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GLUCONEOGENESIS
The liver maintains glucose levels in blood during fasting through either
glycogenolysis or gluconeogenesis. These pathways are promoted by glucagon
and epinephrine and inhibited by insulin. In fasting, glycogen reserves drop
dramatically in the first 12 hours, during which time gluconeogenesis increases.
After 24 hours, it represents the sole source of glucose. Important substrates for
gluconeogenesis are:
• Glycerol 3-phosphate (from triacylglycerol in adipose)
• Lactate (from anaerobic glycolysis)
• Gluconeogenic amino acids (protein from muscle)
Dietary fructose and galactose can also be converted to glucose in the liver. In
humans, it is not possible to convert acetyl-CoA to glucose. Inasmuch as most
fatty acids are metabolized solely to acetyl-CoA, they are not a major source of
glucose either. One minor exception is odd-number carbon fatty acids (e.g., Cl
7), which yield a small amount of propionyl-CoA that is gluconeogenic. The
pathway of gluconeogenesis is diagrammed in Figure I- 14-5. Lactate is
oxidized to pyruvate by lactate dehydrogenase. The important gluconeogenic
amino acid alanine is converted to pyruvate by alanine aminotransferase (ALT or
GPT) . Glycerol 3-phosphate is oxidized to dihydroxyacetone phosphate (DHAP)
by glycerol 3-phosphate dehydrogenase. Most steps represent a reversal of
glycolysis, and several of these have been omitted from the diagram. The 4
important enzymes are those required to catalyze reactions that circumvent the
irreversible steps:
 Pyruvate carboxylase is a mitochondrial enzyme requiring biotin. It is
activated by acetyl-CoA (from P-oxidation). The product oxaloacetate
(OAA), a citric acid cycle intermediate, cannot leave the mitochondria but
is reduced to malate that can leave via the malate shuttle. In the
cytoplasm, malate is reoxidized to OAA.
 Phosphoenolpyruvate carboxykinase (PEPCK) in the cytoplasm is induced
by glucagon and cortisol. It converts OAA to phosphoenolpyruvate (PEP) in
a reaction that requires GTP. PEP continues in the pathway to fructose 1
,6-bisphosphate.
 Fructose-1,6-bisphosphatase in the cytoplasm is a key control point of
gluconeogenesis. It hydrolyzes phosphate from fructose 1 ,6-bisphosphate
rather than using it to generate ATP from ADP. A common pattern to note
is that phosphatases oppose kinases. Fructose- 1 ,6-bisphosphatase is
activated by ATP and inhibited by AMP and fructose 2,6-bisphosphate.
Fructose
2,6-bisphosphate,
produced
by
PFK-2,
controls
both
gluconeogenesis and glycolysis (in the liver). Recall from the earlier
discussion of this enzyme (see Chapter 1 2, Figure I-12-3) that PFK-2 is
activated by insulin and inhibited by glucagon. Thus, glucagon will lower F
2,6-BP and stimulate gluconeogenesis, whereas insulin will increase F 2,6BP and inhibit gluconeogenesis.
 Glucose-6-phosphatase is in the lumen of the endoplasmic reticulum.
Glucose 6-phosphate is transported into the ER, and free glucose is
transported back into the cytoplasm from which it leaves the cell. Glucose6- phosphatase is only in the liver. The absence of glucose-6phosphatase in skeletal muscle accounts for the fact that muscle glycogen
cannot serve as a source of blood glucose .
Although alanine is the major gluconeogenic amino acid, 1 8 of the 20 (all but
leucine and lysine) are also gluconeogenic. Most of these are converted by
individual pathways to citric acid cycle intermediates, then to malate, following
the same path from there to glucose. It is important to note that glucose
produced by hepatic gluconeogenesis does not represent an energy source for
the liver. Gluconeogenesis requires expenditure of ATP that is provided by Poxidation of fatty acids. Therefore, hepatic gluconeogenesis is always dependent
on P-oxidation of fatty acids in the liver. During hypoglycemia, adipose tissue
releases these fatty acids by breaking down triglyceride. Although the acetyl-CoA
from fatty acids cannot be converted to glucose, it can be converted to ketone
bodies as an alternative fuel for cells, including the brain. Chronic hypoglycemia
is thus often accompanied physiologically by an increase in ketone bodies.
