Hepatic Function

Question 1

CHARACTERISTICS OF THE LIVER

Answer 1

• Portal circulation, 25% of cardiac output, gives liver the first choice on absorbed nutrients and toxic substances, higher concentrations of insulin and glucagon. The fenestrated endothelium permits passage of proteins, lipoproteins and chylomicron remnants. Hepatic artery provides oxygen.

Question 2

REPLACEMENT of LIVER FUNCTIONS.

Answer 2

• Artificial Liver systems
• Liver transplant (from dead donors, genetically modified animals?)
• Partial liver transplant (from living donors)
• Portal hepatocyte colonization (from healthy donors, or self after genetic engineering, stem cells?)

Question 3

Mechanisms of Blood Glucose Maintenance

Answer 3

• Storage of Glycogen
• Release of glucose from stored glycogen.
• Conversion of other sugars to glucose
• Gluconeogenesis

Question 4

Storage of Glycogen

Answer 4

• Highly branched glycogen permits rapid mobilization
• Glycogen synthase needs primer: glycogenin
• Insulin does not affect glucose transport in liver
• Liver contains hexokinase (as other tissues)(km<0.l mM), but 80% of phosphorylation is catalyzed by liver-specific glucokinase, sensitive to glucose concentration (Km = 10 mM)
• Contrary to hexokinase, glucokinase is not inhibited by its product (Glc-6-P). Thus, in liver, phosphorylation limits uptake. Once glycogen particles are replenished, Glc-6-P is largely channeled into lipogenesis.

Question 5

Release of glucose from stored glycogen.

Answer 5

• Glucose-6-phosphatase occurs only in liver and kidney. Because of its larger mass and glycogen content, release of glucose between meals is largely a liver function
• Contrary to muscle, in the dephosphorylated state Hepatic glycogen phosphorylase is not stimulated by AMP, but in the phosporylated state it is inhibited by glucose.
•

Question 6

Glycogen storage diseases

Answer 6

• Type I (Von Gierke disease)
• Type II (Pompe’sdisease)
• Type III (Cori’s disease)
• Type IV (Andersen’s disease)
• Type V( McArdle’s disease)
• Type VI (Hers’disease)
• Type VII
• Type VIII

Question 7

Type I (Von Gierke disease)

Answer 7

• Defective enzyme: Glucose-6-phosphatase
• Organs affected: Liver and Kidney
• Clinical features: Large liver, fasting hypoglycemia

Question 8

Type II (Pompe’sdisease)

Answer 8

• Defective enzyme: Lysosomal amylase
• Organs affected: All organs
• Clinical features: Cardiorespiratory failure before 2

Question 9

Type III (Cori’s disease)

Answer 9

• Defective enzyme: Debranching enzyme
• Organs affected: Muscle , Liver
• Clinical features: Like Type I but milder course

Question 10

Type IV (Andersen’s disease)

Answer 10

• Defective enzyme: Debranching enzyme
• Organs affected: Liver
• Clinical features: Cirrhosis: death before 2yrs

Question 11

Type V( McArdle’s disease)

Answer 11

• Defective enzyme: Glycogen phosphorylase
• Organs affected: Muscle
• Clinical features: Cramps after exercise

Question 12

Type VI (Hers’disease)

Answer 12

• Defective enzyme: Glycogen phosphorylase
• Organs affected: Liver
• Clinical features: Like Type I but milder course

Question 13

Type VII

Answer 13

• Defective enzyme: Phosphofructokinase I
• Organs affected: Muscle
• Clinical features: Like Type V but milder

Question 14

Type VIII

Answer 14

• Defective enzyme: Phosphorylase kinase
• Organs affected: Liver
• Clinical features: Mild liver enlargment; mild hypoglycemia

Question 15

Conversion of other sugars to glucose.

Galactose

Answer 15

• Galactose is half of the carbohydrates in milk
• Conversion of Galactose to glucose:
Galactose -(1)-> Gal-1 P -(2)-> Glucose-1 P -(3)-> Glucose-6 P
1) Gal kinase
2) P-gal uridyl transferase
3) Epimerase
• Genetic deficiency of P-gal uridyl transferase is far more serious than Gal kinase because of accumulation of Gal-1 P which is toxic because of possible sequestration of intracellular Pi

Question 16

Conversion of other sugars to glucose.

