The liver is strategically interposed between the general circulation and the digestive tract. It receives 20% to 25% of the volume of blood leaving the heart each minute (the cardiac output) through the portal vein (which delivers absorbed nutrients and other substances from the gastrointestinal tract to the liver) and through the hepatic artery (which delivers blood from the general circulation back to the liver). Potentially toxic agents absorbed from the gut or delivered to the liver by the hepatic artery must pass through this metabolically active organ before they can reach the other organs of the body. The liver’s relatively large size (~3% of total body weight) allows extended residence time in the liver for nutrients to be properly metabolized as well as for potentially harmful substances to be detoxified and prepared for excretion into the urine or feces. Among other functions, therefore, the liver, along with the kidney and gut, is an excretory organ, equipped with a broad spectrum of detoxifying mechanisms. It can, for example, carry out metabolic conversion pathways and utilize secretory systems that allow the excretion of potentially toxic compounds. Concurrently, the liver contains highly specific and selective transport mechanisms for essential nutrients that are required not only to sustain its own energy but to provide physiologically important substrates for the systemic needs of the organism. In addition to the myriad of transport processes within the sinusoidal and canalicular plasma membrane sheets (see below), intracellular hepatocytic transport systems exist in organelles such as endosomes, mitochondria, lysosomes, as well as the nucleus. The sequential transport steps carried out by these organelles include (1) uptake, (2) intracellular binding and sequestration, (3) metabolism, (4) sinusoidal secretion, and (5) biliary excretion. The rate of hepatobiliary transport is determined, in part, by the rate of activity of each of these steps. The overall transport rate is also determined by such factors as hepatic blood flow, plasma protein binding, and the rate of canalicular reabsorption. The various aspects of the major metabolic processes performed by the liver have been discussed in greater detail elsewhere in this text. These previous sources will be referred to as the broad spectrum of the liver’s contributions to overall health and disease is described.
Jean T.’s family difficulties continued, and, in spite of a period of sobriety that lasted 6 months, she eventually started drinking increasing amounts of gin again in an effort to deal with her many anxieties. Her appetite for food declined slowly as well. She gradually withdrew from much of the social support system that her doctors and friends had attempted to build during her efforts for rehabilitation. Upper mid-abdominal pain became almost constant, and she noted increasing girth and distention of her abdomen. Early one morning, she was awakened with pain in her upper abdomen. She vomited dark-brown “coffee ground” material followed by copious amounts of bright red blood. She called a friend, who rushed her to the hospital emergency department.
Amy B., a 23-year-old missionary, was brought to the hospital emergency department complaining of the abrupt onset of fever, chills, and severe pain in the right upper quadrant of her abdomen. The pain was constant and radiated to her right shoulder top. She vomited undigested food twice in the hour before arriving at the emergency department. This did not relieve her pain.
Her medical history indicated that, while serving as a missionary in western Belize, Central America, 2 months earlier, she had several days of mild but persistent diarrhea. A friend of Amy B.’s there, a medical missionary, had given her an unidentified medication for 7 days. Amy B.’s diarrhea slowly resolved, and she felt well again until her current abdominal symptoms began.
On physical examination, she appeared toxic and had a temperature of 101°F. She was sweating profusely. Her inferior anterior liver margin was palpable three fingerbreadths below the right rib cage, suggestive of an enlarged liver. The liver edge was rounded and tender. Gentle fist percussion of the lower posterior right rib cage caused severe pain. Routine laboratory studies were ordered, and a computed tomogram (CT) of the upper abdomen was scheduled to be done immediately.
I. Liver Anatomy
The human liver consists of two lobes, each containing multiple lobules and sinusoids. The liver receives 75% of its blood supply from the portal vein, which carries blood returning to the heart from the small intestine, stomach, pancreas, and spleen. The remaining 25% of the liver’s blood supply is arterial, carried to the liver by the hepatic artery.
