biochem 44 Liver Functions and Transport Systems

• The liver is strategically located between the general circulation and the digestive tract
• Receives 20-25% of blood volume leaving the heart each minute
• Via the portal vein, which delivers absorbed nutrients
• Via the hepatic artery, which delivers blood from general circulation
• The liver's size, about 3% of body weight, allows extended time for nutrient metabolism and detoxification of harmful substances
• Functions as an excretory organ alongside the kidney and gut

Liver Functions

• Equipped with broad spectrum of detoxifying mechanisms
• Performs metabolic conversion pathways
• Utilizes secretory systems for excretion of potentially toxic compounds
• Contains highly specific and selective transport mechanisms
• Transports essential nutrients to sustain its energy and provide substrates for systemic needs

Hepatocytic Transport Systems

• Located in organelles like endosomes, mitochondria, lysosomes, and the nucleus
• Involves sequential steps: uptake, intracellular binding and sequestration, metabolism, sinusoidal secretion, biliary excretion
• The rate of activity of each step affects hepatobiliary transport
• Hepatic blood flow, plasma protein binding, and canalicular reabsorption also are factors

Liver Anatomy

• Consists of two lobes, each with multiple lobules and sinusoids
• Blood supply is 75% from the portal vein, carrying blood from the small intestine, stomach, pancreas, and spleen
• The rest, 25%, is arterial supply via the hepatic artery.
• Portal vein and hepatic artery blood mixes in a common conduit before entering liver sinusoids
• Sinusoids = expandable vascular channels running through the hepatic lobules
• Sinusoids are lined with endothelial cells that are described as "leaky"
• Blood flows through the sinusoids
• Contents of plasma have free access to the hepatocytes, located on the other side of the endothelial cells
• Liver also acts as an exocrine organ, secreting bile into the biliary drainage system
• Hepatocytes secrete bile into the bile canaliculus
• Contents flow parallel to that in the sinusoids but in the opposite direction
• Canaliculi empty into the bile ducts
• Lumina of bile ducts fuse, forming the common bile duct
• The common duct then releases bile into the duodenum
• Some liver effluent is stored in the gallbladder & discharged into the duodenum postprandially for digestion
• The entire liver surface is covered by a connective tissue capsule
• Capsule provides support for the blood vessels, lymphatic vessels, and bile ducts that permeate the liver
• Connective tissue subdivides the liver lobes into smaller lobules

Liver Cell Types and functions

• Hepatocytes: primary cell type, making up 80% of the liver volume and 60% of the liver cells
• Carry out liver functions, representing almost all pathways of metabolism
• Low turnover and long life span, hepatocytes are normally quiescent
• Can be stimulated to grow if liver damage occurs
• Liver mass has a relatively constant relationship to the total body mass of adult individuals
• Hepatic growth rapidly corrects any deviation from the normal or optimal ratio, by proportional increase in hepatocyte replication
• Sinusoidal endothelial cells constitutes the lining cells of the sinusoid -Unlike other body tissues endothelial cells
• Contain fenestrations with a mean diameter of 100 nm
• Do not form a tight basement membrane barrier between themselves and hepatocytes
• Allows small molecule diffusion to hepatocytes, not particles the size of chylomicrons
• Capable of endocytosing many ligands and secreting cytokines when appropriately stimulated
• Liver endothelial cells do not present enough barrier to sinusoid contents movement into hepatocytes
• Fenestrations or pores further promote blood component passage
• Kupffer cells = tissue macrophages in the sinusoidal lining
• Contain almost one quarter of all liver lysosomes
• Possess endocytotic and phagocytic capacity
• Phagocytose substances: denatured albumin, bacteria, and immune complexes
• Protect liver from gut-derived particulate materials and bacterial products
• Secrete potent mediators of inflammatory response on stimulation by immunomodulators
• Role in liver immune defense, releasing cytokines to inactivate foreign substances
• Kupffer cells also remove damaged erythrocytes from circulation
• Stellate cells, also called perisinusoidal or Ito cells
  • 5-20 of these cells per 100 hepatocytes
  • Lipid-filled cells, serve as primary storage site for vitamin A
  • Control turnover of hepatic connective tissue and extracellular matrix
  • Regulate contractility of sinusoids
  • Stellate cells stimulated by various signals, increase synthesis of extracellular matrix material: Cirrhosis
• Pit cells also are known as liver-associated lymphocytes
• Natural killer cells, liver defense mechanism against invasion by potentially toxic agents like tumor cells or viruses

Major Functions of the Liver

• The liver can carry out a multitude of biochemical reactions to fulfill constant monitoring, recycling, modifying, and distributing ingested nutrients
• Retrieves useful portion & converts it to a substrate for hepatic & nonhepatic cell use
• Removes toxic compounds & targets them for excretion (urine or bile)
• Receives nutrient-rich blood from the enteric circulation via the portal vein

