Nutrition - macronutrient metabolism

Nutrition - macronutrient metabolism - technical

Topics covered:

  • Essentials
  • Regulation of macronutrient flux
  • Carbohydrate metabolism in the postabsorptive and postprandial states
  • Fat metabolism in the postabsorptive and postprandial states
  • Inter-relationships between carbohydrate and fat metabolism
  • Protein and amino acid metabolism and their regulation

Essentials

Food intake is sporadic: for most people it occurs in three major boluses each day. Energy expenditure, however, is continuous, with variations during the day that bear no resemblance to the pattern of energy intake, except that over some reasonable period of time (a week or more) the two will, in most people, match almost exactly. Therefore the body has developed complex systems that direct nutrients into storage pools when they are in excess, and that regulate the mobilization of nutrients from these pools as they are needed.

The situation is analogous to the fuel tank of a car and the throttle that regulates fuel oxidation, except that in the car there is just one fuel and just one engine: in humans there are three major nutrients and a variety of tissues and organs, each of which may have its own preferences for fuels, that vary with time. Carbohydrate, fat, and protein (made up of amino acids), are the three sources of energy which are variably stored and assimilated from food each day. The fact that we can carry on our daily lives without thinking about whether to store or mobilize fuels, and which to use, attests to the remarkable efficiency of these control systems.

Regulation of macronutrient flux

The principal macronutrient stores are listed in Table 1 and are related to daily fluxes of energy substrates in the body.

The need for the coordinated control of nutrient storage, mobilization, and flux between tissues and along the many metabolic pathways, is met by a complex series of control mechanisms. These may be viewed on several levels. The simplest involves the effects of substrate concentration, and is dependent upon the kinetic properties of enzymes and transport proteins. .

The next level involves more specific interaction of nutrients, or pathway intermediates, with enzymes, usually through allosteric effects (binding of the effector alters the conformation of the enzyme and hence its catalytic properties). There are many examples in the metabolism of carbohydrate, fat, and protein. The enzyme 6-phosphofructo-1-kinase (EC 2.7.1.11) in the glycolysis pathway is a good example, subject to allosteric regulation by many compounds that relate to the energy status of the cell. For instance, it is activated by AMP (indicating energy shortage) and inhibited by ATP. Such mechanisms undoubtedly provide important fine tuning of flux along various pathways, entirely in accord with the modern view that control of flux does not reside in certain rate-limiting steps but is distributed among many steps along a pathway. Related to this, the enzyme AMP-activated protein kinase responds to the cellular energy status and regulates a number of metabolic pathways accordingly (see ‘Further reading’).

These mechanisms operate essentially within tissues. However, the coordination of nutrient metabolism requires considerable interaction between tissues and organs. This coordination is largely brought about by the hormonal and nervous systems. Certain hormones play a particularly important role in regulation of macronutrient flux (Table 2). The role of the nervous system in metabolic regulation is often difficult to assess. Although the effects of adrenaline are properly regarded as hormonal, liberation of noradrenaline from sympathetic nerve endings in tissues may bring about identical effects and can be difficult to distinguish. The somatic nervous system (motor neurons innervating skeletal muscle) has clear effects, e.g. stimulation of breakdown of muscle glycogen linked to muscle contraction. The autonomic nervous system probably plays multiple roles, but some are indirect, e.g. regulation of blood flow and cardiac output, thus affecting delivery of substrate to tissues, and regulation of the secretion of pancreatic hormones.

The effects of hormones are mediated in many ways, but may be divided into acute effects (usually acting within seconds or minutes), often brought about through reversible phosphorylation of enzymes, and longer-term effects (hours or days), brought about by regulation of gene expression. The former are usually exerted through binding to cell surface receptors linked to a variety of second-messenger systems, the latter through nuclear receptors (e.g. for glucocorticoids and thyroid hormones). However, the distinction is not absolute: e.g. insulin brings about both acute and longer-term effects through binding to the same cell surface receptor.