Fructose

Answer 16

• Fructose can be up to l/3 of total dietary carbohydrates, most as part of sucrose
• A minor portion of that fructose is used by muscle and liver through hexokinase via fructose-6 P and the normal glycolytic pathway
Fructose -(hexokinase)-> fructose-6 P
• The majority is converted to Glyceraldehyde:
Fructose -(1)-> Fructose-1 P -(2)-> Glyceraldehyde
1) Fructokinase
2) Fructose-1-P aldolase
• Genetic deficiency of Fructose-1-P aldolase is far more serious than Fructose kinase because of accumulation of Fructose-1 P which is toxic because of possible sequestration of intracellular Pi

Question 17

Two Warnings about Fructose

One

Answer 17

Ingestion of excessive amounts of fructose (say, children drinking lots of apple cider in a hot summer day) can cause diarrhea because the absorption of fructose through the GLUT5 transporter is not very effective. This characteristic may be a protective mechanism, because excessive absorption of fructose could be damaging to the liver

Question 18

Two Warnings about Fructose

Two

Answer 18

• Decades ago, it had been fashionable in some countries to consider replacing glucose by fructose as an intravenous nutrient in diabetics. Never do that!
• Large amounts of fructose are preferentially taken up by hepatocytes. Fructokinase, however, is a much more efficient enzyme than fructose-1-P aldolase even when this enzyme is not defective. That makes fructose-1-P aldolase rate-limiting. The consequence of the excessive uptake of fructose is the hepatic accumulation of fructose-1-P, which is deleterious to the liver (just as with much smaller amounts of fructose, when there is a genetic defect in fructose-1-P aldolase)

Question 19

Gluconeogenesis

Answer 19

• Glucosw can be made in liver from non-sugar substrates, such as:
1) lactate
2) amino acids
3) glycerol.
• In the postabsorptive state, gluconeogenesis from lactate in the ‘Cori Cycle’, generates about 15% of liver glucose production (increased during anaerobic exercise).
• Gluconeogenesis from amino acids, is only 10% of liver glucose production in postabsorptive conditions, but it is vastly increased after a protein-rich meal (the only source of carbohydrates in carnivorous diets) or during starvation (in which case 50% is from alanine)

Question 20

Gluconeogenesis

Regulation

Answer 20

• Availability of lactate (anaerobic exercise) or excess amino acids (after a protein-rich meal or because of net proteolysis in liver and muscle during starvation).
• Modulation by cAMP of pyruvate kinase and of the conversion of Frc-l,6 P to Frc-6 P (via Frc-2,6 P )

Question 21

Gluconeogenesis and Starvation

Answer 21

• After several days of starvation, hepatic capacity for gluconeogenesis is increased by induction of PEP carboxykinase, glucose-6 phosphatase and many transaminases (by glucocorticoids and the release of the inhibitory effect of insulin through FOXO1 phosphorylation).
• Gluconeogenesis depends on the availability of cytosolic NAD+ for the reoxidation of malate.
• Combination of starvation and ethanol is a frequent cause of hypoglycemia because cytosolic oxidation of ethanol leads to fully reduced cytosolic NADH. Because of competition for the same cytosolic NAD+, starvation decreases ethanol metabolism, ethanol plus starvation leads to production of lactic acid, and ethanol inhibits liver glycolysis.

Question 22

NITROGEN METABOLISM.

Synthesis of urea

Answer 22

Increased urea synthesis results from increased supply of amino acids to the liver (from the diet after a protein-rich meal, from muscle protein degradation during starvation). Increased Glu results in an expansion of intramitochondrial N-ac-Glu, which stimulates carbamyl-P synthetase. Over a period of days liver adapts to protein-rich diets or starvation with induction of all urea cycle enzymes (re: Dr Barlowe). Liver failure or defects in any of the urea cycle enzymes may result in high blood NH3, which is toxic to the brain.

Question 23

NITROGEN METABOLISM.

Catabolism of excess dietary amino acids

Answer 23

Amino acid carbons in excess are used for glucogenesis, lipogenesis or ketogenesis depending on the condition and the nature of the C chain.

Question 24

NITROGEN METABOLISM.

Conversion of extrahepatic amino acid C to glucose in the postabsorptive state

Answer 24

In this state, excess amino acids are generated primarily in muscle by autophagy. The branched chained amino acids (Leu, Ile, Val) are oxidized in muscle . Their amino N is transferred to pyruvate to make alanine (Ala). Ultimately, Ala transports C (from muscle glycolysis) and amino acids N to the liver, where it is converted to glucose and urea.

Question 25

NITROGEN METABOLISM. | Synthesis of non-essential amino acids

Answer 25

A major but not exclusive hepatic function. After a meal, it is in the liver where most of the ‘nitrogen shuffling’ occurs, by which N from abundant dietary amino acids is utilized to synthesize those poorly represented in the diet.

