Urea Cycle Flashcards

1
Q

bilirubin

A

toxic waste product, extracted and biotransformed mainly in the liver, excreted in bile and urine.
= a bile pigment

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

breakdown of RBC

A

go to the spleen, Hb broken down to heme and globin
globin reused as amino acid, heme taken up by reicoendothelial cells of the spleen and called unconjugated bilirubin
bilirubin then transported to liver in bloodstream bound to albumin and is taken up by hepatic cells

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what happens to bilirubin in the liver

A

In liver cells, bilirubin is conjugated by addition of glucuronic acid to produce bilirubin diglucuronide (conjugated or water-soluble bilirubin). This reaction is catalyzed by uridyldiphosphate glucuronyl transferase (UDPGT)
Elevations of serum and urine bilirubin levels are normally associated with Jaundice

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Hemoglobin’s journey to bilirubin (enzymes, etc.)

A

Hemoglobin –> globin and heme
Heme oxygenase turns heme into biliverdin
NADPH joins it and biliverdin reductase forms bilirubin (water insoluble)

In the liver: glucoronyltransferase helps 2 UDP-glucoronic acid join to make bilirubin diglucuronide (water soluble)

–> via bile duct to intestins

Urobilinogen formed by bacteria in the intestin. Stercobilin excreted in feces, Urobilin excreted in urine.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Heme degradation - bilirubin

A

Transfer of bilirubin from blood to bile involves 4 distinct but interrelated steps
Hepatocellular uptake: Uptake of bilirubin by hepatocyte has carrier-mediated kinetics
Intra-cellular binding: Within the hepatocyte, bilirubin is kept in solution as a non substrate ligand to several of glutathione-S-transferases, formerly called ligandins
Conjugation: bilirubin is conjugated with one or two glucuronic acid moieties by a specific UDP-glucuronyl transferase to form bilirubin mono or diglucuronide. Conjugation disrupts the internal hydrogen bonding that limits aqueous solubilty of bilirubin, and the resulting glucuronide conjugates are highly soluble in water. Conjugation is obligatory for excretion of bilirubin acroscc the bile canalicular membrane into bile. The UDP-glucuronosyl transferases have been classified into gene families (UGT1 family)
Biliary excretion: Bilirubin mono- and diglucuronides are excreted across the canalicular plasma membrane into bile canaliculi by an ATP-dependent transport process mediated by a canalicular membrane protein called multidrug resistance-associated protein 2 (MRP2). Mutations of MRP2 result in Dubin-Johnson syndrome.

Unconjugated bilirubin does not reach the gut except in neonates or, by ill-defined alternative pathways as seen in unconjugated hyper-bilirubinemia (e.g. Crigler-Najjar syndrome type I)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

bilirubin metabolism

A

Conjugated bilirubin flows into bile ducts and is secreted with bile into intestines
Converted to urobilinogen by intestinal bacteria
Oxidized urobilinogen is excreted in stool as urobilin and stercobilin.
A small portion is reabsorbed and recycled back into bile.
Another small portion remains in blood and is filtered and excreted by kidney in urine

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Bilirubin excretion

A

Conjugated bilirubins are poorly reabsorbed, but are partly hydrolyzed back
to unconjugated bilirubin by catalytic action of bacterial b glucuronidases

In the distal ileum and colon, anaerobic flora mediate further catabolism of
bile pigments: 50% of conjugated bilirubin is converted into urobilinogen by
intestinal bacteria
Hydrolysis of conjugated bilirubin to unconjugated
bilirubin by bacterial b glucuronidases
Multistep hydrogenation (reduction) of unconjugated bilirubin
to form colorless urobilinogens: and
Oxidation of unconjugated bilirubin to brown colored mesobilifuscins
5% of urobilinogen is excreted by kidney through urine

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Urobilinogens

A

Urobilinogens is a collective term for a 3 group of 3 tetrapyrroles:

- Stercobilinogen
- Mesobilinogen
- Urobilinogen

In the lower intestinal tract, the 3 urobilinogens spontaneously oxidize to
produce the corresponding bile pigments:
- Stercobilin
- Mesobilin and,
- Urobilin

Which are orange-brown color and are the major pigments of stool

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Stercobilinogen

A

Some of the urobilinogen is excreted in feces as stercobilinogen.

In feces, stercobilinogen is oxidized to stercobilin (red-brown color).

