required glucose. Ultimately, even the brain can substitute ketone bodies for about half of its energy

requirements.

The preferred energy substrates for liver are ketoacids derived from amino acid degradation even in

well-fed states. This is designed to allow the consumption of glucose by obligate tissues. Glucose

produced by the dephosphorylation of G6P rapidly diffuses out of the cell and is taken up by the brain,

muscles, and other organs. Hepatic glycolysis is used primarily for the production of intermediates of

metabolism and not for energy. Hepatic fatty acid degradation for energy is also inhibited under most

circumstances and occurs only during adipocyte lipolysis. By way of clinical relevance, alterations in

liver mitochondrial function are now known to be important in two common diseases. Patients with

type 2 diabetes have reduced ATP synthesis and abnormal ATP repletion in response to substrateinduced ATP depletion. Nonalcoholic fatty liver disease is also associated with lowered ATP repletion in

response to oxidative stress.14

Most short-, medium-, and long-chain fatty acids (C8 to C20) are metabolized by mitochondria to

generate ATP. Mitochondria cannot metabolize fatty acids with acyl chains greater than 20, and these

very long-chain fatty acids are metabolized in peroxisomes, predominantly in the liver. Peroxisomes do

not contain elements of the citric acid cycle or electron transport chain and thus do not generate ATP.

Most of the energy is released as heat.15

Adipocyte Phenotype, Thermogenesis, and Obesity

Epidemic obesity has created a surge of interest in the adipocyte, or fat cell. There are three basic types

of adipocytes; white, brown, and beige. White adipocytes differentiate for the storage of fat and contain

few mitochondria. The traditional view is that adult human fat is comprised predominantly of white

adipocytes. These adipocytes accumulate lipid in times of over feeding leading to obesity and insulin

resistance.8

Brown adipocytes are so named because of the brown color created by a high density of cellular

mitochondria. Unlike the mitochondria of other organs, the mitochondria of brown adipocytes contain

uncoupling protein-1 which can catalyze a protein leak across the inner membrane uncoupling electron

transport from ATP synthesis. This results in the dissipation of energy as heat, or thermogenesis. Brown

adipocytes were thought to be present only in human infants, and rare circumstances such as coldclimate outdoor workers or pheochromocytoma patients. Recently, the identification of “beige”

adipocytes has led to the realization that the adipocyte may be a target for therapy of obesity. Beige

adipocytes appear capable of switching from a white phenotype to brown, and the hope that adipocytes

could be switched to utilize excess calories for thermogenesis rather than storage as fat.16

64

Figure 2-13. The electron transport chain. A: NADH pathway. Electrons flow through complex I to complex III via the lipid soluble

molecule CoQ. From complex III, the electrons are transported through the intermembrane space by cytochrome c to complex IV.

For every two electrons that flow from NADH to O2

, 10 protons are pumped to the intermembranous space. B: FADH2

-succinate

pathway. Electrons flow through complex II to complex III via the lipid soluble molecule Coenzyme Q (CoQ). From complex III,

the electrons are transported through the intermembranous space by cytochrome c to complex IV. For every two electrons that

flow from FADH2 and succinate to O2

, six protons are pumped to the intermembranous space.

BIOTRANSFORMATION

Biotransformation is defined as the intracellular metabolism of endogenous organic compounds (e.g.,

heme proteins and steroid hormones) and exogenous compounds (e.g., drugs and environmental

65

compounds). Most biotransformation occurs chiefly in the liver, which contains enzyme systems that

can expose functional groups, such as hydroxyl ions (phase I reactions), or alter the size and solubility

of a wide variety of organic and inorganic compounds by conjugation with small polar molecules (phase

II reactions). A general strategy is to convert hydrophobic, potentially toxic compounds into hydrophilic

conjugates that can then be excreted into bile or urine.

7 Biotransformation of potentially toxic, often hydrophobic, compounds into hydrophilic, excretable

compounds occurs mainly in the liver by the cytochromes P-450, the uridine diphosphate-glucuronyl

(UDP-glucuronyl) transferases, the GSH S-transferases, and the sulfotransferases. Biotransforming

enzymes are not distributed uniformly within the cells of the hepatic lobule. This heterogeneity may

account for the ability of some drugs to cause damage preferentially in zone 3 hepatocytes (those

nearest the central venule).

Cytochromes P-450

The cytochromes P-450 are named for their ability to absorb light maximally at 450 nm in the presence

of carbon monoxide. These enzymes are bound to the ER and collectively catalyze reactions by using

NADPH and oxygen. The P-450 isozymes present in mammalian liver catalyze reactions such as

oxidation, hydroxylation, sulfoxide formation, oxidative deamination, dealkylation, and dehalogenation.

Such reactions allow further phase II conjugation with polar groups such as glucuronate, GSH, and

sulfate. The cytochromes P-450 can also create potentially toxic metabolites. Drugs such as

acetaminophen, isoniazid, halothane, and the phenothiazines can be converted into reactive forms that

cause cellular injury and death. The cytochromes also are responsible for the formation of organic free

radicals, reactive metabolites that can directly attack and injure cellular components or act as haptens in

the generation of an autoimmune response. Several of the most potent known carcinogens are aromatic

hydrocarbons, which are modified by cytochromes P-450.