Table 1 Macronutrient stores in relation to daily intake
Macronutrient Total amount in body Energy equivalent (MJ) Days’ supply if the only energy source Daily intake (g) Daily intake as percentage of store
Carbohydrate 0.6 kg 8.5 <1 300 60
 Free glucose 12 g        
 Liver glycogen 100 g        
 Muscle glycogen 500 g        
Fat (triacylglycerol) 12–18 kg 550 56 100  0.7
 Circulating in plasma 5 g        
 Stored in adipocytes 12–18 kg        
Protein and amino acids 12 kg 200 (20) 100  0.8
 Free amino acids 100 g        
 Protein 12 kg        

Note: These are very much typical rounded figures. Days’ supply is the length of time for which this store would last if it were the only fuel for oxidation at an energy expenditure of 10 MJ/day: the figure for protein is given in parentheses since protein does not fulfil the role of an energy store in this way.

A further level of coordination is through the effects of nutrients themselves, or important cellular components such as cholesterol, upon gene expression (summarized in Table 3). This can be seen as a longer-term mechanism to ensure that metabolism is appropriate to the diet being ingested and the lifestyle followed. A variety of nutrient response elements are known in the promoter regions of genes for enzymes concerned with substrate metabolism. Particular examples are the carbohydrate response element (which up-regulates expression of genes for glucose metabolism such as pyruvate kinase in the glycolysis pathway, and lipogenic genes), the sterol response element (by which insulin activates lipogenic gene expression, and cellular sterols down-regulate expression of the low density lipoprotein receptor and the enzymes of cholesterol biosynthesis) and response elements for fatty acid derivatives. Fatty acids affect gene expression through a family of transcription factors known as the peroxisome proliferator activated receptors, summarized in Table 4. The expression of many genes is also regulated by insulin.

Table 2 Major hormonal effects on intermediary metabolism
Hormone Origin Target tissue Major metabolic effects Comments
Insulin Pancreatic islets (β-cells) Liver Stimulation of glycogen synthesis/suppression of glycogen breakdown Regulates glucose storage in liver
Stimulation of glycolysis/suppression of gluconeogenesis Regulates hepatic glucose output
Suppression of fatty acid oxidation/ketogenesis Via malonyl CoA
Stimulation of triacylglycerol synthesis  
Stimulation of cholesterol synthesis  
Skeletal muscle Stimulation of glucose uptake Via recruitment of GLUT4 (see Fig. 1)
Stimulation of glycogen synthesis  
Net protein anabolic effect Suppression of protein breakdown may be more important than stimulation of protein synthesis
Adipose tissue Activation of triacylglycerol removal from plasma Via lipoprotein lipase
Suppression of fat mobilization Via hormone-sensitive lipase
Glucagon Pancreatic islets (α-cells) Liver Stimulation of glycogen breakdown/suppression of glycogen synthesis In effect the regulation is via the insulin/glucagon ratio
Stimulation of gluconeogenesis/suppression of glycolysis  
Stimulation of fatty acid oxidation/ketogenesis  
Adrenaline Adrenal medulla Adipose tissue Stimulation of fat mobilization Via hormone-sensitive lipase
Skeletal muscle Stimulation of glycogen breakdown Acts in concert with muscle contraction
Tri-iodothyronine Thyroid All oxidative tissues Increase in basal metabolism  
Cortisol Adrenal cortex Liver Stimulation of gluconeogenesis  
Skeletal muscle Generally catabolic effect on protein  
Adipose tissue Promotes site-specific fat deposition (central depots) and fat mobilization (peripheral depots)  
Growth hormone Anterior pituitary Liver Stimulation of gluconeogenesis Direct effect: other effects are mediated indirectly via insulin-like growth factors
Adipose tissue Stimulation of fat mobilization This is an acute effect: chronically, growth hormone promotes mobilization from central fat depots
Insulin-like growth factors (IGF) I and II Liver (IGF-I) and other tissues (both) Several Generally insulin-like acute effects on metabolism Physiological role is probably longer-term effects on growth
Leptin Adipose tissue Hypothalamus Suppression of appetite; possibly stimulation of energy expenditure Latter effect prominent in rodents, may not occur in humans; low leptin levels (signalling starvation) more important than high levels signalling excess
Reproductive system Signals sufficient fat stores for reproduction to be possible As with effects on hypothalamus, low leptin may be a signal of starvation

Carbohydrate metabolism in the postabsorptive and postprandial states

Postabsorptive state

In the overnight-fasted (postabsorptive) state, no glucose enters the plasma from the small intestine. Glucose enters and leaves the plasma at about 2 mg/kg body weight per min (200 g/24 h). About one-half of this will be consumed by the brain. Of the remainder, a considerable proportion will be utilized by blood cells and peripheral tissues by anaerobic glycolysis, thus returning lactate to the liver for reconversion to glucose. This is the Cori cycle.