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Causes of jaundice

A

Causes of jaundice:
- excessive production of bilirubin
- reduced hepatocyte uptake
- impaired bilirubin conjugation
- impaired bile flow
In infants, bilirubin exceeding 15-20 mg/dL causes kernicterus
Kernicterus is a form of brain damage caused by excessive jaundice

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Examples of hyperbilirubinemia

A

a. Hemolytic anemia
increased unconjugated bilirubin (in blood), increased congugated bilirubin (released to bile duct)

b. Hepatitis
increased unconjugated bilirubin (in blood), increased conjugated bilirubin (in blood)

c. Biliary duct stone
increased unconjugated bilirubin (in blood)
increased conjugated bilirubine (in blood)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Prehepatic jaundice

A
excessive bilirubin presented to liver 
     for metabolism, hemolysis 
in most cases, liver function is normal.
↑ in serum unconjugated bilirubin.
total bili usually does not exceed 5 mg/d
negative urine bilirubin 
urinary urobilinogen ↑
dark brown color of feces due to high content of stercobilinogen
Examples:
Malaria
Sickle cell crisis
Thalassemia
G6PD
Autoimmune disorders
Rh- or ABO incompability
Drugs or toxins
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

hepatic jaundice

A

abnormal hepatocyte function
cannot deal with normal load of bilirubin
Cause: enzyme mutation/impaired hepatocellular
uptake (Gilbert’s syndrome)
Findings: serum total bilirubin 5.0 mg/dL; ↑ urinary urobilinogen
Cause: defective secretion by hepatocyte
(Dubin-Johnson syndrome)
Findings: ↑ serum conjugated bili
Cause:hepatitis with lowered conjugation/excretion
Findings: ↑ serum direct and indirect bili with
total levels of 5-10 mg/dL
Darkcoloured urine due to the
excessive excretion of bilirubin and urobilinogen
Pale, clay coloured stools due to the absence of
Stercobilinogen
Increasedactivities of alanine and aspartate
transaminases.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Post-hepatic jaundice

A

impaired excretion of bilirubin
cause: mechanical obstruction of the flow
of bile into the intestines due to gallstones
or tumors
Findings: ↑ serum AND urine conjugated
bilirubin
↓ level of urobilin/stercobilin in
stool (clay-colored stools)
Negative urinary urobilinogen
Increased serum ALP
darkcoloured urine due to elevated
excretion of bilirubin and
clay coloured feces due to absence of stercobilinogen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What’s important to know about findings in various causes of jaundice?

A

Serum bilirubin: unconjugated in pre-hepatic, both in hepatic, conjugated in post hepatic

Urine bilirubin is absent in pre-hepatic (achloric jaundice)

Fecal stercobilinogen - markedly increased in pre-hepatic jaundice (dark brown stool)
reduced in hepatic (pale stool)
Absent in post hepatic (clay colored stool)

Fecal fat increased in hepatic and post hepatic jaundice

Liver function impaired in hepatic jaundice
alkaline phosphatase up in post hepatic

Vonden burg test biphasic in hepatic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Inherited Disorders of Bilirubin Metabolism

A
Gilbert’s Syndrome
Crigler-Najjar (Type I)
Crigler-Najjar (Type II)
Lucey-Driscoll – (exclude)
Dubin-Johnson
Rotor’s Syndrome
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Gilbert’s Syndrome

A

There is compensatory hemolysis, impaired uptake and conjugation

Gilbert’s syndrome is also called the familial non-hemolytic
non-obstructive jaundice

Mild unconjugated hyperbilirubinemia

It affects 3% - 5% of the population.

It is often misdiagnosed as chronic hepatitis

The concentration of bilirubin is serum fluctuates between
1.5 and 3 mg/dl

In this condition the activity of hepatic glucuronyltransferase
is low as a result of mutation in the bilirubin
–UDP-glucuronyltransferase gene (UGT 1A1)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Crigler Najjar Syndrome (Type I)

A

Crigler_Najjar Syndrome (Type I) is a rare genetic disorder caused by complete absence of UDP-glucuronyltransferase and manifested by very high levels of unconjugated bilirubin and often leads tobrain damage in infants.
characterised by aserum bilirubin usually above 345 µmol/l (310–755) (whereas thereference range for total bilirubin is 2–14 μmol/l).
NoUDP glucuronosyltransferase 1-A1 expression in theliver tissue Hence, there is no response to treatment withphenobarbital, which causesCYP450 enzyme induction
It is inherited as an autosomal recessive trait
Most patients die of severe brain damage caused by kernicterus
within the first year of life
Early liver transplant is the only effective therapy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Crigler-Najjar Syndrome (Type II)

A

Crigler-Najjar Syndrome (Type II) is a rare autosomal recessive disorder

It is characterized by partial deficiency of UDP-glucuronyltransferase

Unconjugated bilirubin is usually 5 – 20 mg/dl and some cases are only detected later in life.

Because of lower serum bilirubin, kernicterus is rare in type II.

Unlike Crigler_Najjar Syndrome Type I, Type II responds dramatically to phenobarbital and a normal life can be expected. In fact, this can be used, along with these other factors, to differentiate type I and II.

