Biochemistry
ZOO-700
LABORATORY BIOLOGICAL ANALYSIS
PROFESSIONAL Ph.D.
PROF. AMAL HAMZA
Professor of Biochemistry
Biochemistry & Nutrition Department
2023-2024
Overview of
CHO Metabolism
 Carbohydrates
INTRODUCTION
are the most abundant organic
molecules in nature.

They have a wide range of functions, including
providing a significant fraction of the dietary calories
for most organisms, acting as a storage form of
energy in the body, and serving as cell membrane
components that mediate some forms of intercellular
communication.
 Carbohydrates also serve as a structural component of
many organisms, including the cell walls of bacteria,
the exoskeleton of many insects, and the fibrous
cellulose of plants.
 The empirical formula for many of the simpler carbo
- hydrates is (CH2O)n, hence the name “hydrate of
carbon.
 Metabolism is the set of life-sustaining chemical reactions
in organism.
 The three main purposes of metabolism are:
INTRODUCTION
TO
METABOLISM
1.Conversion of the energy in food to energy available to run
cellular processes;
2.Conversion
of
food
to
building
blocks
for proteins, lipids, nucleic acids, and some carbohydrates.
3. Elimination of metabolic wastes.
 The word metabolism can also refer to the sum of all chemical
reactions
that
occur
in
living
organisms,
including digestion and the transportation of substances into
and between different cells.
 In a pathway, the product of one reaction serves as the
substrate of the subsequent reaction.
INTRODUCTION
TO
METABOLISM
 Different pathways can also intersect, forming an integrated
and purposeful network of chemical reactions.
 These are collectively called metabolism, which is the sum
of all the chemical changes occurring in a cell, a tissue, or
the body.
 Most pathways can be classified as either catabolic
(degradative) or anabolic (synthetic).
 Catabolic reactions break down complex molecules,
such as proteins, polysaccharides, and lipids, to a few
simple molecules, for example, CO2, NH3 (ammonia),
and water.
INTRODUCTION
TO
METABOLISM
 Anabolic pathways form complex end products from
simple precursors, for example, the synthesis of the
polysaccharide, glycogen, from glucose.
 [Note: Pathways that regenerate a component are
called cycles.]
 In this course, we will focuses on the central metabolic
pathways that are involved in synthesizing and
degrading carbohydrates, lipids, and amino acids
A. Metabolic map
 A. Metabolic map It is convenient to investigate
metabolism by examining its component pathways.
 Each
pathway is composed of multienzyme
sequences, and each enzyme, in turn, may exhibit
important catalytic or regulatory features.
 This map is useful in tracing connections between
pathways, visualizing the purposeful “movement” of
metabolic intermediates, and picturing the effect on
the flow of intermediates if a pathway is blocked, for
example, by a drug or an inherited deficiency of an
enzyme.
Important reactions of intermediary metabolism. Several important pathways to be discussed in later chapters are
highlighted. Blue text = intermediates of carbohydrate metabolism; brown text = intermediates of lipid metabolism; green
text = intermediates of protein metabolism.
B. Catabolic pathways
Catabolic reactions serve to capture chemical energy in the form of adenosine
triphosphate (ATP) from the degradation of energy-rich fuel molecules.
Catabolism also allows molecules in the diet (or nutrient molecules stored in cells) to
be converted into building blocks needed for the synthesis of complex molecules.
Energy generation by degradation of complex molecules occurs in three stages as
shown in Figure 8.3.
[Note: Catabolic pathways are typically oxidative, and require coenzymes such as
NAD+]
THREE STAGES OF CATABOLISM
1. Hydrolysis of complex molecules: In the first stage, complex molecules are broken
down into their component building blocks. For example, proteins are degraded to
amino acids, poly - saccharides to monosaccharides, and fats (triacyl glycerols) to free
fatty acids and glycerol.
2. Conversion of building blocks to simple intermediates: In the second stage, these
diverse building blocks are further degraded to acetyl coenzyme A (CoA) and a few
other, simple molecules. Some energy is captured as ATP, but the amount is small
compared with the energy produced during the third stage of catabolism.
3. Oxidation of acetyl CoA: The tricarboxylic acid (TCA) cycle is the final common
pathway in the oxidation of fuel molecules that produce acetyl CoA. Oxidation of
acetyl CoA generates large amounts of ATP via oxidative phosphorylation as electrons
flow from NADH and FADH2 to oxygen.