Blood from both the portal vein and hepatic artery empty into a common conduit, mixing their contents as they enter the liver sinusoids (Fig. 44.1). The sinusoids are expandable vascular channels that run through the hepatic lobules. They are lined with endothelial cells that have been described as “leaky” because, as blood flows through the sinusoids, the contents of the plasma have relatively free access to the hepatocytes, which are located on the other side of the endothelial cells.
The liver is also an exocrine organ, secreting bile into the biliary drainage system. The hepatocytes secrete bile into the bile canaliculus, whose contents flow parallel to that in the sinusoids but in the opposite direction. The canaliculi empty into the bile ducts. The lumina of the bile ducts then fuse, forming the common bile duct. The common duct then releases bile into the duodenum. Some of the liver’s effluent is stored in the gallbladder and discharged into the duodenum postprandially to aid in digestion.
The entire liver surface is covered by a capsule of connective tissue that branches and extends throughout the liver. This capsule provides support for the blood vessels, lymphatic vessels, and bile ducts that permeate the liver. In addition, this connective tissue sheet subdivides the liver lobes into smaller lobules.
The primary cell type of the liver is the hepatocyte. Hepatocytes, also known as hepatic parenchymal cells, form the liver lobules. Eighty percent of the liver volume is composed of hepatocytes, but only 60% of the total number of cells in the liver are hepatocytes. The other 40% of the cells are nonparenchymal cells and constitute the lining cells of the walls of the sinusoids. The lining cells comprise the endothelial cells, Kupffer cells, and hepatic stellate cells. In addition, intrahepatic lymphocytes, which include pit cells (liver-specific natural killer cells), are also present in the sinusoidal lining.
The hepatocyte is the cell that carries out the many functions of the liver. Almost all pathways of metabolism are represented in the hepatocyte and these pathways are controlled through the actions of hormones that bind to receptors located on the plasma membrane of their cells. Although hepatocytes are normally quiescent cells with low turnover and a long life span, they can be stimulated to grow if damage occurs to other cells in the liver. The liver mass has a relatively constant relationship to the total body mass of adult individuals. Deviation from the normal or optimal ratio (caused, e.g., by a partial hepatectomy or significant hepatic cell death or injury) is rapidly corrected by hepatic growth caused by a proportional increase in hepatocyte replication.
B. Endothelial Cells
The sinusoidal endothelial cells constitute the lining cells of the sinusoid. Unlike endothelial cells in other body tissues, these cells contain fenestrations with a mean diameter of 100 nm. They do not, therefore, form a tight basement membrane barrier between themselves and the hepatocytes. In this way, they allow for free diffusion of small molecules to the hepatocytes but not of particles the size of chylomicrons (chylomicron remnants, however, which are smaller than chylomicrons, do have free passage to the hepatocyte). The endothelial cells are capable of endocytosing many ligands and also may secrete cytokines when appropriately stimulated. Because of their positioning, lack of tight junctions, and the absence of a tight basement membrane, the liver endothelial cells do not present a significant barrier to the movement of the contents of the sinusoids into hepatocytes. Their fenestrations or pores further promote the free passage of blood components through this membrane into the liver parenchymal cells.
C. Kupffer Cells
Kupffer cells are located within the sinusoidal lining. They contain almost one quarter of all the lysosomes of the liver. The Kupffer cells are tissue macrophages with both endocytotic and phagocytic capacity. They phagocytose many substances such as denatured albumin, bacteria, and immune complexes. They protect the liver from gut-derived particulate materials and bacterial products. On stimulation by immunomodulators, these cells secrete potent mediators of the inflammatory response and play a role in liver immune defense through the release of cytokines that lead to the inactivation of substances considered foreign to the organism. The Kupffer cells also remove damaged erythrocytes from the circulation.
D. Hepatic Stellate Cells
The stellate cells are also called perisinusoidal or Ito cells. There are approximately 5 to 20 of these cells per 100 hepatocytes. The stellate cells are lipid-filled cells and serve as the primary storage site for vitamin A. They also control the turnover of hepatic connective tissue and extracellular matrix and regulate the contractility of the sinusoids. When cirrhosis of the liver is present, the stellate cells are stimulated by various signals to increase their synthesis of extracellular matrix material. This, in turn, diffusely infiltrates the liver, eventually interfering with the function of the hepatocytes.
