1-20 Liver Metabolism and Urea Cycle Flashcards
What is bilirubin? How is gotten rid of, in a general sense?
Bilirubin is the orange-yellow pigment derived from senescent red blood cells
- It is a bile pigment
It is a toxic waste product in the body
It is extracted and biotransformed mainly in the liver, and excreted in bile and urine
Where does bilirubin come from? How is it disposed of? What organs are involved? What is an important enzyme involved?
RBC when old or damaged are disposed in the spleen
Hb are broken down to heme and globin
Globin are reused as amino acid, and heme are taken up by reticoendothelial cells of the spleen and called unconjugated bilirubin (lipid soluble)
Bilirubin (from RBC breakdown) is transported to liver in blood stream bound to albumin and is taken up by hepatic cells
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 in serum and urine bilirubin levels are normally associated with jaundice
How much bilirubin does an average person produce per day?
4 mg/kg per day, totaling 250-300 mL
What is the breakdown of components of bilirubin?
85% - Hb from senescent RBCs destroyed in reticuloendothelial cells of liver, spleen, and bone marrow
15% - RBC precursors destroyed in bone marrow
Catabolism of heme-containing proteins
How is bilirubin excreted from the liver, and in what form?
Conjugated bilirubin flows into bile ducts and is secreted with bile into intestines
Converted to urobilinogen by intestinal bacteria, glucuronic acid is removed
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, as urobilin.
Where is bilirubin converted to urobilinogen and urobilin?
Conversion to colorless urobilinogen occurs in the terminal ilium and colon
Oxidation to the yellowish urobilin occurs in urine
How does the hepatocyte take up bilirubin?
The hepatocyte takes up bilirubin across its basolateral membrane through an OATP and other unidentified mechanisms.
How does the hepatocyte process bilirubin?
The hepatocyte conjugates the bilirubin with one or two glucuronic acid residues and exports this conjugated form of bilirubin into the bile.
How is hemoglobin broken down?
Hemoglobin -> globin + heme
heme + O2 -> biliverdin (w/ heme oxygenase)
biliverdin + NADPH -> bilirubin (water insoluble)
- also releases NADP+
- uses biliverdin reductase
bilirubin complexed to albumin to go to liver
bilirubin -> bilirubin diglucoronide (water soluble)
- uses 2-UDP glucoronic acid, with glucoronyltransferase
- released via bile duct to intestines
bilirubin diglucoronide -> urobilinogen (bacteria)
- stercobilin excreted in feces
- urobilin excreted in kidneys
What is the van den Berg test?
measures blood levels of conjugated (water soluble) and unconjugated (water insoluble)
What happens to bilirubins initially in the small intestine?
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. How is this process accomplished?
Hydrolysis of conjugated bilirubin to unconjugated bilirubin by bacterial b glucuronidases
Multistep hydrogenation (reduction) of unconjugated bilirubin to form colorless urobilinogens
Oxidation of unconjugated bilirubin to brown colored mesobilifuscins
5% of urobilinogen is excreted by kidney through urine
What is urobilinogens a collective term for?
3 group of 3 tetrapyrroles:
- Stercobilinogen
- Mesobilinogen
- Urobilinogen
What happens to the remaining 50% of conjugated bilirubin in intestines?
Remaining 50% of conjugated bilirubin is reabsorbed from intestine enters the entero-hepatic circulation and re-excreted as bile
A small fraction (2% - 5%) of urobilinogen enters the general circulation and excreted by kidney through urine. In urine, due to exposure to air, the urobilinogen is converted into urobilin by oxidation
What happens to the 3 urobilinogens in the lower GI tract?
In the lower intestinal tract, the 3 urobilinogens spontaneously oxidize to produce bile pigments, which are orange-brown color and are the major pigments of stool
What is the breakdown of total serum bilirubin? How is amount of unconjugated bilirubin determined?
Normally, total serum bilirubin ranges from 0.2-1.0 mg/dL.
Of this total, < 0.2 mg/dL is conjugated (water soluble).
The remainder of the total is unconjugated (water insoluble)
Total bilirubin- conjugated bilirubin = unconjugated bilirubin
What happens when serum bilirubin is elevated?