Glucose is produced by hepatocytes from glycogen breakdown and from gluconeogenesis. Net glycogen breakdown is stimulated by the relatively low insulin/glucagon ratio after overnight fasting. The major substrates for gluconeogenesis are lactate and pyruvate, released from blood cells and peripheral tissues, together with alanine and glycerol. The pathway of gluconeogenesis predominates over that of glycolysis, again because of the relatively low insulin/glucagon ratio.

Table 3 Mechanisms by which nutrients regulate expression of genes involved in macronutrient metabolism
Stimulus Transcription factor Examples of proteins whose expression is regulated at the mRNA transcription level Comments
Glucose Carbohydrate-response element binding protein
  • Pyruvate kinase (liver isoform) (+)
  • Acetyl CoA carboxylase 1 (+)
  • Fatty acid synthase (+)
  • SREBP-1c (see below) (+)
  • Insulin (in the pancreatic β-cell) (+)
See Fig. 2
Insulin Various, binding to a variety of insulin response elements (see ‘Further reading’)
  • GLUT 1, 2, 3, 4 (glucose transporters) (+)
  • Hexokinase, glucokinase (+)
  • Glyceraldehyde-3-phosphate dehydrogenase (+)
  • Glucose-6-phosphatase (–)
  • Acetyl CoA carboxylase 1 (+)
  • Fatty acid synthase (+)
  • SREBP-1c (+)
Glycolysis and lipogenesis are activated, gluconeogenesis suppressed; see ‘Further reading’ for more information.
Cholesterol (and insulin) Sterol regulatory element binding proteins (SREBP)
  • SREBP-1c:
    • Acetyl CoA carboxylase 1 (+)
    • Fatty acid synthase (+)
    • Stearoyl CoA desaturase (+)
  • SREBP2:
    • LDL receptor (+)
    • HMG CoA synthase (+)
    • HMG CoA reductase (+)
The two major isoforms, SREBP-1c and SREBP2, regulate respectively lipogenesis (in response to glucose and insulin) and cellular cholesterol homeostasis (in response to cellular sterol levels; low sterol levels allow mature SREBP2 to migrate to the nucleus)
Fatty acids Peroxisome proliferator activated receptors (PPARs) See Table 4 PPARs act as transcription factors as heterodimers with the retinoid-X receptor; the endogenous ligand is unclear: it might be a fatty acid (weak affinity) or a fatty acid derivate (e.g. a prostaglandin) (higher affinity)
Amino acids
  • Mammalian target of rapamycin (mTOR);
  • Activating transcription factor 4 (ATF4)
  • IGFBP-1 (–)
  • Asparagine synthase (–)
  • Amino acid transporters (–) (especially neutral amino acid transport system A; cationic amino acid transporter CAT-1)
  • Induction of IGFBP-1 (binds IGF-1) when amino acid supply is restricted limits growth;
  • molecular mechanisms are described in ‘Further reading’

Note: (+), indicates gene induction; (–), gene suppression.

Glucose metabolism following a meal

When a meal enters the system, this pattern of metabolism changes rapidly. About 12 g of free glucose are present in the circulation and extravascular space. Typically, a single meal will provide about 100 g of glucose, entering the circulation over perhaps 60 min. In order to minimize variations in plasma glucose concentration, coordinated mechanisms come into play to suppress the production of endogenous glucose and to increase the rate of removal of glucose from the circulation.