E. Pit Cells
The hepatic pit cells, also known as liver-associated lymphocytes, are natural killer cells, which are a defense mechanism against the invasion of the liver by potentially toxic agents, such as tumor cells or viruses.
III. Major Functions of the Liver
A. The Liver Is a Central Receiving and Recycling Center for the Body
The liver can carry out a multitude of biochemical reactions. This is necessary because of its role in constantly monitoring, recycling, modifying, and distributing all of the various compounds absorbed from the digestive tract and delivered to the liver. If any portion of an ingested compound is potentially useful to that organism, the liver retrieves this portion and converts it to a substrate that can be used by hepatic and nonhepatic cells. At the same time, the liver removes many of the toxic compounds that are ingested or produced in the body and targets them for excretion in the urine or in the bile.
As mentioned previously, the liver receives nutrient-rich blood from the enteric circulation through the portal vein; thus, all of the compounds that enter the blood from the digestive tract pass through the liver on their way to other tissues. The enterohepatic circulation allows the liver to first access the nutrients to fulfill specific functions (such as the synthesis of blood coagulation proteins, heme, purines, and pyrimidines) and then to access the ingested toxic compounds (such as ethanol) and potentially harmful metabolic products (such as NH4+ produced from bacterial metabolism in the gut).
In addition to the blood supply from the portal vein, the liver receives oxygen-rich blood through the hepatic artery; this arterial blood mixes with the blood from the portal vein in the sinusoids. This unusual mixing process gives the liver access to various metabolites ingested and produced in the periphery and secreted into the peripheral circulation, such as glucose, individual amino acids, certain proteins, iron–transferrin complexes, and waste metabolites as well as potential toxins produced during substrate metabolism. As mentioned, fenestrations in the endothelial cells, combined with gaps between the cells, the lack of a basement membrane between the endothelial cells and the hepatocytes, and low portal blood pressure (which results in slow blood flow) contribute to the efficient exchange of compounds between sinusoidal blood and the hepatocyte and clearance of unwanted compounds from the blood. Thus, large molecules targeted for processing, such as serum proteins and chylomicron remnants, can be removed by hepatocytes, degraded, and their components recycled. Similarly, newly synthesized molecules, such as very-low-density lipoprotein (VLDL) and serum proteins, can be easily secreted into the blood. In addition, the liver can convert all of the amino acids found in proteins into glucose, fatty acids, or ketone bodies. The secretion of VLDL by the liver not only delivers excess calories to adipose tissue for storage of fatty acids in triacylglycerol, but it also delivers phospholipids and cholesterol to tissues that are in need of these compounds for the synthesis of cell walls as well as other functions. The secretion of glycoproteins by the liver is accomplished through the liver’s gluconeogenic capacity and its access to a variety of dietary sugars to form the oligosaccharide chains, as well as its access to dietary amino acids with which it synthesizes proteins. Thus, the liver has the capacity to carry out a large number of biosynthetic reactions. It has the biochemical wherewithal to synthesize a myriad of compounds from a broad spectrum of precursors. At the same time, the liver metabolizes compounds into biochemically useful products. Alternatively, it has the ability to degrade and excrete those compounds presented to it that cannot be further used by the body.
Each of the liver cells described contains specialized transport and uptake mechanisms for enzymes, infectious agents, drugs, and other xenobiotics that specifically target these substances to certain liver cell types. These are accomplished by linking these agents covalently by way of biodegradable bonds to their specific carriers. The latter then determines the particular fate of the drug by using specific cell recognition, uptake, transport, and biodegradation pathways.