The Enterohepatic Circulation

• Allows the liver to first access the nutrients to fulfill specific functions (i.e. blood coagulation proteins, heme, purines, and pyrimidines)
• Access the ingested toxic compounds like ethanol
• Access harmful metabolic products like NH4+ produced from bacterial metabolism in the gut
• The hepatic artery supplies liver cells with oxygen
• Fenestrations in the endothelial cells, combined with gaps between the cells and the lack of a basement membrane efficient exchange of compounds
• Large molecules being processed (serum proteins & chylomicron remnants) can be removed by hepatocytes, degraded and their components recycled.
• The ability to convert all amino acids found in proteins into glucose, fatty acids, or ketone bodies occurs
• Secretion of VLDL by the liver delivers excess calories to adipose tissues for storage of fatty acids in triacylglycerol, phospholipids and cholesterol to tissues to synthesize cell walls
• Liver performs gluconeogenesis due to access to a variety of dietary sugars, forming the oligosaccharide chains & dietary amino acids with which it synthesizes proteins → large number of biosynthetic reactions
• Liver metabolizes compounds into biochemically useful products
• Degrades and excretes compounds that cannot be further used
• Specific biochemical pathways of the hepatocytes can be designed to determine normalcy of functional characteristics

Liver Cell Specialization and function

• Specialized transport & uptake mechanisms
• Enzymes, infectious agents, drugs, & xenobiotics specifically target substances to certain liver cell types with biodegradable bonds

Inactivation and Detoxification of Xenobiotic Compounds

• Xenobiotics: non-nutritive & potentially toxic compounds that are present as natural components of foods or introduced by additives or processing
• Can also be Pharmacological & recreational drugs
• Liver = site in the body for xenobiotic degradation
• Phase I reactions: oxidation, hydroxylation, or hydrolysis by enzymes introduce or expose hydroxyl groups/other reactive sites (lipophilic to increase excretion)
• Phase II reactions: conjugation reactions add a negatively charged group (i.e., glycine, sulfate, or glucuronic acid) to increase excretion
• Many xenobiotic compound pathways exist
• The liver's conjugation and inactivation pathways are similar to those used to inactivate its own metabolic waste products
• Liver = synthesize precursors for conjugation & inactivation reactions from other compounds
• Sulfation is used by the liver to clear steroid hormones, sulfate from cysteine/methionine degradation
• Major sites in the body for biotransformation of xenobiotic compounds:
  • Liver
  • Kidney
  • Intestine
• The nitrogen-containing rings of nicotine or pyridoxine & aromatic rings (i.e., benzopyrene in tobacco smoke) = structures that are hydrophobic retained in adipose tissue unless sequestered by the liver for biotransformation reactions
• Phase I & II reactions convert harmless hydrophobic molecules to toxins or potent chemical carcinogens

Cytochrome P450 and Xenobiotic Metabolism

• Enzymes that accomplished xenobiotic toxification/detoxification have a broad spectrum of biological activity
• Cytochrome P450–dependent mono-oxygenase system: Determinants in oxidative, peroxidative, and reductive degradation
• Key enzymatic constituents = flavoprotein NADPH-cytochrome P450 oxidoreductase & cytochrome P450
• Omura & Sato found a pigment in 1962, derived from liver microsomes of rabbits
• System named in 1962

Cytochrome P450 Enzymes Major Role

• Oxidize substrates & introduce oxygen to the structure
• Similar reactions by other flavin mono-oxygenases that do not contain cytochrome P450
• Human family cytochromes = 57 functional genes, produce proteins 40% identical, differing specificities
• Enzymes divided into nine major subfamilies, each further subdivided: CYP = cytochromes P450, number = subfamily, letter = ethanol, last number = isozyme
• CYP3A4 isoform accounts for 30% to 40% of CYP450 enzymes in the liver and 70% of cytochrome enzymes in gut wall enterocytes
• Greatest number of drugs in humans are metabolized (substrates for CYP3A4)

Cytochrome P450

• Concomitant ingestion of two CYP3A4 substrates → competition for the binding site
• Binding site competition can alter drug blood levels
• Greatest affinity → preferentially metabolized
• Metabolism of drugs & degradation can be reduced
• Can be impaired by substances
• Drug concentration in the blood rises
• Drugs and substances that impair or inhibit CYP3A4 enzyme, impairing the body's ability to metabolize a drug
• Some statins (HMG-CoA reductase inhibitors) require CYP3A4 for degradation of drug
• Treatment & dose account for drug’s normal degradation pathway
• Grapefruit juice potent inhibitor of CYP3A4-mediated drug metabolism
• Side effects of statins dose-related in regular intake
• Levels in the blood may increase 15-fold

Common Features of Cytochrome P450 Isozymes

• They all contain cytochrome P450, oxidize the substrate, & reduce oxygen
• All have flavin-containing reductase subunit that uses NADPH (not NADH) as a substrate
• All found in the smooth endoplasmic reticulum & referred to as microsomal enzymes
• All bound to the lipid portion of the membrane, probably to phosphatidylcholine
• All inducible by the presence of own best substrate & somewhat less inducible by the substrates for other P450 isozymes
• All generate a reactive free-radical compound as an intermediate in the reaction