20
Q

Dubin Johnson Syndrome

A

It is a benign, autosomal recessive condition characterized by jaundice with predominantly elevated conjugated bilirubin and a minor elevation of unconjugated bilirubin, no elevation of liver enzymes ALT, AST.

Excretion of various conjugated anions
     and bilirubin into bile is impaired, 
     reflecting the underlying defect in
     canalicular excretion (mutation in MRP2)
The liver has a characteristic greenish
     black appearance and the liver biopsy
     reveals a dark brown melanin-like
     pigment in hepatocytes and Kupffer 
     cells due to pigment accumulation
21
Q

Rotor’s Syndrome

A

It is another form of conjugated hyperbilirubinemia

is a rare, relatively benignautosomal recessive bilirubin disorder.

It is a distinct, yet similar disorder toDubin–Johnson syndrome

both diseases cause an increase in conjugated bilirubin

Rotor syndrome has many features in common withDubin–Johnson syndrome, an exception being that the liver cells are not pigmented.

The main symptom is a non-itchingjaundice.

22
Q

Physiologic Neonatal Jaundice

A

Prehepatic, due to high level of hemolysis and less conjugation
Low levels of UDP-Glucuronyl Transferase soon after birth, no conjugation of bilirubin, results in an increase in unconjugated bilirubin
Undeveloped intestinal flora (intestinal flora converts bilirubin to urobilinogen), results in an increase in unconjugated bilirubin
Canalicular mechanisms responsible for bilirubin excretion are also immature at birth and their maturation lag behind that of UGT
As a result, most neonates develop mild unconjugated hyperbilirubinemia between days 2 and 5 after birth.
Peak levels are typically between 85-170 mol/L and decline to normal within 2 weeks
A rapidly rising unconjugated bilirubin (levels above 340 mg/L) puts the infant at risk for bilirubin encephalopathy or kernicterus
Bilirubin crosses an immature blood brain barrier and precipitates in the basal ganglia and other areas of the brain
Consequences range from appreciable neurologic deficits to death

23
Q

Physiologic neonatal jaundice: maternal-fetal blood group incompatibiity

A

This form of jaundice occurs when there is incompatibility between blood types of the mother and fetus

This leads to increased bilirubin levels from the breakdown of the fetus red cells (hemolysis)

24
Q

Breast milk jaundice

A

Breast Milk Jaundice

This form of jaundice occurs in breastfed newborns and usually appears at the end of the first week of life.
Certain chemicals in breast milk are thought to be responsible for inhibition of UDP Glucuronyl transferase.
It is usually a harmless condition that resolves spontaneously

25
Q

treatment for neonatal jaundice

A

Phototherapy: converts insoluble bilirubin (trans bilirubin isomer) into water-soluble (cis-bilirubin isomer) that can be excreted into bile.
Phenobarbitone (inducing agent)
Albumin infusion
Exchange transfusion

26
Q

Kernicterus

A

Kernicterus or brain encephalopathy refers to yellow staining of the deep nuclei (kernel) of the brain (basal ganglia)

It is a form of permanent damage caused by excessive jaundice

The concentration of bilirubin in serum is so high that it can move out of the brain into brain tissue by crossing the fetal blood-brain barrier

This condition develops in newborns with
prolonged jaundice due to:
-Polycythemia
-Rh incompatibility between mother & fetus

27
Q

Ammonia

A

Nitrogen in body from dietary proteins, muscle breakdown and intestinal bacteria
Excess nitrogen and amino acids are not stored and excreted as ammonia
Ammonia is the end product of protein metabolism
Ammonia is toxic in even small amount and must be removed from the body
Ammonia is converted to a harmless water soluble compound called urea
Urea is the major end product in Nitrogen metabolism in humans
Urea cycle is the conversion reactions of NH3 into urea for excretion

28
Q

Urea cycle

A

= ornithine cycle

produces urea ((NH2)2CO) from ammonia (NH3)
takes place primarily in the liver, and to a lesser extent in the kidney
Is the process of removal of ammonia from the body
urea is transported to the kidney where it is excreted, 30 g/day excreted
The overall urea formation reaction is :-
2 NH3 + CO2 + 3ATP —> urea + H2O + 3 ADP

29
Q

Biosynthesis of Urea

A

Urea biosynthesis occurs in four stages:

  1. Transamination (Removal of α-amino group)
    by a coupled process of transamination and deamination
    Transamination forms Glutamate in peripheral cells
    Deamination of glutamate forms ammonia in liver
  2. Oxidative deamination of glutamate
  3. Ammonia transport - Ammonia is transported to liver as glutamate, glutamine or alanine
  4. Reactions of the Urea Cycle
30
Q

Transamination

A

Transfer of amino group to α-ketoglutarate and it is reversible.
There are several aminotransferases specific to different amino acids.
In this step amino group from all the amino acids are transferred to a-ketoglutarate and they exist as glutamate.
Transamination is not restricted to α -amino groups.
The δ -amino group of ornithine and the Ʃ -amino group of lysine readily undergoes transamination.