Question 26

NITROGEN METABOLISM. | Oxidation of hipoxanthine and xanthine to uric acid

Answer 26

Uric acid from endogenous purines is made in the liver. Xanthine oxidase is also present in intestinal epithelium, where most ingested purines are oxidized.

Question 27

NITROGEN METABOLISM. | Fine-tuned regulation of amino pools

Answer 27

Regulation of hepatic protein turnover is the most important factor in the acute regulation of amino acid pools. Rapid changes in protein turnover permit storage of excess amino acids as additional liver protein, and the utilization of hepatic protein as a predominant source of amino acids for the body between meals.

Question 28

LIPID METABOLISM - Oxidation of Fatty Acids.

Answer 28

Most tissues (except brain and blood cells) use FA. Liver uses FA almost exclusively as source of energy, about 20 g/day (except after a CHO-rich meal or alcohol ingestion).

Question 29

LIPID METABOLISM - Ketogenesis

Answer 29

• Hepatocytes can synthesize but not oxidize ketone bodies. The enzymes necessary for β-oxidation of FA, and the conversion of the resulting AcCoA to acetoacetate and ß-OH butyrate are always present in the matrix of hepatic mitochondria. • Ketogenesis results from abundant exogenous supply of FA to the hepatocytes, and increased transport of those FA into the mitochondrial matrix. The resulting beta-oxidation of the FA floods the mitochondria with AcCoA in excess of what can be oxidized in the Krebs cycle. The excess AcCoA is converted to ketone bodies and exported.

Question 30

LIPID METABOLISM - Ketogenesis | Starvation

Answer 30

In starvation, ketogenesis is promoted by cAMP in adipocytes (lipolysis) and hepatocytes (inhibition of AcCoA carboxylase, lower malonyl CoA, increased carnitine-linked mitochondrial transport of FA). After 3 d of starvation production of ketone bodies reaches ca. 150 g/d

Question 31

LIPID METABOLISM - Ketogenesis | Ketogenic Diests

Answer 31

* Ketogenesis may also result from ingestion of lipid-rich, carbohydrate-deprived diets (carnivorous diets, Atkins diet). In this case the FA are provided by the diet, not the adipocytes. * Ketogenic diets such as these have been used to ameliorate refractory epilepsy in children. One version of such diets is rich in coconut oil, which contains medium-chain FA that can bypass the carnitine-linked step in liver mitochondria, and thus may be consumed together with carbohydrates.

Question 32

LIPID METABOLISM | Lipogenesis: de novo synthesis of Fatty Acids

Answer 32

* From excess dietary glucose (in humans mostly in liver, and in lactating mammary gland). Moderate influx of glucose and modest increase in insulin levels favor glycogen synthesis. Larger glucose influx and insulin direct the glucose ‘overflow’ to lipogenesis. * To provide AcCoa for FA synthesis, glucose must be first degraded to AcCoA (via pyruvate dehydrogenase). * The rate-limiting step in lipogenesis is the synthesis of malonyl CoA. Acetyl CoA carboxylase is modulated by citrate (+), insulin (+) and glucagon (-) . * Lipogenic enzymes are induced by insulin (SREBP-1c) and glucose metabolism (CHREBP)

Question 33

LIPID METABOLISM | Desaturation of Fatty Acids

Answer 33

The liver introduces double bonds in FA by a mixed function oxygenase in liver endoplasmic reticulum (desaturase + cytochrome b5 + NADH-cytochrome b5 reductase). Double bonds in any of the last 7 C must be provided in the diet (essential fatty acids)

Question 34

LIPID METABOLISM | Synthesis of VLDL and degradation of lipoprotein remnants

Answer 34

* In the postabsorptive sate, the liver takes up ca. 70g of free fatty acids (FFA) per day, uses 20g as fuel and exports 50g in VLDL. Repackaging FFA to TG in VLDL permits a more efficient delivery to extrahepatic tissues (excessive FFA are potentially toxic to the heart). * Excessive FFA uptake or lipogenesis (obesity, diabetes type II), decreased FA oxidation (ethanol, a preferred alternative fuel), or impaired synthesis of VLDL (choline deficiency, many drugs, liver failure) can result in accumulation of hepatocyte triglycerides (fatty liver).

Question 35

LIPID METABOLISM | De novo synthesis and catabolism of choline

Answer 35

The synthesis of phosphatidyl-choline (=lecithin) from phosphatidyl-serine occurs only in liver. Other cells use choline as such (from the diet) via the CDP-choline salvage pathway, or albumin-bound lysolecithin produced by the action of LCAT on HDL (re: Chang). Liver is also the site of catabolism of excess choline.