B. Inactivation and Detoxification of Xenobiotic Compounds and Metabolites
Xenobiotics are compounds that have no nutrient value (cannot be used by the body for energy requirements) and are potentially toxic. They are present as natural components of foods or they may be introduced into foods as additives or through processing. Pharmacological and recreational drugs are also xenobiotic compounds. The liver is the principal site in the body for the degradation of these compounds. Because many of these substances are lipophilic, they are oxidized, hydroxylated, or hydrolyzed by enzymes in phase I reactions. Phase I reactions introduce or expose hydroxyl groups or other reactive sites that can be used for conjugation reactions (the phase II reactions). The conjugation reactions add a negatively charged group such as glycine, sulfate, or glucuronic acid to the molecule. Many xenobiotic compounds are transformed through several different pathways. A general scheme of inactivation is shown in Figure 44.2.
The conjugation and inactivation pathways are similar to those used by the liver to inactivate many of its own metabolic waste products. These pathways are intimately related to the biosynthetic cascades that exist in the liver. The liver can synthesize the precursors that are required for conjugation and inactivation reactions from other compounds. For example, sulfation is used by the liver to clear steroid hormones from circulation. The sulfate used for this purpose can be obtained from the degradation of cysteine or methionine.
The liver, kidney, and intestine are the major sites in the body for the biotransformation of xenobiotic compounds. Many xenobiotic compounds contain aromatic rings (such as benzopyrene in tobacco smoke) or heterocyclic ring structures (such as the nitrogen-containing rings of nicotine or pyridoxine) that we are unable to degrade or recycle into useful components. These structures are hydrophobic, causing the molecules to be retained in adipose tissue unless they are sequestered by the liver, kidney, or intestine for biotransformation reactions. Sometimes, however, the phase I and II reactions backfire, and harmless hydrophobic molecules are converted to toxins or potent chemical carcinogens.
1. Cytochrome P450 and Xenobiotic Metabolism
The toxification/detoxification of xenobiotics is accomplished through the activity of a group of enzymes with a broad spectrum of biological activity. Some examples of enzymes involved in xenobiotic transformation are described in Table 44.1. Of the wide variety of enzymes that are involved in xenobiotic metabolism, only the cytochrome P450–dependent mono-oxygenase system is discussed here (see Chapter 33). The cytochrome P450–dependent mono-oxygenase enzymes are determinants in oxidative, peroxidative, and reductive degradation of exogenous (chemicals, carcinogens, pollutants, etc.) and endogenous (steroids, prostaglandins, retinoids, etc.) substances. The key enzymatic constituents of this system are the flavoprotein NADPH-cytochrome P450 oxidoreductase and cytochrome P450 (see Fig. 33.5). The latter is the terminal electron acceptor and substrate-binding site of the microsomal mixed-function oxidase complex, a very versatile catalytic system. The system got its name in 1962 when Omura and Sato found a pigment with unique spectral characteristics derived from liver microsomes of rabbits. When reduced and complexed with carbon monoxide, it exhibited a spectral absorbance maximum at 450 nm.
The major role of the cytochrome P450 enzymes (see Chapter 33) is to oxidize substrates and introduce oxygen to the structure. Similar reactions can be carried out by other flavin mono-oxygenases that do not contain cytochrome P450.
The human cytochrome P450 enzyme family contains 57 functional genes, which produce proteins with at least 40% sequence homology. These isozymes have different but overlapping specificities. The human enzymes are generally divided into nine major subfamilies, and each of these is further subdivided. For example, in the naming of the principal enzyme involved in the oxidation of ethanol to acetaldehyde, CYP2E1, the CYP denotes the cytochrome P450 family, the 2 denotes the subfamily, the E denotes ethanol, and the 1 denotes the specific isozyme.
The CYP3A4 isoform accounts for 30% to 40% of CYP450 enzymes in the liver and 70% of cytochrome enzymes in gut wall enterocytes. It metabolizes the greatest number of drugs in humans. Specific drugs are substrates for CYP3A4. The concomitant ingestion of two CYP3A4 substrates could potentially induce competition for the binding site, which, in turn, could alter the blood levels of these two agents. The drug with the highest affinity for the enzyme will be preferentially metabolized, whereas the metabolism (and degradation) of the other drug will be reduced. The latter drug’s concentration in the blood will then rise.