Much of the incoming glucose may be taken up by hepatocytes as described earlier, but some enters the systemic circulation and stimulates pancreatic insulin secretion (and somewhat suppresses glucagon secretion). Insulin is liberated into the portal vein. Thus, the liver is exposed to high concentrations of glucose (from the small intestine) and insulin. The net effect is to reverse glycogenolysis, so that glycogen synthesis begins. In addition, gluconeogenesis is suppressed and glycolysis favoured. Hepatocyte glucose output is therefore rapidly suppressed and converted to an uptake of glucose. At the same time, utilization of glucose by insulin-sensitive peripheral tissues such as skeletal muscle and adipose tissue is increased. The main mechanism of this short-term change is the recruitment of the insulin-regulated glucose transporter GLUT4 to the cell membrane (Fig. 1). However, the reduction in concentration of plasma nonesterified fatty acids (see following paragraphs) will also remove inhibition of glucose uptake caused by fatty acid oxidation. Within muscle, glycolysis and glycogen synthesis will be stimulated by insulin. In adipose tissue, increased glucose uptake provides glycerol 3-phosphate (formed from glycolysis) for esterification of fatty acids (see following paragraphs). Thus, insulin is the key regulator of the rapid changes that occur in glucose metabolism in the postprandial state: it brings about glucose storage as glycogen, and promotes the utilization of glucose at the expense of fatty acids.

Table 4 Peroxisome proliferator activated receptors (PPARs): tissue distribution and effects of activation
Receptor Main tissue distribution Examples of proteins whose expression is regulated at the mRNA transcription level Comments
PPAR-α
  • Liver (main site)
  • Kidney, heart, muscle, brown adipose tissue
  • Positive:
    • Apolipoprotein AI
    • Apolipoprotein AII
    • Enzymes of peroxisomal fatty acid oxidation
    • Liver FABP
    • CPT-1
    • Enzymes of mitochondrial fatty acid oxidation
  • Negative:
    • Apolipoprotein CIII
Target for the fibrate lipid-lowering drugs
PPAR-δa Widespread Positive:
  • Fatty acid transporters (e.g. CD36)
  • Enzymes of mitochondrial fatty acid oxidation
  • Enzymes of mitochondrial oxidation (succinate dehydrogenase, citrate synthase, cytochrome c, cytochrome oxidase)
  • Uncoupling proteins 1, 2, 3
Effects have been documented in adipose tissue, skeletal muscle and heart; agonists are in early clinical trials
PPAR-γ1 Widespread at low levels    
PPAR-γ2 Adipose tissue
  • Positive:
    • Factors involved in adipocyte differentiation
    • Adipose tissue FABP (also known as aP2)
    • Lipoprotein lipase
    • Fatty acid transport protein
    • Acyl CoA synthase
    • GLUT4
    • Phosphoenolpyruvate carboxykinase
  • Negative:
    • Leptin
Target for the thiazolidinedione insulin-sensitizing agents

a Also known as PPAR-β, NUC 1, FAAR (fatty-acid activated receptor).

CPT-1, carnitine palmitoyltransferase-1; FABP, fatty acid binding protein; GLUT4, insulin-regulated glucose transporter; HDL, high-density lipoprotein.

Fat metabolism in the postabsorptive and postprandial states

Forms of fat in the circulation

Fatty acids circulate in various forms: as nonesterified fatty acids, in triacylglycerol (triglyceride), esterified to glycerol in phospholipids, and esterified to cholesterol as cholesteryl esters. The first two are involved in energy metabolism. The main carriers of triacyl-glycerol in the circulation are the triacylglycerol-rich lipoproteins: chylomicrons secreted from the small intestine and transporting dietary fat, and very low density lipoprotein (VLDL) particles secreted from the liver, transporting endogenous triacylglycerol. In the postabsorptive state, chylomicron triacylglycerol secretion is virtually zero. Secretion of VLDL is a means of exporting fat from the liver to peripheral tissues. In these tissues it is hydrolysed by the enzyme lipoprotein lipase (EC 3.1.1.34) situated in the capillaries of skeletal muscle, adipose tissue, mammary glands, and other tissues that use fatty acids. Lipoprotein lipase acts on the circulating triacylglycerol-rich particles to liberate fatty acids which may diffuse into the parenchymal cells (muscle fibres, adipocytes, etc.). Lipoprotein lipase in skeletal muscle is down-regulated by insulin, whereas that in adipose tissue is up-regulated by insulin. In the postabsorptive state, muscle lipoprotein lipase is likely to predominate as the site of removal of triacylglycerol from the VLDL particles. The fatty acids can then be used as an oxidative fuel by the muscle. In this process, VLDL particles lose their triacylglycerol core and become relatively enriched with cholesterol and phospholipids. After several cycles through such capillary beds, they are reduced to simple particles with a core of cholesteryl ester and an outer phospholipid shell: they become low density lipoprotein (LDL) particles, the main carrier of cholesterol in the circulation.