Elevations of serum bilirubin cause deposition in tissues and sclera of eyes- jaundice or icterus
Jaundice appears when bilirubin exceeds 2-3 mg/dL
What causes jaundice?
Liver must be functioning normally to eliminate the bilirubin produced daily
Causes of jaundice:
- excessive production of bilirubin - reduced heaptocyte uptake - impaired bilirubin conjugation - impaired bile flow
What is high bilirubin in infants? What are the values?
In infants, bilirubin exceeding 15-20 mg/dL causes kernicterus
Kernicterus is a form of brain 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
What are 3 examples of hyperbilirubinemia?
- Hemolytic anemia
increased blood unconjugated bilirubin
increased conjugated bilirubin released to bile duct - Hepatitis
increased unconjugated blood bilirubin
increased conjugated blood bilirubin - Biliary duct stone
increased unconjugated blood bilirubin
increased conjugated blood bilirubin
What is a cause of prehepatic jaundice? (simple)
Hemolysis
What are some causes of intrahepatic jaundice?
Infection Chemical/drugs Genetic errors - bilirubin metabolism - Gilbert - Crigler-Najjar - Dubin-Johnson - Rotor's Genetic errors - specific proteins - Wilson's - alpha1 antitrypsin Neonatal
What are some causes of posthepatic jaundice?
Intrahepatic bile ducts
- drugs
- primary biliary cirrhosis
- cholangitis
Extrahepatic bile ducts
- gall stones
- pancreatic tumor
- cholangiocarcinoma
What is the mechanism for prehepatic jaundice?
Prehepatic- excessive bilirubin presented to liver for metabolism and capable of excreting
- Overcomes ability of liver to clear.
Causes: hemolytic process
What are the lab findings for prehepatic jaundice?
Lab findings:
- ↑ in serum unconjugated bilirubin.
- (Total bili usually does not exceed 5 mg/dL)
- Negative urine bilirubin
- Urinary urobilinogen ↑
What is the mechanism for hepatic jaundice?
Hepatic- abnormal hepatocyte function
-Cannot deal with normal load of bilirubin
Cause: enzyme mutation/impaired hepatocellular uptake (Gilbert’s syndrome)
What are the expected lab findings for hepatic jaundice? Why?
Serum total bilirubin < 3.0 mg/dL, primarily composed of unconjugated bili;
↑ urinary urobilinogen
Cause: enzyme mutation/defective conjugation (Crigler-Najjar type I syndrome)
Findings: serum unconjugated bili
often > 5.0 mg/dL; ↑ urinary urobilinogen
Cause: defective secretion by hepatocyte
(Dubin-Johnson syndrome)
Findings: ↑ serum conjugated bili
Cause: hepatitis with lowered conjugation or excretion
Findings: ↑ serum direct and indirect bili with
total levels of 5-10 mg/dL.
What is the mechanism for post-hepatic jaundice?
Posthepatic- impaired excretion of bilirubin
-Cause: mechanical obstruction of the flow of bile into the intestines due to gallstones or tumors
What are the expected lab findings for post-hepatic jaundice?
Findings:
↑ serum AND urine conjugated bilirubin
↓ level of urobilin/stercobilin in stool (clay-colored stools)
Negative urinary urobilinogen
See summary slide
Slide 20
Why does kernicterus develop in newborns?
This condition develops in newborns with
prolonged jaundice due to:
-Polycythemia
-Rh incompatibility between mother & fetus
What are 6 inherited disorders of bilirubin metabolism?
Gilbert’s Syndrome Crigler-Najjar (Type I) Crigler-Najjar (Type II) Lucey-Driscoll Dubin-Johnson Rotor’s Syndrome
What are the 4 steps of the urea cycle?
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)
What is the relative amount of unconjugated bilirubin seen with Gilbert’s, Crigler-Najjar, and Lucey-Driscoll syndrome?
Gilbert’s - 25 mg/dL
What is another name for Gilbert’s syndrome?
Gilbert’s syndrome is also called the familial non-hemolytic non-obstructive jaundice
Mild unconjugated hyperbilirubinemia
What causes Gilbert’s Syndrome?