Anabolic pathways
Anabolic reactions combine small molecules, such as amino acids, to form complex molecules,
such as proteins (Figure 8.4).
Anabolic reactions require energy (are endergonic), which is generally provided by the
breakdown of ATP to adenosine diphosphate (ADP) and inorganic phosphate (Pi ).
Anabolic reactions often involve chemical reductions in which the reducing power is most
frequently provided by the electron donor NADPH.
Note that catabolism is a convergent process—that is, a wide variety of molecules are
transformed into a few common end products.
By contrast, anabolism is a divergent process in which a few biosynthetic precursors form a wide
variety of polymeric or complex products.
OVERVIEW OF GLYCOLYSIS
 The glycolytic pathway is employed by all tissues for the breakdown of glucose to provide energy (in
the form of ATP) and intermediates for other metabolic pathways.
 Glycolysis is at the hub of carbohydrate metabolism because virtually all sugars—whether arising from
the diet or from catabolic reactions in the body—can ultimately be converted to glucose.
 Pyruvate is the end product of glycolysis in cells with mitochondria and an adequate supply of oxygen.
 This series of ten reactions is called aerobic glycolysis because oxygen is required to reoxidize the
NADH formed during the oxidation of glyceraldehyde 3-phosphate.
 Aerobic glycolysis sets the stage for the oxidative decarboxylation of pyruvate to acetyl CoA, a major
fuel of the TCA (or citric acid) cycle. Alternatively, pyruvate is reduced to lactate as NADH is oxidized
to NAD+.
 This conversion of glucose to lactate is called anaerobic glycolysis because it can occur without the
participation of oxygen. Anaerobic glycolysis allows the production of ATP in tissues that lack
mitochondria (for example, red blood cells) or in cells deprived of sufficient oxygen.
. B. Reactions of aerobic glycolysis.
C. Reactions of anaerobic glycolysis
TRANSPORT OF
GLUCOSE INTO
CELLS
Glucose
cannot
diffuse
directly into cells, but enters
by one of two transport
mechanisms:
Na+-independent, facilitated
diffusion transport system or a
Na+-monosaccharide
cotransporter system.
 L. Energy yield from glycolysis Despite the production of some ATP during glycolysis, the end
products, pyruvate or lactate, still contain most of the energy originally contained in glucose. The TCA
cycle is required to release that energy completely

1. Anaerobic glycolysis: Two molecules of ATP are generated for each molecule of glucose converted
to two molecules of lactate. There is no net production or consumption of NADH.
 2. Aerobic glycolysis: The direct consumption and formation of ATP is the same as in anaerobic
glycolysis—that is, a net gain of two ATP per molecule of glucose. Two molecules of NADH are also
produced per molecule of glucose. Ongoing aerobic glycolysis requires the oxidation of most of this
NADH by the electron transport chain, producing approximately three ATP for each NADH molecule
entering the chain .
 [Note: NADH cannot cross the inner mitochondrial membrane, and substrate shuttles are required
Tricarboxylic acid cycles
TRICARBOXYLIC ACID CYCLES
 The tricarboxylic acid cycle (TCA cycle, also called the Krebs cycle or the citric acid cycle) plays
several roles in metabolism.
 It is the final pathway where the oxidative metabolism of carbohydrates, amino acids, and fatty acids
converge, their carbon skeletons being converted to CO2.
 This oxidation provides energy for the production of the majority of ATP in most animals, including
humans.
 The cycle occurs totally in the mitochondria and is, therefore, in close proximity to the reactions of
electron transport ,which oxidize the reduced coenzymes produced by the cycle.
 The TCA cycle is an aerobic pathway, because O2 is required as the final electron acceptor.
 Most of the body's catabolic pathways converge on the TCA cycle.
 Reactions such as the catabolism of some amino acids generate intermediates of the
cycle and are called anaplerotic reactions.
 The citric acid cycle also supplies intermediates for a number of important synthetic
reactions.
 For example, the cycle functions in the formation of glucose from the carbon
skeletons of some amino acids, and it provides building blocks for the synthesis of
some amino acids and heme.
 Therefore, this cycle should not be viewed as a closed circle, but instead as a traffic
circle with compounds entering and leaving as required.
III. ENERGY PRODUCED BY THE TCA CYCLE
❖ Two carbon atoms enter the cycle as acetyl CoA and leave as
CO2.
