanisms of glucose homeostasis (see Chapters 28 and 34).
   
5. The relative rates of protein synthesis and degradation (protein turnover) determine the size of the free amino acid pools available for the synthesis of new proteins and for other essential functions. For example, the synthesis of new proteins to mount an immune response is supported by the net degradation of other proteins in the body.

A. The Kidney

One of the primary roles of amino acid nitrogen is to provide ammonia in the kidney for the excretion of protons in the urine. NH₄⁺ is released from glutamine by glutaminase and from glutamate by glutamate dehydrogenase, resulting in the formation of α-ketoglutarate (Fig. 40.5). α-Ketoglutarate is used as a fuel by the kidney and is oxidized to CO₂, converted to glucose for use in cells in the renal medulla, or converted to alanine to return ammonia to the liver for urea synthesis.

Glutamine is used as a fuel by the kidney in the normal fed state and, to a greater extent, during fasting and metabolic acidosis (Table 40.2). The carbon skeleton forms α-ketoglutarate, which is oxidized to CO₂, converted to glucose, or released as the carbon skeleton of serine or alanine (Fig. 40.6). α-Ketoglutarate can be converted to oxaloacetate by TCA cycle reactions, and oxaloacetate is converted to PEP by PEP carboxykinase. PEP can then be converted to pyruvate and subsequently acetyl CoA, alanine, serine, or glucose. The glucose is used principally by the cells of the renal medulla, which have a relatively high dependence on anaerobic glycolysis because of their lower oxygen supply and mitochondrial capacity. The lactate released from anaerobic glycolysis in these cells is taken up and oxidized in the renal cortical cells, which have a higher mitochondrial capacity and a greater blood supply.

TABLE 40.2 Major Fuel Sources for the Kidney

aGlucose used in the renal medulla is produced in the renal cortex.

B. Skeletal Muscle

Skeletal muscle, because of its large mass, is a major site of protein synthesis and degradation in the human. After a high-protein meal, insulin promotes the uptake of certain amino acids and stimulates net protein synthesis. The insulin stimulation of protein synthesis is dependent on an adequate supply of amino acids to undergo protein synthesis. During fasting and other catabolic states, a net degradation of skeletal muscle protein and release of amino acids occur (see Fig. 40.3). The net degradation of protein affects functional proteins, such as myosin, which are sacrificed to meet more urgent demands for amino acids in other tissues. During sepsis, degradation of skeletal muscle protein is stimulated by the glucocorticoid cortisol. The effect of cortisol is exerted through the activation of ubiquitin-dependent proteolysis. During fasting, the decrease of blood insulin levels and the increase of blood cortisol levels increase net protein degradation.

Skeletal muscle is a major site of glutamine synthesis, thereby satisfying the demand for glutamine during the postabsorptive state, during metabolic acidosis, and during septic stress and trauma. The carbon skeleton and nitrogen of glutamine are derived principally from the metabolism of BCAAs. Amino acid degradation in skeletal muscle is also accompanied by the formation of alanine, which transfers amino groups from skeletal muscle to the liver in the glucose–alanine cycle.

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