The activity of hepatic glucuronyltransferase is low as a result of mutation in the bilirubin
–UDP-glucuronyltransferase gene (UGT 1A1)
What causes Crigler-Najjar syndrome type I?
rare genetic disorder caused by incomplete absence of UDP-glucuronyltransferase and manifested by very high levels of unconjugated bilirubin
How is Crigler-Najjar type I inherited?
It is inherited as an autosomal recessive trait
How is Crigler-Najjar type I treated? What is the outcome if it is untreated?
Early liver transplant is the only effective therapy
Most patients die of severe brain damage caused by kernicterus within the first year of life
What causes Criggler-Najjar syndrome type II?
partial deficiency of UDP-glucuronyltransferase
What are the genetics of Crigler-Najjar syndrome type II?
rare autosomal dominant disorder
What is the treatment and outcome for Crigler-Najjer syndrome type II?
Type II responds dramatically to phenobarbital and a normal life can be expected
What is Dubin-Johnson syndrome?
characterized by jaundice with predominantly elevated conjugated bilirubin and a minor elevation of unconjugated bilirubin (usually benign)
Excretion of various conjugated anions and bilirubin into bile is impaired, reflecting the underlying defect in canalicular excretion
What are the genetics of Dubin-Johnson syndrome?
autosomal recessive
What does the liver look like with Dubin-Johnson syndrome?
The liver has a characteristic greenish black appearance and the liver biopsy reveals a dark brown melanin-like pigment in hepatocytes and Kupffer cells
What is Rotor’s syndrome?
It is another form of conjugated hyperbilirubinemia
It is similar to Dubin-Johnson syndrome but without pigmentation in the liver. Main symptom is non-itching jaundice.
What is the end goal of the urea cycle?
Removes toxic ammonia from the body, by converting it to urea
What makes up urea?
NH3 from biomolecules ->glutamine -> NH4+-> NH2 end
AAs -> glutamate -> glutamine ->NH4+ -> NH2 end
- gives off alpha ketoglutarate
Citrate cycle -> HCO3 -> center portion of urea
AAs-> glutamate -> aspartate-> NH2 end
- gives off alpha ketoglutarate, fumarate
What is removed from amino acids first in urea creation?
amino group
What is the glucose alanine cycle?
Toxic ammonia formed in muscle is transported to liver as alanine. Alanine serves as a carrier of ammonia and of the carbon skeleton of pyruvate from skeletal muscle to liver. The ammonia is excreted and the pyruvate is used to produce glucose, which is returned to the muscle.
How are amino acids usually broken down?
AAs are transported, then made into alpha amino acids.
Alpha ketoglutarate takes -NH3 from amino acids with help of PLP and amino transferase, making alpha ketoacids.
Enzyme-catalyzed transaminations.
In many aminotransferase reactions, α-ketoglutarate is the amino group acceptor. All aminotransferases have pyridoxal phosphate (PLP) as cofactor.
How is glutamine involved in getting rid of nitrogenous waste?
In non-hepatic tissues the linked reactions of glutamate dehydrogenase and glutamine synthetase remove two ammonia molecules from the tissues as a way of ridding the tissues of nitrogen waste. The glutamine deposits the ammonia in the kidney for excretion
What are the reactions for using glutamate for ammonia removal?
alpha ketoglutarate + NH4+ –>
glutamate
NH3 + ATP + glutamate –>
glutamine + ADP
glutamine does ammonia removal by kidney
Where do amino acids for urea creation in the liver come from?
In liver, nitrogen waste from amino acids ends up in urea. Amino acids are derived either from the breakdown of protein in various tissues or from what is synthesized in those tissues
How is ammonia transported from tissue to the liver?
Excess ammonia in tissues is added to glutamate to form glutamine, a process catalyzed by glutamine synthetase. After transport in the bloodstream, the glutamine enters the liver and NH4+ is liberated in mitochondria by the enzyme glutaminase
(Currently unsure if this is purely redundant to alanine/glucose cycle - KMS.)
What enzymes for the urea cycle are located in the mitochondria? In the cytosol?
