+

Functions of liver and its

evaluation.

+FUNCTIONS:

Intermediary Metabolism- a. Protein Metabolism

b. Carbohydrate Metabolism

c. Lipid Metabolism

Coagulation Factors

Erythropoiesis and erythrocytosis

Bilirubin Metabolism

Endocrine function

Immune Function

Drug metabolism and excretion

+Protein Metabolism

Deamination of amino acids

Formation of urea by Krebs-Henseleit cycle (removal of

ammonia)

Interconversion between non-essential amino acids

Formation of plasma proteins

+Plasma Proteins

Albumin: 15% of total liver protein production. 12-15g albumin

is produced every day by healthy adults.

Factors modulating synthesis of albumin: Plasma oncotic

pressure, dietary amino acids and hormones.

Alfa Feto Protein (AFP): Main source is the yolk sac,

hepatocytes and enterocytes. Marker of hepatocellular

carcinoma.(HCC)

Others: Pro coagulants, cytokines, chemokines,acute phase

reactants, and transport proteins.

+Carbohydrate Metabolism

Important homeostatic regulator of blood glucose.

Factors controlling glucose production: Neuroendocrine( insulin, catecolamines, glucagon) and glucose concentration in sinusoidal blood.

Fed state, hepatocytes polymerize glucose and store it as glycogen.

Unfed state, hepatocytes depolymerize glycogen to glucose.

Regulation of glycogen metabolism is by : 1. Glycogen Synthase

2.Glycogen Phosphorylase

+

Glycogen stores are depleted after 24hrs of fasting.

Body then depends on hepatic gluconeogenesis to replenish

blood glucose.

Substrates for gluconeogenesis:

1.Lactate

2.Glycerol from hydrolysis of triglycerides

3.Glucogenic amino acids such as alanine and glutamine

+Regulation of gluconeogenesis

Glucagon and catecholamines stimulate gluconeogenesis

Glucagon via cAMP dependant protein kinases

Insulin inhibits gluconeogenesis and blocks the stimulatory

effect of glucagon and catecolamines on this pathway.

+Lipid Metabolism

Retains releases and oxidizes fatty acids in accord with

nutritional and endocrine influences.

Major source of intra hepatic fatty acids include:

1.Free fatty acids extracted from blood

2.de novo lipogenesis

3.hydrolysis of cytoplasmic triglycerides

4.hepatocellular uptake and metabolism of blood borne

lipoproteins.

+

Acetyl coA is a building block for lipids and a product of the oxidative breakdown of fatty acids and carbohydrates.

Mitochondria oxidise acetyl groups to yield carbon dioxide, water and ATP.

The surplus is metabolized to ketone bodies- acetoacetate, b-hydroxybutyrate and acetone.

Hepatocytes lack ketoacyl CoA transferase so they cannot extract energy from ketones.

Insulin modulates ketogeneis by inhibiting lipolysis in adipocytes.

+Bile Metabolism

Daily production of bile is 600-800ml.

Composition of bile:

97% water

<1% bile salts

Lipids-Cholestrol

Fatty Acids,

Pigments, Inorganic salts, Lecithin,Alkaline Phosphatase

+Functions of bile salts

Activate lipases

Promote micelle formation

Intestinal uptake of fat soluble vitamins, cholestrol and other

lipids

Excretion of numerous lipophilic substances, exogenous and

endogenous.

+Coagulants and Procoagulants

Hepatocytes make most of the procoagulants with the

exception of factor III, IV and VII.

It also makes protein regulators of coagulation and the

fibrinolytic pathway such as protein S, protein C, protein Z,

plasminogen activator inhibitor and antithrombin III.

+Vitamin K Cofactor andCarboxylation

+Heme Metabolism

+ Bilirubin Metabolism

+Endocrine Functions

Hepatocytes produce a wide variety of endocrine substances :

Angiotensinogen

Thrombopoeitin

Insulin-like-growth factor (IGF-1)

They take up thyroxine T4 and may activate it to T3 or

inactivate it.

The liver also inactvates aldosterone, ADH, estrogens,

androgens and insulin.

+Immune and Inflammatory

Responses

Liver is the largest reticuloendothelial organ in the human body.

Hepatic macrophages i.e Kupffer cells account for nearly 10%

of total liver mass.

They degrade toxins, process antigens, and phagocytose

bacteria. They are important modulators of inflammation.

The mediators released by the kupffer cells include cytokines,

chemokines, leukotrienes, proteases, nitro radicals and

reduced oxygen species.

+Xenobiotic Metabolism and

Excretion

The liver is the epicenter of biotransformation reactions.

Liver chemically transforms drugs in ways that increase their

water solubility.

