An illustration of the position and general structure of the liver.

While the liver serves a variety of functions, the most crucial is its role in the body's metabolism. There is no organ that is more important to healthy metabolism than the liver - in many ways, it is as central to metabolism as the heart is to the circulation of blood. The liver plays a critical role in four key areas of metabolism: fuel management, nitrogen excretion, the regulation water distribution between the blood and tissues, and the detoxification of foreign substances.

Because of the crucial importance of healthly metabolism to overall health, diseases of the liver, such as hepatitis C, can be devastating, leading to fatigue, malaise, and even to death.

Liver cells from a healthy human liver.

Energy Production in Biological Systems Our society depends on energy derived from burning fuels of various kinds in "engines" designed to release, capture, and deliver that energy in a useful form. Typically, wood, oil or coal are burned in the presence of oxygen (air) to liberate energy with the production of CO2 and water. These combustion processes typically occur quite rapidly and generally require relatively high temperatures.

Biological systems operate on the basis of similar energetic principles. There are three types of fuel consumed by a living cell, carbohydrate, fat, and protein and living cells derive large amounts of energy from the "burning" (oxidation) of these fuels. These biological fuels are closely related to those used in mechanical engines and their consumption produces the same waste products, CO2 and water. Carbohydrate and wood are very similar (starch and cellulose are both composed entirely of glucose), fat and oil are both hydrocarbons, and protein is converted to a complex set of carbon based molecules.

While the overall energetics are similar, the molecular processes of energy production in living cells are profoundly different. Unlike mechanical engines, living cells can produce energy very efficiently at temperatures of less than 100 degrees F while capturing that energy and coupling it to systems that carry out useful work with very little waste or loss.

A detailed treatment of the reactions involved in energy production is not necessary for our purposes here. However, it is useful to note that all biological fuels ultimately are converted to one of a small number of intermediates in what is known as the Tricarboxylic Acid Cycle (TCA cycle; sometimes called the Citric Acid Cycle.) The name TCA was given to this cycle because a key intermediate is citric acid, a molecule that contains three carboxylic acid (COOH) groups.

The TCA cycle can be viewed as starting with a reaction in which a four carbon molecule (oxaloacetate) combines with a two carbon molecule (acetyl- CoA) to form a six carbon molecule (citrate). The following steps of the cycle involve a series of reactions in which two carbons are released as carbon dioxide (CO2) and the remaining four carbons are used to reform the original four carbon starting material, oxaloacetate. With every turn of the cycle, two

Mitochondria in a human liver cell.

carbons are oxidized to CO2 and the material (oxaloacetate) needed for another turn of the cycle is regenerated.

The oxidation of carbon to CO2 is coupled to the reduction of other compounds. These in turn are oxidized (donate electrons) by a series of reactions coupled to a final step that consumes molecular oxygen to produce water. These reactions are all carried out in a small subcellular "organelle," the mitochondrion, and taken together lead to the production of a great deal of useful energy.

An adequate level of dietary protein is an essential for good health. Protein molecules are composed of a family of twenty different smaller molecules. These are known collectively as "amino acids" since each contains both an acid group (COOH) and an amino group (NH3). While the twenty amino acids share this common structure, they differ from one another in that each has a different "side chain." The side chain gives each amino acid a different molecular shape.

There are thousands of different kinds of protein molecules, each composed a a linked chain of amino acids arranged in a particular unique sequence. The particular sequence of differently shaped amino acids drives the folding of the chain in a highly specific manner. The unique folding pattern, determined by the unique sequence of amino acids, produces a protein molecule with a unique shape, and as a consequence a unique surface. Proteins have the ability to form the structural elements of cells by a kind of "self assembly" process in which particular proteins "recognize" one another, and because of their unique surfaces, come together to form more complex structures.

In a similar way, the particular specialized surfaces of proteins can recognize small molecules and bring them together in a way that makes possible chemical reaction between these molecules. Such proteins carry the name "enzyme" and serve as very effective catalysts, making it possible for chemical reactions to occur at high rates at physiological temperatures (98.6 degrees F in humans.) In the absence of catalysis by a particular enzyme, a most reactions would occur only very slowly, or not at all. In addition, the protein surface provides a powerful kind of "guidance" to a particular reaction since it recognizes only two particular molecules, holds the molecules near one another in a highly specific way, and thus promotes only a single chemical reaction.

Electron micrograph of a human liver cell. Protein compounds are used to form distinctive receptor sites on the surface of the cell membrane.