Uridine Diphosphate-glucuronyl Transferases

Glucuronidation is the conjugation of UDP-glucuronic acid to a wide variety of xenobiotics by either

ester (acyl) or ether linkages. The transferases catalyzing these reactions reside in the ER. Many

common compounds are metabolized in this way, including bilirubin, testosterone, aspirin,

indomethacin, acetaminophen, chloramphenicol, and oxazepam. Clinically significant loss of activity can

occur with acute ethanol exposure or acetaminophen overdose, when formation of UDP-glucuronic acid

from UDP-glucose is outstripped by use. Some acyl linkages lead to the generation of electrophilic

centers that can react with other proteins. The covalent linkage of conjugated bilirubin to albumin is

believed to occur by this mechanism.

Glutathione S-transferases

The GSH transferases are more selective in the biotransformations they perform. GSH conjugation

occurs only with compounds that have electrophilic and potentially reactive centers. The role of GSH

conjugation catalyzed by the GSH S-transferases is demonstrated by acetaminophen. In metabolism of

this drug, cytochromes P-450 create an electrophilic center that reacts with protein thiol groups or

GSH.17 The presence of GSH S-transferase allows the preferential detoxification of acetaminophen rather

than its potentially injurious binding to thiol groups. A class of GSH S-transferases, known as ligandins,

appears to facilitate the uptake and intracellular transport of bilirubin, heme, and bile acids from plasma

to liver. In addition to the detoxification of potential toxins, GSH is a substrate for GSH peroxidase, an

enzyme important in the metabolism of hydrogen peroxide.

Sulfotransferases

The sulfotransferases catalyze the transfer of sulfate groups from 3'-phosphoadenosine-5'-phosphosulfate

(PAPS) to compounds such as thyroxine, bile acids, isoproterenol, α-methyldopa, and acetaminophen.

They are located primarily in the cytosol. Although many P-450 derivatives can be further conjugated

by either the sulfotransferases or the glucuronyl transferases, a limited ability of the liver to synthesize

PAPS makes glucuronidation the predominant mechanism.

HEME AND PORPHYRIN METABOLISM

Heme is formed from glycine and succinate and is the functional iron-containing center of hemoglobin,

66

myoglobin, cytochromes, catalases, and peroxidases. From glycine and succinate precursors, δaminolevulinic acid (δ-ALA) is synthesized by the rate-limiting enzyme ALA synthase. The

porphyrinogens are intermediates in the pathway from δ-ALA to heme, and porphyrins are oxidized

forms of porphyrinogen (Fig. 2-14). Inherited enzyme defects in the heme synthetic pathway cause the

overproduction of various porphyrinogens, which can in turn cause clinical manifestations known as the

porphyrias.18 Acquired porphyria can be caused by heavy metal intoxication, estrogens, alcohol, or

environmental exposure to chlorinated hydrocarbons.

Bilirubin IXa is the predominant heme degradation product in humans and is derived mostly from

hemoglobin. The enzyme heme oxygenase, located in cells of the reticuloendothelial system, is

primarily responsible for this conversion. Heme oxygenase resides in the ER and requires NADPH as a

cofactor. Hepatic processing of bilirubin is further detailed in the section on bile formation.

METAL METABOLISM

Iron uptake appears to occur by two distinct processes: (a) receptor-mediated endocytosis of iron–

transferrin complexes and (b) facilitated diffusion across the plasma membrane. More iron is taken up

and stored by the liver than by any other organ, with the exception of the bone marrow. Transferrin is

synthesized in the liver and has specific plasma membrane receptors on a number of different tissues.

After endocytosis, the transferrin and iron dissociate and the transferrin and transferrin receptors return

to the cell surface for recycling. A pathway appears to involve the dissociation of iron and transferrin at

the plasma membrane and subsequent internalization by carrier-mediated diffusion. Once internalized,

iron is stored and forms a complex with apoferritin. Each apoferritin molecule is capable of storing

several thousand iron molecules. The iron–apoferritin complex, called ferritin, is responsible for iron

storage under physiologic conditions. Iron storage in a protein-bound form is essential because free iron

can catalyze free radical formation, leading to cell injury.19

Figure 2-14. The heme biosynthetic pathway. Inherited defects of each of the heme biosynthetic enzymes except δ-aminolevulinic

acid synthase have been described and lead to the clinical disorders known as the porphyrias.

Copper is transported to the liver bound to albumin or histidine and enters the hepatocytes by a

process of facilitated diffusion. Once inside the cell, copper can bind to several intracellular proteins for

storage or as a necessary enzyme cofactor. Copper-binding proteins include metallothionein,

monoamine oxidase, cytochrome c oxidase, and superoxide dismutase. Ceruloplasmin is a liver-derived

protein that binds hepatic copper for transport to other tissues. The low levels of ceruloplasmin seen in

patients with Wilson disease suggest a pathogenetic defect.

Zinc is taken up by and competes for the same binding sites as copper. In hepatocytes, zinc binds

predominantly to metallothionein and is excreted into bile, in which it enters the enterohepatic

circulation. Other metals, usually found in trace amounts, are lead, cadmium, selenium, mercury, and

nickel. These metals are usually bound to metallothionein or GSH, and intoxication is associated with

free radical formation and liver injury.

SUMMARY

67