❖ The cycle does not involve net consumption or production of
oxaloacetate or of any other intermediate.
❖ Four pairs of electrons are transferred during one turn of the
cycle: three pairs of electrons reducing three NAD+ to NADH
and one pair reducing FAD to FADH2.
❖ Oxidation of one NADH by the electron transport chain leads to
formation of approximately three ATP, whereas oxidation of
FADH2 yields approximately two ATP.
❖ The total yield of ATP from the oxidation of one acetyl CoA is
shown in Figure 9.7.
GLUCONEOGENESIS

Some tissues, such as the brain, red blood cells, kidney medulla, lens and cornea
of the eye, testes, and exercising muscle, require a continuous supply of glucose
as a metabolic fuel. Liver glycogen, an essential postprandial source of glucose,
can meet these needs for only 10–18 hours in the absence of dietary intake of
carbohydrate

During a prolonged fast, however, hepatic glycogen stores are depleted, and
glucose is formed from precursors such as lactate, pyruvate, glycerol (derived
from the backbone of triacylglycerols,), and α-ketoacids (derived from the
catabolism of glucogenic amino acids,).

The formation of glucose does not occur by a simple reversal of glycolysis,
because the overall equilibrium of glycolysis strongly favors pyruvate
formation. Instead, glucose is synthesized by a special pathway,
gluconeogenesis, that requires both mitochondrial and cytosolic enzymes.

During an overnight fast, approximately 90% of gluconeogenesis occurs in the
liver, with the kidneys providing 10% of the newly synthesized glucose
molecules. However, during prolonged fasting, the kidneys become major
glucose-producing organs, contributing an estimated 40% of the total glucose
production.

Figure 10.1 shows the relationship of gluconeogenesis to other important
reactions of intermediary metabolism.
GLUCONEOGENESIS
SUBSTRATES FOR GLUCONEOGENESIS
 Gluconeogenic precursors are molecules that can be used to produce a net synthesis of glucose.
 They include intermediates of glycolysis and the tricarboxylic acid (TCA) cycle.
 Glycerol, lactate, and the α-keto acids obtained from the transamination of glucogenic amino acids are the most important
gluconeogenic precursors.
 A. Glycerol
 Glycerol is released during the hydrolysis of triacylglycerols in adipose tissue and is delivered by the blood to the liver.
Glycerol is phosphorylated by glycerol kinase to glycerol phosphate, which is oxidized by glycerol phosphate
dehydrogenase to dihydroxy acetone phosphate—an intermediate of glycolysis.
 B. Lactate Lactate is released into the blood by exercising skeletal muscle, and by cells that lack
mitochondria, such as red blood cells. In the Cori cycle, bloodborne glucose is converted by exercising
muscle to lactate, which diffuses into the blood. This lactate is taken up by the liver and reconverted to
glucose, which is released back into the circulation (Figure 10.2).
 C. Amino acids Amino acids derived from hydrolysis of tissue proteins are the major sources of glucose
during a fast. α-Ketoacids, such as α-keto - glutarate, are derived from the metabolism of glucogenic amino
acids.
 These α-ketoacids can enter the TCA cycle and form oxaloacetate (OAA)—a direct precursor of phosphoenol
- pyruvate (PEP).
 [Note: Acetyl coenzyme A (CoA) and compounds that give rise only to acetyl CoA cannot give rise to a net
synthesis of glucose.
 This is due to the irreversible nature of the pyruvate dehydrogenase reaction, which converts pyruvate to
acetyl CoA). These compounds give rise instead to ketone bodies (see p. 195) and are therefore termed
ketogenic.
Summary of the
reactions of glycolysis
and gluconeogenesis
 Of the 11 reactions required to convert pyruvate to free
glucose, seven are catalyzed by reversible glycolytic
enzymes (Figure 10.7).
 The irreversible reactions of glycolysis catalyzed by
hexokinase, phosphofructokinase-1, and pyruvate
kinase are circumvented by glucose 6-phosphatase,
fructose
1,6-bisphosphatase,
and
pyruvate
carboxylase/PEP-carboxykinase.
 In gluconeogenesis, the equilibria of the seven
reversible reactions of glycolysis are pushed in favor of
glucose synthesis as a result of the essentially
irreversible formation of PEP, fructose 6-phosphate,
and glucose catalyzed by the gluconeogenic enzymes.