You can see that protein molecules play critical roles in the cell. As enzymes and catalysts, they provide the molecular surfaces that determine the particular reactions that can occur within the cell, and thus regulate the overall set of reactions that make up cellular metabolism. As structural components they provide the molecular scaffolding for the cell and assist in arranging and ordering the spatial relationships between the different enzymes.

Living cells have a constant and very active "maintenance and housekeeping" program where they are constantly synthesizing new protein molecules and degrading old ones. This process has two functions. First, it assures that cellular protein is in good repair as damaged and defective protein molecules are somehow (mechanisms are unknown) selectively identified and degraded. Second, the overall metabolism of the cell needs to be constantly adjusted and "tuned" as the body adjusts to changing levels of fuels and nutrients (fed and fasting states), levels of activity, time of day, etc.

There is a large amount of protein turnover. In a human, approximately one pound of protein is degraded and resynthesized each day. Most of the amino acid liberated by protein degradation is used again in the resynthesis of new protein. But about 10% of the total amino acid is lost as it is converted to other important molecules involved in nervous system function, pigments, various hormones, and a variety of other essential activities. Amino acids are also used as fuel and, when present in excess in the diet, can be converted to fat for storage of excess calories.

Because of the universal importance of protein molecules to living cells, both plant and animal tissues can provide dietary protein. During digestion, the long chains of amino acids that make up complex protein molecules are disassembled to produce the twenty different single amino acids. These are taken up by cells in our digestive system, mostly in small intestine, and released to the blood where they are transported to all of the cells of the body.

1) Amino Acid Metabolism in the Fed State

Amino acids from the diet are used in three ways. They are uniquely used in the synthesis of new protein and in the fed state cells are actively synthesizing the structural and enzymatic proteins required for healthy functioning. As described in the section "DNA, RNA, Protein, and the Code of Life" the synthesis of these proteins is closely regulated by the expression of particular genes. And, of course, it is this selective regulation that determines which proteins are to be synthesized, and in a more global sense, the characteristics, abilities, and activities of each individual cell.

When present in excess, amino acids are also used as fuel. The twenty different carbon skeletons of the twenty different amino acids are each metabolized through a more or less unique series of reactions. Said differently, the degradation of each amino acid occurs by means of a specific pathway. However, the end products of these pathways are the same as various intermediates in the breakdown of glucose. Thus, overall, amino acid degradation results in the production of acetyl-CoA or its precursors and several of the organic acids involved the the TCA cycle (tricarboxylic acid cycle) discussed above. This means that, like carbohydrate, the carbon atoms that make up the amino acids can be converted to CO2 with the production of energy need to support the life of the cell and the organism.

Excess amino acids can also be converted to fat. Again the picture is similar to that for carbohydrate in that carbon structures derived from the amino acids can be converted to Citrate (a TCA cycle component.) Recall that citrate is the required first intermediate in the synthesis of fat. Since liver is the major site of fat synthesis, excess amino acids are taken up by the liver, converted to fat, packaged into transport structures (VLDL) and stored as fat in adipose tissue.

2) Amino Acid Metabolism in the Fasting State

We have seen that carbohydrate and fat can be stored by cells, and by the organism, for use at a later time. Glycogen represents the storage form for carbohydrate and is present in many types of cells, particularly in the liver. Triglyceride represents the storage form for fatty acids synthesized in the liver and stored in adipose (fat) tissue.

There is no storage form for amino acids. They are either converted into protein or they are converted into other compounds. As a consequence, during the fasting state the body begins to break down protein to obtain the amino acids that to support the synthesis of new protein molecules needed for maintenance or to change metabolic activities.

Each individual kind of protein molecule, each particular type of enzyme for example, appears to have a particular rate of turnover. Some proteins are degraded rapidly, such that half of the total amount of the enzyme in a single cell is broken down every 15 minutes or so. Others are degraded more slowly, where the time it takes to degrade half is perhaps an hour, a few hours, or in some instances, several days or weeks.

In addition to their essential role in supporting the synthesis of needed proteins, during the fasting state the amino acids liberated by protein breakdown also assist in energy production. This occurs both at the level of the individual cell in which protein degradation occurs and in whole body metabolism.

a) Amino Acid Breakdown and Energy Production in Peripheral Tissue

Earlier we discussed the importance of sugars, particularly glucose, as a source of TCA cycle intermediates and the essential role of the TCA cycle in the production of energy. Recall that oxaloacetate (OAA) is a critical intermediate in the TCA cycle and that the first step in the cycle involves the combination of OAA and acetyl-CoA to form citrate.