Question 36

LIPID METABOLISM | De novo synthesis of cholesterol

Answer 36

The liver is the major but not exclusive site. Inhibition of hepatic synthesis by dietary cholesterol (via chylomicron remnants) or cholesterol re-uptake from the blood (via LDL receptors) are crucial to the control of total body cholesterol. A major point of regulation is the activity of hepatic HMG-CoA reductase (the main therapeutic target of statins). The synthesis of this enzyme and of the LDL receptors is induced by SREBP-2

Question 37

LIPID METABOLISM | Elimination of cholesterol

Answer 37

Cholesterol is not broken down in the body. The liver eliminates it in the bile, as such or as bile acids. The bile acids are largely recovered in the enterohepatic circulation. Binding to insoluble food fiber may promote fecal elimination of bile acids and depletion of body cholesterol

Question 38

SECRETION OF BILE AND ENTEROHEPATIC CIRCULATION.

Answer 38

Bile is secreted by hepatocytes and concentrated up to 30-fold in the gall bladder. The secretion of bicarbonate is stimulated by secretin. Reabsorbed bile acids stimulate their further secretion. Cholecystokinin stimulates gal bladder discharge. Most bile acids are conjugated. Together with P-lipids, they keep up to 1% cholesterol in micellar suspension. Cholesterol precipitation may result in gallstones (re: Chang). The bile serves also for the excretion of many other conjugated hydrophobic substances.

Question 39

CONJUGATION AND EXCRETION OF HYDROPHOBIC SUBSTANCES

Answer 39

The liver contains a number of enzymes capable of attaching hydrophilic moieties (amino acids and their derivatives, glutathione or modified sugars) to undesirable hydrophobic substances or catabolites. The major one is bilirubin. Heme is degraded to bilirubin in spleen macrophages. Bilirubin is transported in the plasma bound to serum albumin, taken up by hepatocytes, conjugated with two molecules of glucuronic acid and the conjugate is eliminated in the bile. Bilirubin is further modified by enteric bacteria to darker fecal pigments (stercobilin and urobilin)

Question 40

CONJUGATION AND EXCRETION OF HYDROPHOBIC SUBSTANCES | Jaundice

Answer 40

Serum levels of unconjugated (alcohol soluble or indirect) bilirubin increase in hemolytic or hepatic jaundice. Conjugated (water soluble or direct) bilirubin predominates in the serum in obstructive jaundice., and also after hepatocellular damage, when debris clog biliary canaliculi. Fecal pigments tend to increase in hemolytic jaundice (except in the neonate, where bacteria have yet not colonized the intestine). They decrease or disappear in hepatic and obstructive jaundice

Question 41

OXIDATION OF LIPID-SOLUBLE SUBSTANCES

Answer 41

``` A major pathway for the hydroxylation of hydrophobic substances is catalyzed by a large group of enzymes known as heme-containing cytochrome P450s (CYPs). They use molecular oxygen (O2) and a reactive Fe++ in the heme (which is kept in the reduced form by a NADPH-dependent enzyme). CYPs comprise dozens of different proteins, each with a characteristic broad specificity for a class of hydrophobic organic molecules. In nature, the substrates are primarily undesirable components of plants or soil, as well as endogenous sterol hormones. By hydroxylation, the liver makes these compounds more soluble, which facilitates their elimination in the bile or the urine. ```

Question 42

OXIDATION OF LIPID-SOLUBLE SUBSTANCES | Metabolism of Drugs

Answer 42

CYPs are of great importance in medicine because they are often involved in the metabolism of drugs and hormones. In most cases these are inactivated by hydroxylation. Sometimes, however, hydroxylation results in the activation of an inactive precursor (for instance, codeine -> morphine). The situation is further complicated by the fact that many CYPs are inducible by these or other drugs, substances in the food, smoking, etc.. The increased activity of induced CYPs may modify drastically the sensitivity of the body to drugs and potential toxic agents. This important topic will be thoroughly addressed in your Pharmacology course.

Question 43

SYNTHESIS AND DEGRADATION OF PLASMA PROTEINS.

Answer 43

A major portion of total liver protein synthesis is devoted to the synthesis of most plasma proteins, included albumin, globulins (except immunoglobulins), fibrinogen and coagulation factors, as well as lipoproteins. The liver is also the site for the lysosomal degradation of plasma glycoproteins.

Question 44

STORAGE OF IRON, VIT B12, D AND A

Answer 44

In addition to the storage of glycogen and some of the excess amino acids as hepatic proteins, the liver plays a major role in the storage of iron as ferritin and vitamins (A, D, B12)