Prof. Amal Hamza
Prof. Amal Hamza
Introduction
• Inborn errors of carbohydrate metabolism
discussed in this chapter include:
• 1. Disaccharidase deficiencies
• 2. Disorders of monosaccharide metabolism
• 3. Glycogen storage diseases (GSDs)
• 4. Gluconeogenic disorders.
Prof. Amal Hamza
1. Disaccharidase Deficiencies
• The major sources of dietary carbohydrate in humans are starch and the
disaccharides lactose and sucrose.
• In adults, starch constitutes 60% of the carbohydrate ingested, however, in
newborns and young infants, the primary carbohydrate is lactose (milk sugar).
• Sucrose consumption varies widely with the choice of infant formula and other
eating habits. The normal digestive process involves the splitting of
disaccharides by intestinal hydrolytic enzymes (lactase, sucrase, isomaltase, and
maltase) into monosaccharides before absorption.
• Defective intestinal digestion of dietary sugars leads to symptoms of
flatulence, abdominal cramps, diarrhea, and perianal irritation.
Prof. Amal Hamza
• Levels of enzymes involved in the hydrolysis of disaccharides
may be depressed on either a genetic or an acquired basis.
• The latter situation results from damage to the brush border cells
of the small intestine, consequent to infection or other injuries.
• When enzymatic hydrolysis is impaired, ingested disaccharide
accumulates and provides a growth medium for intestinal bacteria
that produce carbon dioxide, hydrogen gas, and organic acids.
• The stools tend to be sour, foamy, loose, and watery with an
acidic pH.
Prof. Amal Hamza
• A diagnosis of disaccharidase deficiency may be suspected from the
history of symptoms developing in association with the ingestion of a
particular sugar and a laboratory finding of disaccharides in the urine.
• Direct confirmation may be obtained by enzyme assays in peroral
small bowel biopsy. Indirect confirmation of the diagnosis can be
made by a disaccharide tolerance test.
• Disaccharidase deficiency is suggested if the blood glucose curve is
flat upon ingestion of the suspect disaccharide and the breath hydrogen
concentration is increased.
• Genetic testing provides an accurate method for determining
disaccharidase deficiency.
Prof. Amal Hamza
1.1. Lactase Deficiency
• Congenital lactase deficiency is a rare autosomal recessive
inherited disorder that appears to have a higher frequency
in the Finnish population .
• Affected infants present with a severe gastrointestinal
problem characterized by watery diarrhea as soon as milk
or lactose-containing foods are introduced, then
dehydration, acidosis, and weight loss follow.
Prof. Amal Hamza
• Late-onset (adult-type) lactase deficiency, also known as hypolactasia or lactase non
persistence, differs from that encountered in infants at the age of onset, clinical
severity and molecular defect.
• Symptoms usually do not occur until adult life but occasionally may start at an earlier
age.
• This form of lactase deficiency is common in individuals from Mediterranean
countries, African- Americans, Asians, Native Americans and Eskimos.
• Treatment for patients with lactase deficiency usually consists of a diet free of lactose
or inclusion of exogenous lactase. In infants, a lactose-hydrolyzed human milk has
been used.
• Treatment usually results in a rapid clinical response.
Prof. Amal Hamza
• On the molecular level, both adult-type hypolactasia and
congential lactase deficiency stem from variants affecting the
lactase gene localized on chromosomes 2q21–22; however, the
molecular mechanisms causing the two disorders are different.
• Congenital lactase deficiency appears to be caused by mutations
that directly affect the lactase polypeptide.
• In the normal process of human development lactase activity
declines. However, mutations have resulted in lactase
persistence, which is inherited as an autosomal dominant
condition.
• Often attributed to evolutionary circumstances, the frequency of
these mutations appears to be different in various populations.
2.GLUCOSE–GALACTOSE
MALABSORPTION
• This is a rare disorder in which acute, profuse, watery diarrhea develops in newborn infants
following initial feeding. Intestinal disaccharidase activities are normal.
• Fructose is absorbed normally, but glucose and galactose are not. There is no significant rise in
blood glucose levels following an oral glucose–galactose tolerance test.
• However, a breath hydrogen test reveals glucose–galactose malabsorption.
• The stool usually contains large amounts of reducing sugars (>2 g%). Diarrhea stops when
free glucose and galactose are removed from the diet.
• Nephrocalcinosis and nephrolithiasis have been reported. All patients have mild defects in
renal tubular reabsorption of glucose.