Moreover, many substances or drugs impair or inhibit the activity of the CYP3A4 enzyme, thereby impairing the body’s ability to metabolize a drug. Some of the lipid-lowering agents known as the statins (HMG-CoA reductase inhibitors) require CYP3A4 for degradation. Appropriate drug treatment and dosing take into account the normal degradative pathway of the drug. However, a component of grapefruit juice is a potent inhibitor of CYP3A4-mediated drug metabolism. Evidence suggests that if certain statins are regularly taken with grapefruit juice, their level in the blood may increase as much as 15-fold. This marked increase in plasma concentration could increase the muscle and liver toxicity of the statin in question because side effects of the statins appear to be dose-related.
- They all contain cytochrome P450, oxidize the substrate, and reduce oxygen.
- They all have a flavin-containing reductase subunit that uses NADPH, and not NADH, as a substrate.
- They are all found in the smooth endoplasmic reticulum and are referred to as microsomal enzymes (e.g., CYP2E1 is also referred to as the microsomal ethanol-oxidizing system, MEOS).
- They are all bound to the lipid portion of the membrane, probably to phosphatidylcholine.
- They are all inducible by the presence of their own best substrate and somewhat less inducible by the substrates for other P450 isozymes.
- They all generate a reactive free-radical compound as an intermediate in the reaction.
2. Examples of Cytochrome P450 Detoxification Reactions
a. Vinyl Chloride
The detoxification of vinyl chloride provides an example of effective detoxification by a P450 isozyme (ethanol detoxification was discussed in Chapter 33). Vinyl chloride is used in the synthesis of plastics and can cause angiosarcoma in the liver of exposed workers. It is activated in a phase I reaction to a reactive epoxide by a hepatic P450 isozyme (CYP2E1), which can react with guanine in DNA or other cellular molecules. However, it also can be converted to chloroacetaldehyde, conjugated with reduced glutathione, and excreted in a series of phase II reactions (Fig. 44.3).
b. Aflatoxin B1
Aflatoxin B1 is an example of a compound that is made more toxic by a cytochrome P450 reaction (CYP2A1). Current research suggests that ingested aflatoxin B1 in contaminated food (it is produced by a fungus [Aspergillus flavus] that grows on peanuts that may have been stored in damp conditions) is directly involved in hepatocarcinogenesis in humans by introducing a G → T mutation into the p53 gene. Aflatoxin is metabolically activated to its 8,9-epoxide by two different isozymes of cytochrome P450. The epoxide modifies DNA by forming covalent adducts with guanine residues. In addition, the epoxide can combine with lysine residues in proteins and thus is also a hepatotoxin.
Acetaminophen (Tylenol) is an example of a xenobiotic that is metabolized by the liver for safe excretion; however, it can be toxic if ingested in high doses. The pathways for acetaminophen metabolism are shown in Figure 44.4. As shown in the figure, acetaminophen can be glucuronylated or sulfated for safe excretion by the kidney. However, a cytochrome P450 enzyme produces the toxic intermediate N-acetyl-p-benzoquinoneimine (NAPQI), which can be excreted safely in the urine after conjugation with glutathione.
NAPQI is a dangerous and unstable metabolite that can damage cellular proteins and lead to the death of the hepatocyte. Under normal conditions, when acetaminophen is taken in the correct therapeutic amounts, <10% of the drug forms NAPQI, an amount that can be readily handled by the glutathione detoxifying system (phase II reactions). However, when taken in doses that are potentially toxic, the sulfotransferase and glucuronyl transferase systems are overwhelmed, and more acetaminophen is metabolized through the NAPQI route. When this occurs, the levels of glutathione in the hepatocyte are insufficient to detoxify NAPQI, and hepatocyte death can result.
The enzyme that produces NAPQI, CYP2E1, is induced by alcohol (see Chapter 33, MEOS). Thus, individuals who chronically abuse alcohol have increased sensitivity to acetaminophen toxicity, because a higher percentage of acetaminophen metabolism is directed toward NAPQI, compared with an individual with low levels of CYP2E1. Therefore, even recommended therapeutic doses of acetaminophen can be toxic to these individuals.