Nonesterified fatty acids and ‘energy transport’

Fat is mobilized from adipose tissue stores in the form of nonesterified fatty acids (Fig. 3). The adipocyte has a central droplet of triacylglycerol, which is hydrolysed by intracellular enzymes, hormone-sensitive lipase and the newly identified adipose triglyceride lipase (ATGL), releasing glycerol and nonesterified fatty acids. These fatty acids are liberated into the plasma bound to albumin for transport to other tissues, including liver and skeletal muscle. Hormone-sensitive lipase is stimulated by catecholamines but powerfully suppressed by insulin, each exerting control over reversible phosphorylation: insulin leads to dephosphorylation and deactivation. Thus, fat mobilization is active in the postabsorptive state when there is a call upon the body’s fat stores. It is also activated during exercise, mainly by catecholamine stimulation. The turnover of nonesterified fatty acids in the plasma is rapid. They are the major oxidative fuel in muscle after overnight fast (glucose supplies only around 5% of the oxidative fuel for resting skeletal muscle in the postabsportive state). In the liver, fatty acids are both a fuel for oxidation and a substrate for synthesis of triacylglycerol that will be exported as VLDL. A typical concentration of nonesterified fatty acids in the plasma after overnight fast is 500 µmol/litre, one-tenth that of glucose, but because of their rapid turnover and their larger molecular mass fatty acids account for about twice the energy turnover of glucose in the circulation.

Disposition of dietary fat

Dietary fat is almost entirely (typically 95% or more) in the form of triacylglycerol. A typical meal might contain 30 to 40 g of fat. The typical plasma triacylglycerol concentration in a healthy subject is 1 mmol/litre, confined to the vascular space; this means that about 3 g of triacylglycerol is present in the circulation. Therefore, as in the case of glucose, the amount in a meal could overwhelm the system unless coordinated mechanisms come into play to ensure its rapid dispersion.

Dietary triacylglycerol is digested in the stomach and small intestine and packaged by the enterocytes of the duodenum and proximal jejunum into chylomicrons, which enter the circulation via the lymphatics (Fig. 3). Therefore, unlike other nutrients absorbed from the small intestine, they bypass the liver on first passage. The chylomicrons also carry other lipid constituents of food, including cholesterol and fat-soluble vitamins. In the circulation their fate is similar to that of VLDL particles, although the tissue-specific regulation of lipoprotein lipase ensures that adipose tissue (where lipoprotein lipase is up-regulated by insulin) is a major site of clearance of their triacylglycerol. The pathway of triacylglycerol synthesis in adipocytes, as in the liver, is stimulated by insulin. Therefore, there is a short and energy-efficient pathway for storage of dietary fatty acids in adipose tissue. The half-life of chylomicron triacylglycerol in the circulation is about 5 min. After hydrolysis of most of the triacylglycerol, the remnant particles are removed by receptors in the liver and other tissues. Thus dietary cholesterol, which remains in the remnant particles along with fat-soluble vitamins, is transported mainly to the liver.

Provided that a meal contains carbohydrate or protein, stimulation of insulin secretion will rapidly suppress the mobilization of adipose tissue fat stores, and concentrations of nonesterified fatty acids in the plasma fall after a meal. Therefore utilization of fatty acids by tissues such as skeletal muscle and liver will be reduced simply by lack of availability. This reduces competition for oxidation in muscle, further increasing glucose utilization. In liver, the lack of nonesterified fatty acids is likely to decrease the secretion of VLDL triacylglycerol. Insulin appears also to suppress VLDL triacylglycerol secretion directly. This is somewhat controversial, and the effects of insulin may be different in the acute, postprandial situation from the situation of prolonged hyperinsulinaemia (as in insulin resistance). Within the liver, insulin powerfully stimulates esterification of fatty acids (for triacylglycerol synthesis) at the expense of oxidation of fatty acids (see following paragraphs), so the suppressive effect of insulin on VLDL triacylglycerol secretion can only be short term. Nevertheless, it seems an exact parallel with the suppression of hepatic glucose output by insulin.