Prof. Amal Hamza
Prof. Amal Hamza
• The basic defect is in the transport protein (Na+/glucose
cotransporter) localized in the brush border membrane of the small
intestinal epithelial cells.
• The protein is encoded by the SGLT1 gene that is localized on
chromosome 22q.
• More than 40 mutations have been identified. Certain mutations
appear to be recurrent in specific populations like the Old Order
Amish, suggesting the role of founder effect.
• A novel homozygous nonsense mutation that results in
hypercalcemia, nephrocalcinosis and proximal tubular dysfunction
has been reported.
Prof. Amal Hamza
Each of the 14 glucose transporter proteins that have been
identified to date targets specific tissues in order to facilitate
the diffusion of glucose across the plasma membrane.
The genes for these transporter proteins have also been
localized . Of known clinical significance are GLUT1, GLUT2
and GLUT10.
GLUT1 aids in the diffusion of glucose across the blood–
brain barrier.
The classic presentation for GLUT1 deficiency includes
epilepsy, developmental delay, acquired microcephaly,
spasticity, and ataxia.
The expanded phenotype includes paroxysmal movement
disorders and atypical childhood absence epilepsy, although there
has been one case without epilepsy.
2.1.Glucose
Transport
Defects
Prof. Amal Hamza
• The accompanying severe seizure disorder can be partially or fully remedied
with a ketogenic diet.
• Low CSF glucose values (relative to blood glucose) and low CSF lactate
levels are virtually diagnostic. The gene for GLUT1 is located on
chromosome 1p.34.2.
• GLUT1 defects are normally inherited in an autosomal dominant manner,
with de novo mutations producing haploinsufficiency and conferring
symptomatic heterozygosity, but recent studies have shown that the defects
may also be inherited in an autosomal recessive manner.
Prof. Amal Hamza
• Molecular analyses have identified a wide spectrum of
heterozygous mutations, including nonsense, missense,
insertion, deletion, splice-site mutations and microdeletions.
More than 200 cases have been identified.
• GLUT2 defects cause Fanconi–Bickel syndrome. More than 100 cases
have been reported, and 34 different mutations have been identified,
with none of them being particularly frequent.
Prof. Amal Hamza
- It is characterized by
proximal renal tubular
dysfunction, as well as
hepatic dysfunction,
which result in glycogen
accumulation in the liver.
The affected patients present with
rickets, hepatomegaly, and growth
failure associated with impaired
glucose and galactose tolerance,
increased renal clearance of glucose,
amino acids, protein, phosphate, and
uric acid as well as glycogen
accumulation in the liver.
Due to the latter,
Fanconi–Bickel
syndrome has also been
classified as a type of
glycogen storage disease.
GLUT3 transports
glucose in human
neural, brain, and
muscular tissues.
Prof. Amal Hamza
• GLUT4 is the insulin responsive glucose transporter involved in the
signaling pathway that regulates glucose metabolism in muscle cells an
adipocytes, and the gene is localized in 17p.
• States of insulin resistance such as in obesity, type II diabetes, and
long-term fasting, trigger a drop in GLUT4 transcription and
expression in adipocytes.
• GLUT5 has been identified as a key molecule in fructose
transportation and GLUT9 has been classified as a urate transporter.
Prof. Amal Hamza
• GLUT10, instead of transporting glucose like the other GLU
transporters, functions as a mitochondrial transporter for the
oxidized form of vitamin C.
• Loss of GLUT10 function causes arterial tortuosity syndrome, an
autosomal recessive disorder characterized by tortuosity, elongation,
stenosis, and aneurysm formation in the major arteries .
• It is expected that we will learn more about the roles of other
glucose transporters in carbohydrate metabolism through research.
Prof. Amal Hamza
• Galactokinase Deficiency
• Clinical Aspects.
• The major clinical manifestations are cataracts and pseudotumor cerebri, both appearing early in
life.
• In contrast to the transferase defect, hepatomegaly, jaundice and mental retardation are
not usually the features of this disorder, yet isolated reports of one or more of these
findings have been made in patients with galactokinase deficiency.
• It has been suggested that carriers of galactokinase deficiency may be at risk for presenile cataracts.
Prof. Amal Hamza
Prof. Amal Hamza
• Biochemical Aspects.
• Ingestion of lactose-containing diets will raise blood galactose
concentrations to values as high as 100 mg/dL in patients with
galactokinase deficiency.