Coordinate Regulation of Pyruvate Carboxylase
and Pyruvate Dehydrogenase by Acetyl-CoA
The two major mitochondrial enzymes that use pyruvate, pyruvate carboxylase
and pyruvate dehydrogenase, are both regulated by acetyl-CoA. This control is
important in these contexts:
 Between meals, when fatty acids are oxidized in the liver for energy,
accumulating
acetyl-CoA
activates
pyruvate
carboxylase
and
gluconeogenesis and inhibits PDH, thus preventing conversion of lactate
and alanine to acetyl-CoA.
 In the well-fed, absorptive state (insulin), accumulating acetyl-CoA is
shuttled into the cytoplasm for fatty acid synthesis. OAA is necessary for
this transport, and acetyl-CoA can stimulate its formation from pyruvate
Cori Cycle and Alanine Cycle
During fasting, lactate from red blood cells (and possibly exercising skeletal
muscle) is converted in the liver to glucose that can be returned to the red blood
cell or muscle. This is called the Cori cycle. The alanine cycle is a slightly
different version of the Cori cycle, in which muscle releases alanine, delivering
both a gluconeogenic substrate (pyruvate) and an amino group for urea
synthesis.
Alcoholism and Hypoglycemia
Alcoholics are very susceptible to hypoglycemia. In addition to poor nutrition and
the fact that alcohol is metabolized to acetate (acetyl-CoA), the high amounts of
cytoplasmic NADH formed by alcohol dehydrogenase and acetaldehyde
dehydrogenase interfere with gluconeogenesis. High NADH favors the formation
of:
• Lactate from pyruvate
• Malate from OAA in the cytoplasm
• Glycerol 3-phosphate from DHAP
The effect is to divert important gluconeogenic substrates from entering the
pathway
Accumulation of cytoplasmic NADH and glycerol 3-P may also contribute to lipid
accumulation in alcoholic liver disease. Free fatty acids released from adipose in
part enter the liver where β-oxidation is very slow (high NADH). In the presence
of high glycerol 3-P, fatty acids are inappropriately stored in the liver as
triglyceride.
Extreme Exercise and Alcohol Consumption
Immediately after completing a 26-mile marathon race, a healthy 24-yearold
man was extremely dehydrated and thirsty. He quickly consumed a 6-pack of
ice-cold beer and shortly thereafter became very weak and lightheaded and
nearly fainted. He complained of muscle cramping and pain.
Although the effect of alcohol is unrelated to the hormonal control of
gluconeogenesis, excessive consumption of alcohol can result in severe
hypoglycemia after running a marathon. In exercising muscle, lactic acid builds
up in muscle due to anaerobic glycolysis, causing muscle cramping and pain.
The lactate spills into blood and is converted to glucose in the liver, as part
of the Cori cycle. But to carry out gluconeogenesis, NAD is required by lactate
dehydrogenase to oxidize lactate to pyruvate. However, much of the available
NAD is being used for ethanol metabolism and is unavailable for lactate
oxidation. The result is metabolic acidosis and hypoglycemia .
HEXOSE MONOPHOSPHATE SHUNT
The hexose monophosphate (HMP) shunt (pentose phosphate pathway) occurs
in the cytoplasm of all cells, where it serves 2 major functions:
• NADPH production
• Source of ribose 5-phosphate for nucleotide synthesis
The first part of the HMP shunt begins with glucose 6-phosphate and ends with
ribulose 5-phosphate and is irreversible. This part produces NADPH and involves
the important rate-limiting enzyme glucose 6-phosphate dehydrogenase
(G6PDH). G6PDH is induced by insulin, inhibited by NADPH, and activated by
NADP. The second part of the pathway, beginning with ribulose 5-phosphate,
represents a series of reversible reactions that produce an equilibrated pool of
sugars for biosynthesis, including ribose 5-phosphate for nucleotide synthesis.