Enzymes in mitochondria:
1. Carbamoyl phosphate synthetase 2. Ornithine Trans carbamylase
Enzymes in cytosol:
3. Arginino-Succinate Synthase 4. Arginino-succinase 5. Arginase
What is the overall reaction for the urea cycle?
NH4+ + CO2 + aspartate + 3 ATP —>
urea + fumarate + 2 ADP + AMP + 4 Pi
What does the urea cycle accomplish for the organism?
Urea synthesis provides an efficient mechanism for land animals to remove excess nitrogen from the body. Urea is synthesized in the liver and exported to the kidneys where it enters the bladder.
What is the commitment step of the urea cycle?
Carbamoyl phosphate synthetase I – catalyzes the commitment step in the urea cycle; the activity of this mitochondrial enzyme is activated by N-acetylglutamate in response to elevated levels of glutamate and arginine.
What is an example of the urea cycle in real life?
A deficiency in the enzyme argininosuccinase inhibits flux through the urea cycle and causes hyperammonemia and neurological symptoms. This metabolic disease can be treated with a low protein diet that is supplemented with arginine, thereby resulting in argininosuccinate excretion a substitute for urea.
What do deficiencies in urea cycle enzymes cause?
Complete loss of a urea cycle enzyme causes death shortly after birth
Deficiencies in urea cycle enzymes results in hyperammonemia (elevated ammonia levels in the blood).
Most urea cycle disorders also lead to a build-up of glutamine and glutamate which function as osmolites that can cause brain swelling -neurological symptoms
How can some deficiencies in urea cycle enzymes be treated?
Urea cycle disorders treated by restricting dietary protein as a means to limit nitrogen intake
Metabolic substrates can be provided that increase biosynthesis of nitrogen containing compounds that can be excreted
For example, argininosuccinase deficiency can be treated effectively by putting patients on a protein-depleted diet that is supplemented with high doses of L-arginin
What is deficient in phenylketonuria? What strange products are present?
No phenylalanine hydroxylase
- no phe conversion to tyr, epinephrine
- no tetrahydrobiopterin conversion to dihydrobiopterin
Phenylpyruvate present
- products phenyllactate and phenylacetate seen too
- all harmful to CNS
What happens if PKU is untreated?
Severe mental retardation
- ADHD seen with those that ingest too much phe
What are the normal products of phe?
Phenylalanine –> Tyrosine –> L-Dopa –> Dopamine –> Norepinephrine –> Epinephrine
What are some reasons that liver function tests are done?
- To screen for liver infections, such as hepatitis
- To monitor the progression of a disease like viral or alcoholic hepatitis and determine how well a treatment is working
- To measure the severity of a disease, particularly cirrhosis
- To monitor possible side effects of certain medications
What are some common liver function tests?
Alanine transaminase (ALT). An enzyme found mainly in liver cells, to metabolize protein. Normally, ALT levels in the blood are low. When the liver is damaged, ALT is released in the bloodstream and levels increase.
Aspartate transaminase (AST). The enzyme AST plays a role in metabolism of alanine. AST is found in high concentrations in liver cells. An increase in AST levels may indicate liver damage.
Alkaline phosphatase (ALP). ALP is an enzyme in the liver and bile ducts, as well as some other tissues. Higher than normal levels of ALP may indicate liver damage or disease.
Albumin and total protein. Levels of albumin — a protein made by the liver — and total protein show how well your liver is making proteins that your body needs to fight infections and perform other functions. Lower than normal levels may indicate liver damage or disease.
Bilirubin. Bilirubin results from the breakdown of red blood cells. Normally, bilirubin passes through the liver and is excreted in your stool. Elevated levels of bilirubin (jaundice) may indicate liver damage or disease.
Gamma-glutamyltransferase (GGT). This test measures the amount of the enzyme GGT in the blood. Higher than normal levels may indicate liver or bile duct damage.
L-lactate dehydrogenase (LDH). LDH is an enzyme found in many body tissues, including the liver. Elevated levels of LDH may indicate liver damage.
Prothrombin time (PT). This test measures the clotting time of plasma. Increased PT may indicate liver damage