Inter-relationships between carbohydrate and fat metabolism

Links between carbohydrate and fat

In mammals, fat cannot be converted to glucose in a net sense. Glucose can, however, be converted to fat. Acetyl CoA produced by pyruvate dehydrogenase leaves the mitochondrion (it is transported across the mitochondrial membrane as citrate), and is then a substrate for the pathway of de novo lipogenesis, which begins with the enzyme acetyl CoA carboxylase (EC 6.4.1.2), forming malonyl CoA. At one time it was believed that de novo lipogenesis was a major route for laying down storage fat. Although this may be true in rodents, recent measurements have shown that this pathway makes a quantitatively small contribution to triacylglycerol synthesis in humans. Instead, almost all the triacylglycerol that we deposit in adipose tissue arises from dietary fatty acids, taken up from circulating triacylglycerol-rich lipoproteins by the lipoprotein lipase pathway.

The lack of quantitatively significant interconversion of carbohydrate and fat has led to the suggestion that we may view carbohydrate and fat balance as independent. This view is entirely erroneous. Despite the lack of interconversion, carbohydrate balance strongly influences fat balance, and vice versa. These influences occur at a number of levels.

Principal amongst these is carbohydrate-induced insulin secretion. Insulin, as outlined earlier, acutely suppresses the release of nonesterified fatty acids from adipose tissue. Therefore, when carbohydrate is readily available, fat stores are conserved. In the longer term, ingestion of a high-carbohydrate diet will induce enzymes of fat synthesis and down-regulate enzymes of fatty acid oxidation, through insulin- and carbohydrate-response elements in the promoter regions of the relevant genes (see Table 3).

Glucose–fatty acid cycle

Beyond this, there are specific cellular mechanisms that regulate the relative oxidation of carbohydrate and fat. These probably operate in a number of tissues, although they have been most studied in skeletal and heart muscle and in liver. In 1963, Philip Randle and colleagues described the glucose–fatty acid cycle, which encompasses one aspect of this mutual relationship between carbohydrate and fat oxidation. The concept was based upon observations that availability of fatty acids reduced the oxidation of glucose in skeletal and cardiac muscle. The precise mechanism has been disputed, but the basic observation has been confirmed many times. The glucose–fatty acid cycle describes both the normal interplay between fat and carbohydrate oxidation, and also pathological situations involving excess availability of fat and insulin resistance (e.g. type 2 diabetes and obesity).

Glucose and the regulation of fatty acid oxidation

An additional mechanism was first described in 1977 by Denis McGarry and Daniel Foster. They were following up a long-standin observation that the generation of ketone bodies by the liver was suppressed by insulin. They showed that malonyl CoA, the first committed intermediate in the pathway of de novo lipogenesis (produced by acetyl CoA carboxylase; see above), strongly inhibits fatty acid oxidation. This inhibition is mediated via the enzyme carnitine palmitoyltransferase-1 in the mitochondrial membrane. Carnitine palmitoyltransferase-1 is responsible for the transport of fatty acids from the cytoplasm to the mitochondrion for β-oxidation. Acetyl CoA carboxylase is activated by insulin (both by increased genetranscription and by reversible dephosphorylation). Hence, in a carbohydrate-replete state, malonyl CoA will be formed and fatty acid oxidation inhibited.

This is now recognized as a widespread regulatory mechanism. There are two isoforms of acetyl CoA carboxylase. Acetyl CoA carboxylase 1, expressed in lipogenic tissues such as liver and adipose tissue, is involved in de novo fatty acid synthesis. Acetyl CoA carboxylase 2 is expressed more in tissues oxidizing fatty acids such as heart and skeletal muscle and is thought to produce malonyl CoA for regulatory, rather than synthetic, purposes. Muscle carnitine palmitoyltransferase-1 is more sensitive to inhibition by malonyl CoA than is the liver enzyme. The ability of glucose to inhibit the oxidation of fatty acids in muscle has been clearly demonstrated in vivo, and has been termed the ‘reverse glucose–fatty acid cycle’.