During breakdown of amino acids, the carbon skeleton of many of the amino acids is converted to one on the intermediates of the TCA cycle. Because of its cyclic character, once these intermediates enter the TCA cycle they are easily converted to OAA. The production of OAA from amino acids means that the cell no longer needs to use as much glucose to maintain adequate levels of OAA in the TCA cycle. This, in turn, means that blood glucose is used more sparingly.

b) Amino Acid Breakdown and Glucose Production in the Liver

In the fasting state, a significant portion of the amino acid produced by the breakdown of protein in peripheral tissues, such as muscle, is released to the blood. Because of its very rich blood supply, the liver has excellent access to these circulating amino acids.

These free amino acids are used for two major purposes. The first, just as in peripheral tissue, is for the support of the synthesis of proteins needed by the liver to maintain its own structures and processes. The second is the synthesis of additional glucose for use by other tissues. As discussed earlier (gluconeogenesis - see above), this is a process that is unique to liver. The importance of glucose synthesis is easy to appreciate in the light of the critical role that glucose in supporting the production of energy from other fuels, particularly fat.

Glucose can be synthesized from several key intermediates in metabolism. One of these is from one of the components of the TCA cycle, malate. Just as for OAA, all of the TCA cycle intermediates can be converted to malate. Since the carbon skeletons of many of the amino acids are converted into TCA cycle intermediates, they also serve as starting material for the synthesis of glucose.

With that in mind, it is easy to see that amino acid released from peripheral tissues can be converted to glucose in the liver. (Amino acids are released, taken up by the liver, converted into TCA cycle intermediates that are converted to malate, with malate then being used for the synthesis of glucose.) As we have seen, this newly synthesized glucose can be released to the blood for use by the central nervous system and by other tissues.

In our discussion of amino acid metabolism we concentrated on the use of amino acids in synthesis of new cellular proteins and the way in which the various carbon skeletons derived from amino acids could be used to support energy production in the peripheral tissues and the synthesis of glucose in the liver.

I. Overview

Recall that in addition to the carbon structures, as indicated by their name, amino acids also contain an amino group (NH2.) During the breakdown of amino acids, this amino group raises certain problems for the cell.

Fish release nitrogen as NH3 (free ammonia) through their gills. The gills simultaneously carry out two processes: obtaining oxygen from the water, and releasing ammonia to the water.

The simplest way to deal with the amino group would be to remove it from the amino acid directly, releasing the nitrogen as free ammonia (NH3). Unfortunately, ammonia is extremely toxic and would quickly kill the organism if not promptly removed. Fish are the only members of the animal kingdom to excrete nitrogen in this way. Since they are surrounded by water, they can convert the amino acid amino group to ammonia, release ammonia to the blood, and excrete the ammonia into the surrounding water. Fish circulate blood through the delicate structures of the gills where the blood and water come into close contact. The gills simultaneously carry out two processes. One is obtaining oxygen from the water, the other is releasing ammonia to the water.

Land dwelling animals must deal with nitrogen in a completely different way. Mammals avoid the production of toxic ammonia by converting the amino group into a water soluble non-toxic compound known as urea. Urea is a compound in which two amino groups are attached to a single oxidized carbon atom (CO).

Again the liver plays a central role in metabolism because the synthesis of urea occurs only in liver cells. Once synthesized, urea is released from the liver to the blood and transported to the kidneys where it is concentrated and released from the body in the urine.

II. Nitrogen Metabolism in the Fed State

As described above, free amino acids are obtained by breaking down dietary protein and enter the blood for transport throughout the body. We have seen that the carbon skeletons of excess amino acids are used either as fuel or are converted to fat.

The first step in the breakdown of excess amino acids involves the removal of the amino group and the production of the first intermediate. Since there are 20 different amino acids, there are twenty different compounds that result from removal of the amino group. These are each degraded through a series of reactions unique to that compound.

If excess amino acid is being degraded in a peripheral cell or tissue, the amino acid must be transferred to the liver so that the nitrogen can be converted to urea and excreted from the body. The removal of the amino group from an amino acid is achieved by first transferring the group to an acceptor molecule. There are several possible acceptors, but the bulk of the nitrogen is transferred to a three carbon intermediate formed during the breakdown of glucose.