Because fructose 6-phosphate and glyceraldehyde 3-phosphate are among the
sugars produced, intermediates can feed back into glycolysis; conversely,
pentoses can be made from glycolytic intermediates without going through the
G6PDH reaction. Transketolase, a thiamine-requiring enzyme, is important for
these interconversions. Transketolase is the only thiamine enzyme in red blood
cells.
Functions of NADPH
Cells require NADPH for a variety of functions, including:
Biosynthesis
Maintenance of a supply of reduced glutathione to protect against
reactive oxygen species ( ROS)
Bactericidal activity in polymorphonuclear leukocytes (PMN)These important
roles are cell specific .
Glucose 6-Phosphate Dehydrogenase Deficiency
Deficiency of G6PDH may result in hemolytic anemia and, in rare cases,
symptoms resembling chronic granulomatous disease (CGD). The disease shows
significant allelic heterogeneity (over 400 different mutations in the G6PDH gene
are known). The major symptom is either an acute episodic or (rarely) a chronic
hemolysis. The disease is X-linked recessive. Female heterozygous for G6PDH
deficiency have increased resistance to malaria. Consequently, the deficiency is
seen more commonly in families from regions where malaria is endemic.
Because red blood cells contain a large amount of oxygen, they are prone to
spontaneously generate ROS that damage protein and lipid in the cell. In the
presence of ROS, hemoglobin may precipitate (Heinz bodies) and membrane
lipids may undergo peroxidation, weakening the membrane and causing
hemolysis. As peroxides form, they are rapidly destroyed by the glutathione
peroxidase/glutathione reductase system in the red blood cell, thus avoiding
these complications. NADPH required by glutathione reductase is supplied by the
HMP shunt in the erythrocyte.
Persons with mutations that partially destroy G6PDH activity may develop an
acute, episodic hemolysis. Certain mutations affect the stability of G6PDH, and,
because erythrocytes cannot synthesize proteins, the enzyme is gradually lost
over time and older red blood cells lyse. This process is accelerated by certain
drugs and, in a subset of patients, ingestion of fava beans. In the United States,
the most likely cause of a hemolytic episode in these patients is overwhelming
infection, often pneumonia (viral and bacterial) or infectious hepatitis.
In rare instances, a mutation may decrease the activity of G6PDH sufficiently to
cause chronic nonspherocytic hemolytic anemia. Symptoms of CGD may also
develop if there is insufficient activity of G6PDH ( <5% of normal) in the PMN to
generate NADPH for the NADPH oxidase bactericidal system.
Clinical Correlate
Favism
Broad beans, commonly called fava beans, are common to diets in
Mediterranean countries (Greece, Italy, Spain, Portugal, and Turkey), in which
their ingestion may cause severe hemolysis in G6PDH individuals. Clinically, the
condition presents as pallor, hemoglobinuria, jaundice, and severe anemia 24 48 hours after ingestion of the beans.
Clinical Correlate
CGD
Chronic granulomatous disease is most frequently caused by genetic deficiency
of NADPH oxidase in the PMN. Patients a resusceptible to infection by catalasepositive organisms such as Staphylococcus aureus, Klebsie/la, Escherichia coli,
Candida, and Aspergi/lus. A negative n itroblue tetrazolium test is useful in
confirming the diagnosis.
Bridge to Microbiolog
Many parasites, such as Plasmodium, are deficient in antioxidant mechanisms,
making them particularly susceptible to oxygen radicals. In G6PDH deficiency,
the ability of erythrocytes to detoxify oxygen radicals is impaired. Ironically, the
accumulation of the radicals in erythrocytes in G6PDH deficiency gives protection
against malaria.