Pathways of drug metabolism:

Phase 1 metabolism

Phase 2 metabolism

Phase 3 elimination

+Phase 1 Metabolism

Phase 1 utilizes CYP (cytochrome P450)and mixed function oxidases to increase the polarity of drugs.

The reactions include oxidation, reduction, deamination, sulfoxidation, dealkylation, methylation.

Oxidases promote the formation of highly active chemicals including reactive oxygen species and free radical intermediates, which can aggravate liver injury.

P-450 can be induced by certain drugs like ethanol, barbiturates and ketamine. This leads to increased tolerance to the drugs effect. Conversely some agents like cimetidine and chloramphenicol can prolong the effect of other drugs by inhibiting these enzymes.

+Phase 2 Metabolism

These reactions conjugate xenobiotics with endogenous hydrophilic molecules like :

Glucuronic acid

Acetate

Sulfates

Amino acids

Glutathione

Compared to their precursors, conjugated xenobiotics are usually less efficacious, less toxic, more hydrophilic and more readily excreted in bile.

+Phase 3 Elimination

Phase 3 reactions involve specific molecular transporters

known as ABC- ATP-binding-cassette transport proteins.

They facilitate the excretion of xenobiotics and endogenous

compounds.

These proteins usually use ATP hydrolysis to drive molecular

transport.

Dysfunction of ABC transport proteins can disrupt the flow of

bile, impair excretion of xenobiotics and endogenous

compounds and induce cholestatic liver injury.

+Determinants of Drug Metabolism

Conditions leading to up regulation of CYP proteins- obesity,

fasting, diabetes.

Conditions causing down regulation of CYP proteins- systemic

inflammatory disorders, fever, hepatic cirrhosis.

PHARMOKINETICS:

Intrinsic hepatic clearance of drug/Hepatic blood flow =(ER)

ER- Extraction Ratio

+ Efficiently Extracted Drugs Poorly Extracted Drugs

Amitriptyline Acetaminophen

Desipramine Amobarbital

Imipramine Antipyrine

Labetalol Aspirin

Lidocaine Clindamycin

Meperidine Diazepam

Metoprolol Digitoxin

Morphine Ethanol

Nortriptyline Hexobarbital

Pentazocine Phenobarbital

Propoxyphene Phenytoin

Propranolol Tolbutamide

Ranitidine Valproic Acid

Verapamil Warfarin

Zidovudine

+

Type of Hepatic

Elimination

Extraction Ratio (ER Rate of Hepatic Drug

Metabolism

Flow-dependent

elimination

High ER: At clinically

relevant

concentrations, most

of the drug in the

afferent hepatic blood

is eliminated on first

pass through the liver

Rapid: Because drugs

with a high ER are

metabolized so rapidly,

their hepatic

clearances roughly

equal their rates of

transport to the liver

(i.e., hepatic blood

Capacity-limited

elimination (also

referred to as dose-

dependent, nonlinear,

saturable, or zero-

order elimination)

Low ER: Hepatic

elimination of these

drugs is determined by

their plasma

concentration

Slow: When the capacity

of the liver to eliminate a

drug is less than the

dosing rate, a steady

state is unachievable;

plasma levels of drug will

continue to rise unless the

dosing rate is decreased.

Drug clearance has no

real meaning in such

settings

+EVALUATION OF LIVER

FUNCTIONS

Detection of hepatocellular injury: Aminotransferases, Lactate Dehydrogenase, Glutathione-S-transferase.

Assessment of hepatic protein synthesis: Serum Albumin, Prothrombin time.

Detection of cholestatic disorders: Alkaline Phosphatase, 5’neucleotidase, Glutamyl Transpeptidase, Serum Bilirubin

Testing for specific diseases: Viral markers

Quantitative liver tests

Measurement of liver blood flow

Radiologic and Endoscopic methods

+Aminotransferases

ALT- Alanine aminotransferase SGPT

AST- Aspartate aminotransferase SGOT

Both enzymes are involved in gluconeogenesis

They are indicators of hepatocellular injury

Ratios of aminotransferases hold clues to diagnose various hepatic diseases:

AST/ALT ratio >4 :Wilsons disease

2-4:Alcoholic liver disease

<1:Non Alcoholic steatohepatitis

+AST ALT LEVELS CAUSES

Mild 100-249 IU/L steatosis, medications, alcohol

consumption, hemochromatosis,

cholestasis, chronic viral hepatitis,

neoplasms, and cirrhosis

Moderate 250-999 IU/L acute viral hepatitis, drug-induced

liver injury, and flare-ups of

chronic liver disease (e.g., viral

hepatitis, steatohepatitis)

Large 1000-1999 IU/L acute hepatitis superimposed on

chronic active liver disease.