Here you can see that, once again, glucose is playing a central role in whole body metabolism. This intermediate, pyruvate, is converted to a three carbon compound known as alanine. Alanine is released to the blood and transferred to the liver. In the liver, the nitrogen is moved to another acceptor molecule, regenerating pyruvate. Pyruvate can be used as a precursor for glucose (can enter gluconeogenesis.) It is easy to see a cyclical process (known as the Alanine Cycle) in which the newly synthesized glucose is released from the liver to the blood, transported to the peripheral tissue where the glucose is reconverted to pyruvate, the pyruvate converted once again to alanine and released to the blood to carry nitrogen to the liver where pyruvate is formed, supporting the production of glucose. The net effect of the alanine cycle is to move amino groups from peripheral tissue to the liver with the regeneration of the intermediates needed to carry more amino groups.

Excess amino acid can also be metabolized by liver cells. In this instance there is no need for nitrogen transport since the liver cells can make urea themselves. In the fed state, the carbon skeletons derived from the amino acids enter the TCA cycle (directly or indirectly) where they are either used as fuel (oxidized to CO2) or the excess calories they provide are stored as fat (converted to citrate and then to triglyceride - see above).

III. Nitrogen Metabolism in the Fasting State

Most of the nitrogen in the body is in the form of amino groups on amino acids making up protein molecules. We have seen that the bulk of the bodyÕs protein, particularly the protein that is broken down during fasting, is contained in peripheral tissues.

Once again, if amino acids are to be used as fuel, or to support the TCA cycle so that fat can be used as fuel, it is essential to transport the amino groups to the liver so that toxic nitrogen can be converted to non-toxic urea, released to the blood and then concentrated and excreted by the kidneys.

Transport is achieved by two mechanisms. The first, of central importance to the peripheral tissue itself, is the Alanine Cycle described above. The second, of central importance to the whole body, is the release of free amino acids from the peripheral tissues to the blood. Once in the blood, these amino acids can serve the full range of functions that we have discussed. They can be used directly to support the synthesis of needed protein by all of the body's tissues. They can be taken up by other cells and converted to TCA cycle intermediates (nitrogen transport from these cells to liver will be required, the Alanine Cycle again being significant.) Finally, they can be taken up by the liver and excess amino acid converted to glucose (gluconeogenesis) for release to the blood and the support of whole body metabolism.

At first glance, the regulation of water distribution in the body does not seem particularly important. In fact, the control of the amount of water contained by the body and the distribution of the water within the body are critically important to a wide variety of processes.

Water moves easily through the various compartments of our body, passing quickly through the membrane surrounding each cell, through the spaces between cells, and through the connective tissues and structures that provide the scaffolding that supports us. And of course this is advantageous because many of the essential nutrients required by the body are soluble in water, and the movement of water assures the effective distribution of these nutrients. The movement of water is so important that we have a particularly sophisticated system, the heart and blood vessels, to assure that delivery is fast and that every cell is well served.

At the cellular level, there is a central problem that must be managed. There is a fundamental law that says that substances always move from a region of higher concentration to a region of lower concentration. This principle is so common that we hardly are surprised when we see it in operation, for example we expect that heat will always flow from a warm area to a cold area.

Cell membranes act as a kind of filter, only allowing small molecules to pass through them. As a result, the large enzyme and structural protein molecules described earlier cannot leave the cell. Once they are synthesized inside the cell, they remain there. The interior concentration of protein and other substances that cannot cross the membrane is quite high.

An electron micrograph of human liver cells. Water moves easily through the components of the body, passing through cell membranes and through the space between cells; note the porous appearance of the cell structure.

Since cells contain a high concentration of protein and other complex molecules they also must contain a relatively lower concentration of water. Said differently, all molecules take up space. The more non-water molecule are present, the fewer water molecules there are in that space. Because water can pass easily through the membranes of cells and tissues, it will always flow from an area where the water concentration is high to one where it is lower. This means that if you place a cell in water it will rapidly swell as water enters the cell, and the swelling will continue until the cell bursts.

Having our cells burst is a very bad idea! The only way this can be avoided in animal cells it to have the circulating fluids of the body contain a concentration of molecules that is equal to the concentration of molecules inside the cell. And, of course, these circulating molecules must not be able to pass through the cell membrane. Said differently, the water concentration of the blood, lymph, plasma, and fluids in the spaces between cells, must be the same as the water concentration of the cells.

To maintain water balance, the blood contains a high level of protein that never enters the cellular spaces. Some of these proteins, the various kinds of antibodies produced in response to invasion by foreign substances or organisms, are associated with immunity. Others are involved in repair processes, like blood clotting. But the bulk of the circulating protein is composed of a single protein known as Albumin.