• As a consequence, galactose appears in the urine. Galactitol and
galactonic acid are produced in increased amounts owing to
diversion of galactose into these secondary pathways.
Prof. Amal Hamza
• It is thought that the accumulation of galactitol is the cause of
cataract formation and cerebral edema.
• In contrast to classic galactosemia (transferase deficiency),
aminoaciduria and proteinuria are usually absent.
• The diagnosis of galactokinase deficiency can be confirmed by
measurement of the enzyme activity in erythrocytes.
Prof. Amal Hamza
• One should be aware that erythrocyte galactokinase activity is
much higher in newborns and decreases with age.
• The enzyme is not stable in erythrocytes stored at room
temperature, but it is stable in washed red blood cells frozen for
more than 1 week.
• Treatment. Early treatment is important to prevent progression of
cataract formation.
• Treatment consists of exclusion of lactose and other sources of
galactose from the diet.
• Clinical Aspects.
• Galactosemia was probably first described in 1908 by von
Reuss, but it was not until 1956 that Kalckar and his associates
established the defect in activity of the enzyme galactose-1phosphate uridyl transferase.
• The infant appears normal at birth, and symptoms usually do not
develop until milk feedings are given.
• Food may be refused; vomiting is common, and diarrhea occurs
occasionally.
Prof. Amal Hamza
Prof. Amal Hamza
• Other manifestations include lethargy, hypotonia, jaundice,
hepatomegaly, and susceptibility to infection with Gram-negative
organisms.
• In untreated patients, cataracts become evident, and physical and
mental retardation occur.
• The clinical course of many infants is fulminant, and death
occurs early from inanition, infection and hepatic failure.
Prof. Amal Hamza
In some patients, the course is
much milder and may even
escape early detection. Late
clinical manifestations have
been reported in both
untreated and treated
galactosemia patients.
These include
hypergonadotropic
hypogonadism in about 80%
of the affected women,
speech defects in about half,
and, to a lesser extent,
neurologic sequelase .
Prof. Amal Hamza
• Although many female patients develop ovarian hypofunction,
gonadal function in adult males appears to be normal.
• It appears that small amounts of transferase activity in some female
patients with galactosemia may preserve normal gonadal function;
those with the p.Ser135Leu genotype are shown to have a lower
likelihood of ovarian dysfunction.
• Galactosemic men and women have had normal offspring .
Difficulties in school are frequently encountered.
Prof. Amal Hamza
• Biochemical Aspects.
• Classic galactosemia (G/G) is defined as GALT activity below 5% and
gal-1-P buildup greater than 20 mg/dL accompanying with severe
mutations, although a few less serious mutations may also be included.
• Galactosemia may be suspected on clinical grounds, but laboratory
confirmation is essential.
• Diagnostic tests, which are based on response to galactose ingestion,
should not be employed because they are hazardous for the infant
patients.
•
Prof. Amal Hamza
• Direct enzyme assay using erythrocytes can be readily carried out
to confirm the diagnosis.
• On a galactose-containing diet, affected individuals excrete large
amounts of galactose, galactitol, and galactonic acid in the urine.
Gross generalized aminoaciduria and proteinuria are also evident.
• The erythrocyte galactose-1-phosphate level is elevated. It is
believed that this compound produces hepatic damage, whereas
galactitol accounts for the formation of cataracts.
Prof. Amal Hamza
• There are many reliable methods for the measurement of
erythrocyte transferase activity; affected individuals exhibit
either little or no activity in their red blood cells.
• Neonatal screenings for galactosemia have been carried out in
many countries effectively, reducing morbidity and mortality.
• Methods depend on the measurement of galactose and/or galactose-1phosphate by microbiologic assays, by galactose dehydrogenase, by
measurement of transferase activity with a fluorescent spot test, or
modified by automated fluorescent analysis.
Treatment:
Prof. Amal Hamza
• Treatment is directed toward minimizing the accumulation of
galactose and its metabolites in body tissues by excluding milk and
milk-containing products from the diet.
• Various milk substitutes are available (casein hydrolysates,
soybean formulas).
• Although a galactose-free diet is the basis of treatment,
supplementary measures are often required in the neonate to
correct secondary manifestations, such as hyperbilirubinaemia,
hypoprothrombinaemia, sepsis with Gram-negative organisms, and
anemia.
Prof. Amal Hamza
• The infections often respond poorly to antibiotic
therapy unless restriction of galactose is also carried
out.