Extreme >2000 IU/L massive hepatic necrosis;

fulminant viral hepatitis, severe

drug-induced liver injury (e.g.,

from acetaminophen), shock liver

or hypoxic hepatitis, and in rare

instances, autoimmune hepatitis

or acute biliary obstruction

+Lactate Dehydrogenase

Elevated levels may refect hepatocellular injury, extrahepatic

disorders or both.

Extreme increases signify massive liver damage

Prolonged concurrent elevations in LDH and Alkaline

Phosphatase suggest malignant infiltration of the liver.

An elevation in LDH that is solely due to hepatocellular injury is

accompanied by elevations in AST and ALT.

+Glutathione-S-Transferase

Sensitive and specific test for some drug induced liver injuries.

The enzyme has a brief plasma half life

GST localizes in acinar zone 3(centrilobular region). This zone

contains hepatocytes with highest susceptibility to injury from

hypoxia or reactive drug metabolites.

+Serum Albumin

Normal concentration 3.5-5.5g/dl

Half life is about 2-3 weeks

Albumin <2.5g/dl are generally indicative of:

Chronic Liver Disease

Acute Stress

Severe Malnutrition

Increased loss in urine- Nephrotic syndrome

Increased loss in gastrointestinal tract- Protein losing enteropathy

+Prothrombin Time

Liver-derived procoagulants have short half-lives, which range from 4 hours for factor VII to 4 days for fibrinogen.

Plasma levels of such procoagulants start to descend shortly after the liver begins to fail.

A prolonged PT secondary to liver failure generally reflects a low blood level of factor VIIa, which has the shortest plasma half-life of hepatic coagulant factors. (6h)

PT > 3-4 secs from control are considered significant

PT is of value for patients with drug-induced liver failure or those with active liver disease and a surgical condition that needs immediate attention.

+Alkaline Phosphatase

It lacks specificity for liver disease since it is produced by liver, bone, kidneys and placenta.

Extreme increases in AP suggest

(1) a major block in biliary flow as a result of disorders such as primary biliary cirrhosis and choledocholithiasis

(2) a hepatic malignancy (primary or metastatic) that is compressing small intrahepatic bile ducts.

5′-Nucleotidase and γ-Glutamyl Transpeptidase are used to distinguish between the hepatic and extra-hepatic sources of AP.

+Serum Bilirubin

Total bilirubin is normally below 1gm/dl.

At 3gm/dl scleral icterus is detectable with natural light.

Conjugated Hyperbilirubinemia: (usually associated with

increased urinary urobilinogen)

(1) flow of bile is blocked within the hepatobiliary tree,

(2) hepatocytes produce more bilirubin conjugates than they can

efficiently transport into the canalicular space.

Unconjugated Hyperbilirubinemia: Hemolysis,congenital or

acquired defects in bilirubin conjugation,

+Blood Test Bilirubin Overload

(Hemolysis)

Hepatocellular Injury Cholestasis

Aminotransferases Normal Increased—may be normal

or decreased in advanced

stages

Normal- may be increased in

advanced stages

S. Albumin Normal Decreased—may be normal

in acute fulminant hepatic

failure

Normal- may be decreased

in advanced stages

Prothrombin Time Normal Prolonged Normal- prolonged in

advanced stages

Bilirubin Unconjugated, (mild increase

in conjugated also)

Conjugated Conjuated

ALP Normal Normal/Increased Increased

GGTP & 5’NT Normal Normal Increased

BUN Normal Normal Normal

BSP/ICG Normal Retention type of dye Normal/ Retention type

+Testing for specific disease

(1) serologic testing to identify viral, microbial, and autoimmune

causes.

(2) genetic testing to diagnose heritable metabolic disorders

(3) tumor marker assays to detect hepatic malignancies

+Quantitative Liver Tests

Measuring the clearence of a substance that is avidly extracted

by the liver: Bromsulphalein, Indocyanine Green (ICG) or Rose

Bengal.

Drug-metabolizing capacity of the liver can be measured by

several methods, such as caffeine clearance, galactose

elimination capacity, aminopyrine breath test, antipyrine

clearance, and monoethylglycinexylidide (MEGX).

Quantitative tests are expensive and time consuming, with no

compelling evidence of being superior,

+Measurement of Liver Blood Flow

Clearance Techniques

Indicator Dilution Techniques

Direct Measurements

+Radiologic and Endoscopic

Methods

ERCP

PTHC

Esophagogastroscopy

Splenoportography

Portal Venography

+

Millers Anesthesia 7th ed

Morgans Clincal Anesthesiology 5th ed

Harper’s Illustrated Biochemistry

Robbins pathologic basis of disease