Effective treatment for acetaminophen poisoning or overdose involves the use of N-acetyl cysteine. This compound supplies cysteine as a precursor for increased glutathione production, which, in turn, enhances the phase II reactions, which reduces the levels of the toxic intermediate.
C. Regulation of Blood Glucose Levels
One of the primary functions of the liver is to maintain blood glucose concentrations within the normal range. The manner in which the liver accomplishes this has been the subject of previous chapters (Chapters 19, 28, and 34). In brief, the pancreas monitors blood glucose levels and secretes insulin when blood glucose levels rise and glucagon when such levels decrease. These hormones initiate regulatory cascades that affect liver glycogenolysis, glycogen synthesis, glycolysis, and gluconeogenesis. In addition, sustained physiological increases in growth hormone, cortisol, and catecholamine secretion help to sustain normal blood glucose levels during fasting (see Chapter 41).
When blood glucose levels drop, glycolysis and glycogen synthesis are inhibited, and gluconeogenesis and glycogenolysis are activated. Concurrently, fatty acid oxidation is activated to provide energy for glucose synthesis. During an overnight fast, blood glucose levels are maintained primarily by glycogenolysis and, if gluconeogenesis is required, the energy (six ATP molecules are required to produce one molecule of glucose from two molecules of lactate) is obtained by fatty acid oxidation. On insulin release, the opposing pathways are activated such that excess fuels can be stored either as glycogen or fatty acids. The pathways are regulated by the activation or inhibition of two key kinases, the cyclic adenosine monophosphate (cAMP)–dependent protein kinase and the AMP-activated protein kinase (see Fig. 34.8 for a review of these pathways). Recall that the liver can export glucose because it is one of the only two tissues that express glucose 6-phosphatase.
D. Synthesis and Export of Cholesterol and Triacylglycerol
When food supplies are plentiful, hormonal activation leads to fatty acid, triacylglycerol, and cholesterol synthesis. High dietary intake and intestinal absorption of cholesterol compensatorily reduces the rate of hepatic cholesterol synthesis, in which case the liver acts as a recycling depot for sending excess dietary cholesterol to the peripheral tissue when needed as well as accepting cholesterol from these tissues when required. The pathways of cholesterol metabolism were discussed in Chapter 32.
E. Ammonia and the Urea Cycle
The liver is the primary organ for synthesizing urea and thus is the central depot for the disposition of ammonia in the body. Ammonia groups travel to the liver on glutamine and alanine, and the liver converts these ammonia nitrogens to urea for excretion in the urine. The reactions of the urea cycle were discussed in Chapter 36.
Table e-44.1 lists some of the important nitrogen-containing compounds that are primarily synthesized or metabolized by the liver.
F. Formation of Ketone Bodies
The liver is the only organ that can produce ketone bodies, yet it is one of the few that cannot use these molecules for energy production. Ketone bodies are produced when the rate of glucose synthesis is limited (i.e., substrates for gluconeogenesis are limited) and fatty acid oxidation is occurring rapidly. Ketone bodies can cross the blood–brain barrier and become a major fuel for the nervous system under conditions of starvation. Synthesis and metabolism of ketone bodies have been described in Chapter 30.
G. Nucleotide Biosynthesis
The liver can synthesize and salvage all ribonucleotides and deoxyribonucleotides for other cells to use. Certain cells have lost the capacity to produce nucleotides de novo but can use the salvage pathways to convert free bases to nucleotides. The liver can secrete free bases into the circulation for these cells to use for this purpose. Nucleotide synthesis and degradation are discussed in Chapter 39.
H. Synthesis of Blood Proteins
The liver is the primary site of the synthesis of circulating proteins such as albumin and the clotting factors. When liver protein synthesis is compromised, the protein levels in the blood are reduced. Hypoproteinemia may lead to edema because of a decrease in the protein-mediated osmotic pressure in the blood. This, in turn, causes plasma water to leave the circulation and enter (and expand) the interstitial space, leading to edema.