31
Q

Clinical significance of Transaminases

A

Serum aminotransferases such as:
Aspartate aminotransferase, AST and
Alanine transaminase, ALT
have been used as clinical markers of tissue damage, with increasing serum
levels indicating an increased extent of damage

AST is found in the liver, cardiac muscle, skeletal muscle, kidneys, brain,
pancreas, lungs, leukocytes, and erythrocytes
Normal serum activity is 0-41 IU/L.
The concentration of the enzyme is very high in myocardium.
The enzyme is both cytoplasmic as well as mitochondrial in nature

ALT is found primarily in the liver.
The normal serum activity ranges between 0-45 IU/L

32
Q

Oxidative deamination of glutamate

A

The amino group of glutamate is released as ammonia, regenerating α -ketoglutarate, by an enzyme glutamate dehydrogenase

33
Q

Ammonia transport: sources of ammonia

A
  1. Ammonia is produced in body from different tissues by Amino acid catabolism.
  2. Purine and Pyramidine catabolism.
  3. The other source is from the dietary proteins and from urea present in fluids
    secreted into the GI tract.
34
Q

The importance of glutamine

A

GLUTAMINE is the important plasma transport form of nitrogen from muscle.

Glutamine synthetase takes glutamate, NH4, and ATP to make glutamine and ADP + Pi

35
Q

Transport of excess ammonia by glutamine

A

Excess ammonia is toxic to animal tissues.

Glutamine synthetase catalyses the synthesis of glutamine by adding the ammonia to glutamate at the expense
of ATP hydrolysis.

Glutamine is a non-toxic carrier of ammonia. It is transported to liver or kidney via blood.

In the liver & kidney, glutamine is reconverted to glutamate and ammonia by glutaminase.

Ammonia is incorporated in urea cycle to form urea and then it will be excreted through kidneys

36
Q

synonyms for direct bilirubin

A

water soluble
conjugated
( cis form)

37
Q

Urea Cycle occurs?

A

mainly in the liver. Consists of two mitochondrial followed by 3 cytosolic reactions.

38
Q

overview of the urea cycle- what is spared and what is consumed

A

Since the Ornithine consumed in 2nd reaction is regenerated in last reaction, so there is no net loss or gain of Ornithine, Citrulline, argininosuccinate, or arginine.
Ammonium ion, CO2, ATP, and aspartate are, however, consumed.
Aspartate can however be resynthesized from the released fumarate by a series of reactions

39
Q

four steps of urea cycle

A
  1. Carbamoyl phosphate combined with ornithine to form citrulline (ornithine transcarbamoylase) in matrix, passes into the cytosol.

2 Citrulline is converted to arginino- succinate (argininsuccinate synthetase)

3 Argininosuccinate is cleaved (argininosuccinase) to yield fumarate & arginine which enters citric acid cycle.

4 Formation of urea: arginine is converted to urea & ornithine (arginase)
Ornithine – product of the last reaction & substrate of first reaction (same as oxaloacetate in TCA)

40
Q

Urea cycle enzymes in the mitochondria

A

carbamoyl phosphate synthetase I,

ornithine transcarbamylase

41
Q

Urea cycle enzymes in cytosol

A

arginosuccinate synthetase
arginosuccinate lyase
arginase

42
Q

Which enzyme releases urea and ornithine?

A

arginase

43
Q

Urea Cycle Facts to remember

A

Found primarily in liver and lesser extent in kidney
L-glutamine can be used to synthesize new amino acids, or it can dispose of excess nitrogen as ammonia toxic ammonia is quickly recaptured into carbamoyl phosphate and passed into the urea cycle
Nitrogen added to the urea cycle via carbamoyl phosphate and aspartate
Enzymes in mitochondria:
1. Carbamoyl phosphate synthetase
2. Ornithine Trans carbamylase
Enzymes in cytosol:
3. Arginino-Succinate Synthase
4. Arginino-succinase
5. Arginase

44
Q

urea cycle formula

A

NH4+ + CO2 + aspartate + 3 ATP —> urea + fumarate + 2 ADP + AMP + 4 Pi

45
Q

What is the fate of urea from the liver?

A

Urea from liver transported into blood and reaches kidneys where it is filtered and excreted into urine
A portion of urea diffuses from blood into intestine, decomposed by intestinal bacterial urease to CO2 and ammonia
The ammonia is partly lost in the feces and is partly absorbed into blood through the portal circulation, majority reaches the liver and detoxified into urea
A small portion of ammonia goes back into systemic circulation