Albumin is produced by only one organ, and of course that organ is the liver. Throughout your life, the liver is sensing the level of complex molecules in the blood and, if the concentration of those molecules is too low, synthesizing and releasing albumin to the blood.

Liver disease and failure is often signaled by decreased albumin synthesis and release resulting in progressive entry of water into cells and tissues. This produces characteristic swelling and a state known as "edema." Obviously, this kind of water retention by cells and tissues can easily become a serious problem, interfering with cellular processes, causing disruption of cell structure and, if water movement into cells is too great, death.

We are constantly being exposed to materials that interfere with cellular processes or that are poisonous. Such materials are described as "toxic" and are generally termed toxins. Some toxins are naturally occurring substances. These may be produced by plants or animals as a means of discouraging other organisms from destroying or eating them. Some toxins are dangerous to most other organisms, others are selective in their effect.

Increasingly, the toxins in our environment are chemicals introduced as a result of industrial processes or as part of products that we buy. There is an enormous, and growing, number of these compounds, and we are producing and using these substances in larger and larger amounts. Ranging from fertilizers and insecticides used in agriculture, to chemicals and solvents used in manufacturing, to plastics, packaging materials, dyes, food additives, and a host of other materials that we have in our homes, handle, and eat on a daily basis. Various drugs and pharmaceutical products are also potentially toxic. Indeed, in many instances it is their toxicity that makes them useful.

In many instances, the potential toxicity of new chemicals and compounds is unknown. However, if they are useful in some part of our economy, they are being produced and released in very large quantities. It is not an overstatement that we are currently running a massive experiment, asking in a rather crude way whether the natural systems, and nature itself, can tolerate, eliminate, or in some other way manage these substances.

Organisms, including humans, have a complex system of enzymes that act on substances that are not a part of our normal metabolic system. These systems are not well understood. The chemicals and substances that they affect, the reactions that they carry out, and the products they produce are largely unknown.

A major portion of this system is built into a complex membrane system within each cell called the "smooth endoplasmic reticulum." Endoplasmic indicates that the membrane is contain in the cell space outside of the nucleus, and reticulum suggests that the membrane system is a complex web of fine tubular structures. The membrane system is described as smooth to distinguish it from the rough endoplasmic reticulum, the site of much of the cells protein synthesis machinery. The smooth endoplasmic reticulum (SER) contains a large number of different enzymes. Among these is a group of enzymes that absorb light of particular frequencies, and are known as "cytochrome p450" enzymes ("cyto" for cell, "chrome" for color).

There are a number of p450 cytochromes. While they are different enzymes they share a common activity, that of adding oxygen (or otherwise oxidizing) the compounds that they modify. There are a number of different oxidation mechanisms and each cytochrome appears to act on a different set of compounds. While much is unknown, the alteration of compounds by the p450 cytochromes changes their chemical nature, and thus their toxicity to cells.

In many instances, cytochrome induced oxidation destroys or diminishes the toxicity of a compound. However, in some cases, cytochrome action may actually produce or increase toxicity. Indeed, some drugs are ineffective until they are activated by the p450 system. The general, if not universal, effect of p450 action is to alter a compound in a way that makes it possible for the body to eliminate it from the body. Elimination is frequently carried out by the kidneys and the compound appears in the urine, hence the common use of urine samples in drug testing.

Once again, the liver plays a central role. Liver cells have very highly developed SER and contain a large proportion of the bodyÕs p450 cytochromes. They also have a rich blood supply with good access to circulating compounds and ability to release modified compounds into the blood. While effective, the liverÕs ability to detoxify chemicals is limited. Chronic exposure to some toxins, such as alcohol, leads to liver disease and can cause death from liver failure. Similarly, the liver can be damaged by brief exposure to high levels of some organic compounds.

A healthy human liver (top) contrasted with a liver from an individual that died from hepatitis C (bottom). Note the extensive damage and scarring from chronic liver disease.

1) The ability of the body to store and synthesize glucose, compromising:

Accumulation of nitrogen wastes with resulting toxicity to many tissues

Human liver cells infected by the hepatitis C virus. Note the killed liver cells scattered throughout the field.

4) Disturbances in water distribution with
 

A healthy human liver (top) contrasted with a liver from an individual that died from hepatitis C (bottom). Note the extensive damage and scarring from chronic liver disease.

1) The ability of the body to store and synthesize glucose, compromising:

Accumulation of nitrogen wastes with resulting toxicity to many tissues

Human liver cells infected by the hepatitis C virus. Note the killed liver cells scattered throughout the field.

4) Disturbances in water distribution with