• The effects of dietary treatment are dramatic, with
immediate reversal of the acute manifestations.
• Galactose restriction, with adequate calcium
supplementation, is compatible with good general
health and normal patterns of physical development.
Prof. Amal Hamza
Genetic Aspects.
• Galactosemia has been found in all races, but the incidence
appears to be higher among Caucasians than among Asians.
• The frequency of classic galactosemia based on newborn
screening in the United States is approximately 1:47,000. The
disorder is transmitted as an autosomal recessive trait.
• Several biochemical variants of erythrocyte transferase have been
described. Some have been associated with disease, and others
have not.
Prof. Amal Hamza
• Prenatal diagnosis of galactosemia is feasible and has been
performed successfully in a number of instances by measuring
transferase activity in cultured amniocytes and chorionic villi, by
galactitol concentration in amniotic fluid, or more recently, by
molecular mutation analysis.
A- DISORDERS OF FRUCTOSE METABOLISM
Fructose Metabolism
 Fructose is a monosaccharide found in honey, fruits, vegetables, and plants.
 In combination with glucose, it forms the disaccharide sucrose.
 It also exists in a number of oligosaccharides, such as raffinose (a
trisaccharide) and stachyose (a tetrasaccharide).
 The oligosaccharides raffinose and stachyose, which also contain galactose
and glucose, are not digested in humans.
Prof. Amal Hamza
The liver plays a dominant role in the metabolism of
fructose; other organs metabolize fructose but to a lesser
extent.
The overall process results in conversion of the sugar to
glycolytic intermediates, leading to the formation of either
glucose or lactic acid.
In the liver, fructose is phosphorylated to fructose-1phosphate (F-1-P) in the presence of fructokinase.
The enzyme is also present in kidney and in intestinal
mucosa.
Prof. Amal Hamza
Fructokinase is not present in muscle, adipose tissue and blood
cells, and in these tissues, fructose is phosphorylated to
fructose-6-phosphate by hexokinase.
In the liver, F-1-P is further metabolized to d-glyceraldehyde
and dihydroxyacetone phosphate by F-1-P aldolase or aldolase
B.
Aldolase B differs from aldolase A and C in that the latter
isozymes act principally on fructose-1,6-diphosphate.
Prof. Amal Hamza
Prof. Amal Hamza
1.A. Hereditary Fructose Intolerance (HFI)
Clinical Aspects.
The clinical manifestation in individuals with HFI may vary with
the age at which fructose or sucrose is introduced into the diet and
with the quantity of sugar ingested.
In infants, ingestion of fructose may produce findings similar to
those found in galactosemia.
Because many formulas contain sucrose, the opportunities of an
affected infant for exposure to fructose are increased accordingly.
Prof. Amal Hamza
In older children and in adults with HFI, ingestion of fructose,
sucrose or sorbitol causes:
 Abdominal pain.
 lowers the blood glucose level.
 Pallor, vomiting, sweating, and even coma may occur.
 It is typical for these individuals to develop a strong aversion for all
sweets and to be free of dental caries.
Prof. Amal Hamza
Biochemical Aspects.
The biochemical defect in HFI is a deficiency of liver F-1-P
aldolase (aldolase B).
Enzyme activity is usually less than 10% of normal when F-1-P
is used as the assay substrate.
The enzyme deficiency can also be demonstrated in intestinal
mucosa.
 Blood cells cannot be used for diagnosis because the enzyme is
not present in leukocytes or erythrocytes.
Untreated patients ingesting fructose in their diet may excrete large
amounts of this sugar in their urine and also show a gross
generalized aminoaciduria.
Prof. Amal Hamza
A fructose tolerance test, administered with caution, can be a
useful first step in facilitating diagnosis.
Assay of the enzyme requires either a biopsy sample of liver or
intestinal mucosa.
Administration of fructose, either orally or parenterally, is followed
by a fall in the blood glucose level and serum inorganic phosphate.
The uric acid level in blood rises from rapid degradation of purine
nucleotides to uric acid.
Prof. Amal Hamza
Treatment:
 The clinical manifestations in young infants with HFI may be severe, and
prompt elimination of fructose and sorbitol from the diet is important.
 Major sources of fructose include cane sugar, honey, fruits, and formula
using sucrose as the source of carbohydrate should be prevented.
 The prognosis for treated patients is good.