Protein and amino acid metabolism and their regulation

Since there are 20 different amino acids incorporated into protein, and a variety of other amino acids that have important biological roles, it is essential here to generalize somewhat about amino acid and protein metabolism. Insulin exerts a net anabolic role on body protein, mainly in skeletal muscle, whereas thyroid hormones and cortisol are generally catabolic. Anabolism is also stimulated by anabolic steroids, by physical training, and during growth by the insulin-like growth factors.

Dietary protein, digested in the small intestine and absorbed as free amino acids and short peptides, enters the portal vein. In the enterocytes of the small intestine, some amino acids, especially glutamine, are removed for use as an oxidative fuel. The remaining products of digestion next enter the liver, where further preferential extraction takes place. Amino acid oxidation is, under most circumstances, the major oxidative pathway in the liver. About 60% of incoming amino acids may be directed into immediate oxidation. The rate of hepatic protein synthesis is also high, and since much of the protein is secreted (e.g. albumin), this represents a net loss of amino acids from the liver (perhaps a further 20% of the incoming amino acids). The remaining mixture of amino acids, around 20% of those absorbed, enters the systemic circulation. This mixture is enriched in the branched chain amino acids leucine, isoleucine, and valine, which have a special role in muscle.

Urea synthesis takes place only in the liver. (The pathway is present in the brain, but this is not a significant site of blood urea production.) Therefore, amino acids released from proteolysis in peripheral tissues must transfer their amino nitrogen to the liver. This results in considerable interaction between the pathways of amino acid, carbohydrate, and fat metabolism. Measurements of arteriovenous differences across muscle and adipose tissue show that the release of the amino acids alanine and glutamine predominates. Since glutamine carries two nitrogens it is, under most circumstances, the predominant carrier of nitrogen. Arteriovenous difference measurements across the splanchnic bed (by catheterization of the hepatic vein) show an almost identical pattern for uptake: removal of alanine and glutamine far exceeds that of other amino acids. Therefore amino acids in tissues including muscle and adipose tissue must transfer their amino nitrogen to alanine (by transamination with pyruvate) and glutamine (formed from glutamate, itself arising by transamination with 2-oxoglutarate). Aminotransferases (transaminases) bring about this transfer. It is important that the 2-oxoacid acceptors, pyruvate and 2-oxoglutarate, are common metabolic intermediates and thus readily available.

Much of the alanine released from skeletal muscle comes from transamination of pyruvate formed in glycolysis. Within the liver, the amino group can be transferred further, e.g. to oxaloacetate, forming aspartate, which is one of the immediate donors of nitrogen to the urea cycle. The pyruvate thus formed may be a substrate for gluconeogenesis, producing glucose that can be recycled to peripheral tissues. This metabolic cycle has been called the glucose–alanine cycle. It closely parallels the Cori cycle.

The other route of entry of nitrogen into the urea cycle is via ammonia. In peripheral tissues ammonia may be formed by the oxidative deamination of glutamate, catalysed by glutamate dehydrogenase (EC 1.4.1.2). This reaction, in combination with the aminotransferases, can be seen to capture amino nitrogen from a number of amino acids. However, blood ammonia concentrations are very low (it is highly toxic) and instead it seems to be fixed in the amido group of glutamine by the enzyme glutamine synthetase. In the liver, the ammonia required for the urea cycle may be formed from the amido nitrogen of glutamine, removed by the enzyme glutaminase (EC 3.5.1.2), or by the oxidative deamination of glutamate. There is also a supply of ammonia from the small intestine.

An important aspect of the large store of muscle protein is that it represents a potential source of synthesis of new glucose during fasting. In that situation, while the brain continues to require glucose for oxidation, and as glycogen reserves are depleted, new glucose can only be formed from glycerol, released in adipose tissue lipolysis, and from amino acids. The pathways described earlier are for the transfer of nitrogen, but not necessarily of carbon, to the liver. To explain the latter, pathways must exist whereby amino acid carbon can also be exported. Amino acids whose 2-oxoacid can enter the tricarboxylic acid cycle may generate pyruvate (which can also accept amino nitrogen to become alanine). Pairs of amino acids can provide all the carbons necessary for glutamine synthesis.

tags: