liver.ppt Biochemistry and clinical functions

Liver function biochemistry and clinical

1. Biochemistry of liver
Alice Skoumalová

2. The liver-introduction
importance in intermediary metabolism and in the
detoxification and elimination of toxic substances
one of the largest organs in the human body
it constitutes only 2-3% of the body mass, it accounts for
20-30% of the total oxygen consumption

3. Diagram of a hepatocyte
There are approximately 300 billion cells in the liver (80% of them
are hepatocytes)
The hepatocytes are central to the intermediary metabolism of the
body

4. Functions of the liver
The uptake of nutrients delivered from the digestive tract via the portal vein
The synthesis, storage, interconversion, and degradation of metabolites (metabolism)
The regulated supply of energy-rich intermediates and building blocks for biosynthetic
reactions
The detoxification of harmful compounds by biotransformation
The excretion of substances with the bile, as well as the synthesis and degradation of many
blood plasma constituens
Fighting infections (Kuppfer cells-macrophages)

5. Liver metabolism
The primary function is the maintenance of sufficient plasma levels of the metabolites
(homeostasis)
Carbohydrate metabolism
Lipid metabolism
Amino acid and protein metabolism
Biotransformations

6. Liver metabolism
The primary function is the maintenance of sufficient plasma levels of the metabolites
(homeostasis)
Carbohydrate metabolism:
1. The liver takes up glucose and other monosaccharides from the blood plasma
-These sugars are then converted to glucose 6-phosphate and other
intermediates of glycolysis (subsequently, they are either stored as the reserve
carbohydrate glycogen or degraded)
-Another large part is converted into fatty acids
-Only a small fraction is used for the generation of ATP
2. The liver secretes glucose when there is a major decline in the blood glucose level
-This glucose is derived from the glycogen store
-If the glycogen store is already exhausted, glucose can also be synthesized
by gluconeogenesis from lactate, glycerol, or the backbones of amino acids
Lipid metabolism
Amino acid and protein metabolism
Biotransformations

7. Liver metabolism
The primary function is the maintenance of sufficient plasma levels of the metabolites
(homeostasis)
Carbohydrate metabolism
Lipid metabolism:
1. The liver synthesizes fatty acids from acetate units
-The fatty acids formed are then used for the synthesis of fats and
phospholipids (these are subsequently released into the blood as complexes
with proteins-lipoproteins)
2. The liver can take up fatty acids from the plasma
-The conversion of fatty acids into ketone bodies, which are also excreted
3. The liver also uses acetate units to synthesize cholesterol
-It is then transported to other organs as a constituent of plasma lipoproteins
4. Excess cholesterol is converted into bile acids or excreted with the bile
Amino acid and protein metabolism
Biotransformations

8. Liver metabolism
The primary function is the maintenance of sufficient plasma levels of the metabolites
(homeostasis)
Carbohydrate metabolism
Lipid metabolism
Amino acid and protein metabolism:
1. The liver controls the amino acid level of the plasma
-It breaks down excess amino acids
-The nitrogen thus released is converted to urea and transported to the
kidneys
-The carbon skeletons of the amino acids enter intermediary metabolism,
where they are consumed either in the synthesis of glucose or in the
production of energy
2. The liver is the site of the synthesis and degradation of most proteins and peptides in
the blood plasma
Biotransformations

9. Liver metabolism
The primary function is the maintenance of sufficient plasma levels of the metabolites
(homeostasis)
Carbohydrate metabolism
Lipid metabolism
Amino acid and protein metabolism
Biotransformations
1. Steroid hormones and degradation products of the blood pigment
hemoglobin are taken up by the liver
-They are inactivated and converted into highly polar metabolites
2. Drugs, ethanol, and other foreign substances (xenobiotics) are
metabolized by similar reactions
-Preparation for excretion

10. The role of the liver in nutrition
The tissues are dependent on a constant supply of energy-rich metabolites to
provide energy and as precursors for the synthesis of complex
macromolecules
The liver:
1. It compensates the variation of the level of these fuels in the diet
(buffering organ)
2. It supplies with metabolites (storage organ)
Biochemistry of nutrition:
1. Well-fed (= absorptive state)
-The energy requirements of the tissues are predominantly
met by glucose
2. Starvation (= postresorptive state)
-The energy requirements of the tissues are met mainly by fatty
acids, amino acids, and ketone bodies (exceptions: nerve tissue
and erythrocytes)
The transition between these two states depends on the levels of energy-rich
metabolites in the blood plasma (it is brought about by the joint action of
hormones and signals from the nervous system)

11. Liver metabolism in the well-fed state

12. Liver metabolism during starvation

13. The role of the liver in carbohydrate metabolism
The concentration of glucose in the blood is maintained constant
(at 4-6 mM)
-this is achieved by the precise regulation of glucose-
forming and glucose-utilizing pathways
In the liver, glucose is derived from:
1. Glycogenolysis
2. Gluconeogenesis
3. Fructose and galactose metabolism
4. Cori cycle (from lactate)
5. Alanine cycle (from alanine)

14. Liver glycogen
Glycogen serves as a carbohydrate reserve, from which glucose can be released
The human body can store up to 450 g of glycogen (a third in the liver)
Liver glycogen serves in the maintenance of the blood glucose level, it declines to zero
in periods of starvation that last more than one day

15. Liver gluconeogenesis
The glycogen reserves are already depleted after one day
The blood glucose level begins to fall
An increase of gluconeogenesis

16. Liver gluconeogenesis
Gluconeogenesis occurs predominantly in the liver(90%)
Precursors:
1. Amino acids derived from the muscles (prolonged fasting results in a
massive degradation of muscle protein)
2. Lactate formed in erythrocytes and in muscles
3. Glycerol produced from the degradation of fats
Cortisol, glucagon, epinephrine: promote gluconeogenesis
Insulin:inhibits gluconeogenesis

17. Cori cycle:
Lactate(anaerobic glycolysis)
-transported to the liver
-converted back into glucose via
gluconeogenesis
Glucose-is sent back to its site of utilization
Alanine cycle:
Degradation of proteins: -the amino groups are
transferred to pyruvate, giving rise to alanine
Alanine is transported to the liver: -the carbon
skeleton is converted into glucose, -nitrogen is
converted into urea
Cori and alanine cycles

18. The role of the liver in lipid metabolism
The liver is the most important site for the formation of fatty acids, fats,
ketone bodies, and cholesterol
Absorptive state:
-The liver converts glucose via acetyl-CoA into fatty acids
-The liver can also retrieve fatty acids from lipids supplied with chylomicrons
from the intestine
-The fatty acids are converted into neutral fats and phospholipids, VLDL are
formed (transport of fatty acids to other tissues)
Postresorptive state:
-The adipose tissue releases fatty acids
-Fatty acids are taken up by the liver and oxidatively degraded to acetyl-CoA
-Acetyl-CoA is converted to ketone bodies

19. Biosynthesis of ketone bodies
Acetyl-CoA (ß-oxidation):
-can be oxidized by the citric acid cycle
-when the acetyl-CoA production exceeds the energy
requirements of the hepatocytes (starvation, diabetes
mellitus) they form ketone bodies (supply other tissues
with energy)
Ketone bodies:
They are released by the liver into the blood, in which they are readily soluble (their
levels are elevated during period of starvation)
3-Hydroxybutyrate and acetoacetate serve as the key metabolites in energy
production (after 1-2 weeks of starvation, the nerve tissue also begins to
utilize ketone bodies as energy sources)
Acetone is exhaled via the lungs
Ketonemia, ketosuria, ketoacidosis: when the production of ketone bodies exceeds
their use outside the liver (prolonged starvation, diabetes mellitus)

20. Cholesterol metabolism
Sources of cholesterol:1. the diet, 2. de novo synthesis from acetyl-CoA (liver)
Utilization of cholesterol: 1. the synthesis of bile acids, 2. building block for cell
membranes, 3. stored in the form of lipid droplets, following esterification with fatty acids,
4. formation of VLDL (supply other tissues)
The liver takes up from the blood and degrades lipoprotein complexes containing
cholesterol(HDL)

21. The role of the liver in amino acid metabolism
The main site of amino acid degradation is the liver(significant quantities of ammonia are
released)
Ammonia:
-is toxic to cells (at higher concentrations, it is especially damaging to the nerve cells)
-has to be rapidly inactivated and excreted (in human, this occurs primarily through the
formation of urea)
NH
3metabolism of the liver:
1.Amino nitrogen, which
accumulates in the tissues, is
transported in the blood
incorporated into glutamine (Gln)
and alanine (Ala)
2.Gln is deaminated by glutaminase
to yield NH
3and glutamate (Glu)
3.The amino group of alanine is
transferred to 2-oxoglutarate by
alanine transaminase
4.NH
3is released from glutamate by
oxidative deamination (catalyzed
by glutamate dehydrogenase)
5.Aspartate (Asp), the second amino
group donor in the urea cycle, also
arises from glutamate (catalyzed by
aspartate transaminase)

22. Urea cycle
Urea:
-It is neutral and non-toxic
-It can cross membranes by diffusion (small, uncharged molecule)
-It is highly soluble in water
-It can be easily transported by the blood and excreted in the urine
Urea synthesis:
-It occurs predominantly in the liver
-Urea is formed in a cyclic series of reactions; its nitrogen atoms are
derived from ammonia and aspartate, and the carbonyl carbon from
hydrogen carbonate

23. Bile acid metabolism
Bile acids:
-steroids with 24 carbons (from one to three α-hydroxyl groups and a side chain with a
carboxylate group)
-synthesized in the liver from cholesterol
-amphipathic (have a polar and an apolar side); act as detergents
-linked with an amino acid (glycine or taurine)-bile salts
-primary bile acids are formed in the liver, secondary bile acids in the intestine (by
dehydroxylation of the primary bile acids)
Lipid digestion:
-facilitate the solubilization of dietary lipids during the process of digestion by
promoting micelle formation (this action increases the effectiveness of pancreatic lipases,
and promotes the absorption of lipids from the intestine)

24. 1.The biosynthesis of bile acids from cholesterol
2.The conjugation with the amino acids
3.The concentration in the gallbladder by removal of
water
4. a 5.Intestinal bacteria in the colon produce
enzymes that can attack and alter the bile salts
6.Most of the bile acids are reabsorbed from the
intestine and, following transport to the liver,
returned once again into the bile (enterohepatic
circulation)
Metabolism of bile salts (exlusively in the liver)

25. Biotransformations (= the detoxification and elimination; predominantly in the liver)
Foreign substances: Self-substances
Xenobiotics (natural origin) Steroid hormones
Pharmaceuticals Bile pigments
Synthetics
Phase
I
Transformation
products
Transformation reactions
Either incorporate new functional
groups into apolar compounds, or
alter groups that are already present
in the molecule
The result is an increase in the
polarity and a decrease in the
biological activity or toxicity of the
substance
The most important reactions:
1.Oxidative reactions: hydroxylation,
epoxide formation, dealkylation,
deamination
2.Reductive reactions
3.Methylation
4.Desulfuration
Poorly soluble, biologically
active, some toxic

26. Biotransformations
Transformation
products
Phase
II
Conjugate
Bile Urine
Conjugate formation:
These reactions couple
substrates to highly polar, often
negatively charged molecules
The enzymes involved are
transferases
1. Glucuronidation
2. Esterification with sulfate
3. Amidation with glycine and
glutamate

27. Cytochrome P450 system
Monooxygenases that contain a heme group
Enzymatic hydroxylation (phase I reactions): drugs, ethanol
Also involved in the biosynthesis of steroid hormones, bile acids, eicosanoids,
and unsaturated fatty acids
Low specificity
Found in the liver, adrenal cortex
Cytochrome P450-dependent
monooxygenases: reactions
The reductive cleavage of molecular oxygen
(one atom is incorporated into the substrate, the
other is released as water)
The reducing equivalents are derived from
NADPH + H
+

28. Cytochrome P450 system
Examples of the Cyt P450-dependent reactions:
1.The hydroxylation of aromatic rings (drugs, steroids)
2.Aliphatic methyl groups can be oxidized to hydroxyl functions
3.The epoxidation of aromatics yields products that are toxic (benzopyrene)
4.Dealkylations

29. Ethanol metabolism
The alcohol content of alcoholic beverages
Given as percent by volume (conversion to
gram ethanol, density = 0,79 kg/l)
A bottle of beer: 0,5l with 4% alcohol contains
16g of ethanol
A bottle of wine: 0,7l with 12% alcohol
contains 66g of ethanol
Ethanol is rapidly taken up from the digestive
tract by diffusion
The maximum blood alcohol level is reached
within 60-90 minutesAlcohol is rapidly
distributed throughout the organism(a large
amount is taken up by muscles and the brain)
The lethal blood alcohol level is 0.3-0.4%

30. Ethanol degradation (in the liver)
Alcohol dehydrogenase oxidizes ethanol to
ethanal (acetaldehyde)
It is futher oxidized to acetate by aldehyde
dehydrogenase
Acetate is then converted to acetyl-CoA by
acetate-CoA ligase in an ATP-dependent reaction
Other enzymes are also involved in
ethanol degradation (catalase, Cyt
P450)

31. Liver damage
Elevated consumption of ethanol over many years may lead to liver damage (the limit for a
man is about 60g daily, and for a woman about 50g); these values are strongly dependent on
body weight, health and the use of medication
The high levels of NADH and acetyl-CoA inhibit citric acid cycle activity and ketogenesis,
whereas they exert a stimulatory effect on the synthesis of neutral fats and cholesterol (an
increase in the storage of lipids leads to a fatty liver-it is reversible)
In chronic alcoholism-liver cell death (cirrhosis of the liver, an irreversible, progressive loss
of liver function)

32. Bilirubin metabolism and excretion
Bilirubin (degraded product of
hemoglobin) is transported in the blood
bound to albumin
The liver takes up bilirubin and conjugates
it with glucuronic acid
Conjugated bilirubin is excreted via the
bile ducts into the gut
It is converted to urobilinogen (most of it
is oxidized to urobilin in the colon and
excreted)
Some urobilinogen is absorbed from the
small intestine and enters the enterohepatic
circulation (most is excreted in the bile,
some reached the systemic circulation and is
excreted in the urine)

33. Hyperbilirubinemia
Elevated bilirubin levels in the blood (>10 mg/l); bilirubin may diffuse into
peripheral tissues, giving them a yellow color (jaundice)
Cause:
1. Pre-hepatic: excessive formation of bilirubin by increased
degradation of erythrocytes (icterus neonatus, hemolytic anemia)
2. Hepatic:insufficient processing of bilirubin as a result of
liver defects (hepatitis, liver toxic damage, cirrhosis, hepatic failure)
3. Post-hepatic:by impaired excretion of gall (obstructive
jaundice due to gallstones, inflammation of biliary tract)
Unconjugated bilirubin can cross the blood-brain barrier, leading to brain
damage
Jaundice in neonates (increased bilirubin degradation+immaturity of the
conjugation enzymes): phototerapy –isomerization of bilirubin to more
soluble pigments

34. Bilirubin
blood urine deriv. in feces
Urobilinogen
blood urine
Prehepatic ↑↑(UC)N ↑ ↑ ↑
Intrahepatic ↑↑(both)↑ N ↑↑ ↑↑
Posthepatic ↑↑(C) ↑ ↓ ↓ ↓

35. Biochemical assessment of liver function
Basic tests: albumin, ALT, AST, GGT, ALP, bilirubin, U-urobilinogen,
U-bilirubin
Special tests: serum protein electrophoresis, prealbumin, glutamate
dehydrogenase, cholinesterase, lactate dehydrogenase, oral glucose
tolerance test (oGTT), P-Fe
Very special tests: ceruloplasmin, S-Cu, α
1-antitrypsin, ammonia, P-bile
acids, serological tests in hepatitis B, assessment of clotting factors

36. ALT (alanine aminotransferase):
increased when cells of the liver are inflamed or undergo cell death
rises dramatically in acute liver damage, such as viral hepatitis
AST (aspartate aminotransferase):
less specific for liver disease
raises in acute liver damage
GGT (gamma-glutamyltransferase):
produced by the bile duct (sensitive marker for cholestatic damage)
raises in alcohol toxicity
ALP (alkaline phosphatase):
associated with the biliary tract
raises in biliary tract damage and inflammation

37. Bilirubin
-total and direct (=conjugated)
Urine urobilinogen:
-insufficient function of the liver
Urine bilirubin:
-when plasma direct bilirubin is elevated (obstructive and hepatocellular
icterus)