 Liver and kidney damage is reversed, and neurologic residuals are
uncommon.
 The use of fructose infusion as a source of calories in hospitalized adult
patients must be approached with caution until it is known that the patient
does not have HFI.
Prof. Amal Hamza
Fructose-1,6-Diphosphatase Deficiency
Clinical Aspects.
 Symptoms usually begin in early infancy with:
 hypoglycemia,
 hyperlactic academia,
ketoacidosis.
Hypoglycemia occurs when glycogen reserves are limited or exhausted.
The onset of symptoms often follows an infection. With age, the
frequency of hypoglycemia attacks decreases.
The most common physical finding is hepatomegaly.
Prof. Amal Hamza
Despite the metabolic defect, growth and intellectual
development may proceed normally.
It differs from HFI in that patients usually have no
aversion to sweets as well as normal renal tubular and
liver functions.
 Confirmation of diagnosis is based on the measurement of
fructose-1,6-diphosphatase activity in liver tissue or
mutation analysis.
Prof. Amal Hamza
Biochemical Aspects.
Fructose-1,6-diphosphatase is the key regulator of gluconeogenesis,
during which it catalyzes the hydrolysis of fructose-1,6-bisphosphate
to fructose-6-phosphate and inorganic phosphate.
This enzyme is expressed in several tissues with maximum activity in
the liver and kidney. Prolonged fasting in the affected patient induces
severe hypoglycemia, lactic acidosis, and hyperalaninemia.
The ingestion of glycerol, fructose, and alanine produces
hypoglycemia, an increase in lactate, and a fall in serum inorganic
phosphate.
Prof. Amal Hamza
Serum uric acid may also increase.
Lactate,
pyruvate,
beta-hydroxybutyrate,
alphaketoglutarate, and glycerol-3-phosphate accumulate in the
urine .
 Fructose-1,6-diphosphatase is normally present in liver,
kidney, and intestinal tissue.
Prof. Amal Hamza
 Treatment. Treatment consists of frequent feedings and avoidance of extended
fasting periods in order to prevent hypoglycemia and limiting fructose and
sorbitol intake.
 It has been suggested that folic acid may increase synthesis of fructose-1,6diphosphatase.
 Genetic Aspects. Fructose-1,6-diphosphatase deficiency is a rare condition
inherited as an autosomal recessive trait.
 In parents of affected children, intermediate values of enzyme activity were
found in the liver and in cultured lymphocytes.
Prof. Amal Hamza
Pyruvate Carboxylase Deficiency
 Clinical Aspects:
 Metabolic acidosis due to lactic acid,
 failure to thrive
 hypotonia
 anorexia
 Death within 1–2 months of age may occur.
 Patients surviving the initial problems exhibit retarded growth, seizures,
hypotonia, and continuing metabolic acidosis.
 Three forms have been described thus far: Type A (infantile or North
American form), Type B (neonatal or French form) and Type C (benign
form).
Prof. Amal Hamza
These phenotypes constitute a continuum spanning from the
most severe (Type B) to the least severe form (Type C).
Laboratory findings include:
 severe lactic acidosis,
ketonemia,
with normal lactate/pyruvate ratio (exception is type B)
in some cases, hyperammonemia,
citrullinemia,
hyperlysinemia and hypoglycemia.
Prof. Amal Hamza
 Biochemical Aspects. Pyruvate carboxylase (PC) is
a biotin-containing enzyme that catalyzes the
formation of oxaloacetate in the presence of an
allosteric activator, acetyl CoA, from pyruvate.
 Not only is PC critical in this reaction, it plays a
critical role in conjunction with oxaloacetate in
gluconeogenesis, glycogen synthesis, lipogenesis,
glycerogenesis, amino acid and neurotransmitter
synthesis, and glucose-dependent insulin secretion.
 The enzyme deficiency has been demonstrated in
liver tissue, leucocytes, and cultured skin fibroblasts.
 The enzyme activity is intermediate in known
carriers.
Prof. Amal Hamza
Treatment. Drastic supportive measures are usually needed.
Metabolic acidosis (lactic acidosis) should be treated promptly.
Peritoneal dialysis, intravenous fluids with glucose, and general
hydration may be helpful in reversing the clinical consequences of the
metabolic defects.
Biotin, thiamine and lipoic acid have been used in some patients, as well
as inclusion of glutamine and aspartic acid with questionable benefit.
Prof. Amal Hamza