Vinyl Chloride & Aflatoxin B1

• Vinyl chloride detoxification provides effective detoxification by a P450 isozyme
• Used for plastics synthesis cause angiosarcoma
• Activated in a phase I: reactive epoxide by a hepatic P450 isozyme (CYP2E1) that can react guanine
• Also can be converted to chloroacetaldehyde, conjugated with reduced glutathione, & excreted in series of reactions
• Aflatoxin B1: an example of a compound that is made more toxic by cytochrome P450 reaction(CYP2A1)
• A current research: ingested aflatoxin B1 (contaminated food) is directly involved liver cancer p53 gene
• Metabolically activated to its 8,9-epoxide
• Epoxide alters DNA: covalent adducts with guanine residues.
• Can combine with lysine residues in proteins
• Is also a hepatotoxin

Acetaminophen

• Tylenol is metabolized by the liver for safe excretion
• Can be toxic when high doses are ingested pathways
• Acetaminophen can be glucuronylated/sulfated for kidney excretion
• Cytochrome P450 produces the dangerous intermediate N-acetyl-p-benzoquinoneimine. (NAPQI)
• NAPQI can be excreted in urine with glutathione

Regulation of Blood Glucose Levels

• Maintain blood glucose concentrations within the normal range
• Monitor glucose and secrete insulin to compensate when levels rise, glucagon when glucose decreases
• Hormones affect liver glycogenolysis, glycogen synthesis, glycolysis, gluconeogenesis
• Sustained physiological increases in growth hormone, cortisol, and catecholamine secretion help to sustain normal blood glucose levels

Key Regulatory steps during fasting

• Glycolysis and glycogen synthesis inhibited
• Gluconeogenesis and glycogenolysis activated
• Fatty acid oxidation activated to provide energy for glucose synthesis
• Glucose levels are maintained primarily by glycogenolysis and gluconeogenesis
• Involves 6 ATP molecules to produce one molecule of glucose from two molecules of lactate
• Activation or inhibition of two key kinases (cyclic adenosine monophosphate [cAMP]–dependent protein kinase and the AMP-activated protein kinase
• Glucose can be exported by the liver, due to expressing glucose 6-phosphatase

Synthesis & Export of Cholesterol & Triacylglycerol

• Hormonal activation, fatty acid, triacylglycerol, & cholesterol synthesis occur when food supplies = plentiful
• High dietary intake & intestinal absorption of cholesterol reduces hepatic cholesterol synthesis
• The liver acts as a recycling depot for sending excess dietary cholesterol to the peripheral tissue

Liver Is the Primary Organ Where Cholesterol & Urea Synthesis Occurs

• Primary organ that synthesizes urea
• Central depot for ammonia’s disposition
• The urea cycle is the way ammonia from the liver is converted to urea

Liver Forms Ketone Bodies

• The only organ for ketone bodies
• Ketone body molecules cannot be used for energy production
• Synthesis Occurs when rates of of glucose are limited or restricted conditions occurs or the rates of fatty acid oxidation are accelerated due to limited rates of keto genesis
• Cross the blood-brain barrier
• Fuels the nervous system to cross the barrier under conditions of starvation
• Synthesis & metabolism of ketone bodies

Nucleotide Biosynthesis

• Synthasizes for other use
• Nucleotides de novo (new)
• Secretes free base from the circulation

Synthesis of Blood Proteins

• Synthesis of blood proteins primarily
• Decreased plasma osmotic pressure in the blood and causes water in the plasma to go to tissues, causing edema

The Synthesis of Glycoproteins and Proteoglycans

• Synthesis of most of the blood proteins
• Need high for the sugars which include mannose, fructose, galactose
• Liver isn't dependent on liver glucose or dietary sugar
• Generate carbs from amino acids (gluconeogenesis)
• Lactate, glycerol, amino acids enter glyconeogensis
• O-linked carbs bound through a gylcosidic linked carbohydrate
• N-linked has an N-glycosyl to asparganine-linked residue
• Amino acid sialic acid
• Synthesis of circ proteins leads
• Cell surface binds proteins which endocytoes the complexes amino acids recycled

Pentose Phosphate Pathway

• Major functions include generating NADPH & five-carbon sugars + red blood cells
• Catalyzes conversion of oxidized glutathione (GSSG) back to reduced glutathione (GSH) can be maintained
• Free-radical injury is gone
• Cells need the pathway to generate ribose
• Has a greater demand for NADPH than other organs
• Uses fatty acids, cholesterol

Fuels Used By NADH, Liver, Mitochondira

• Fatty synthesis needed for phospholipds
• cholesterol
• Liver participates in more reactions
• Uses more glutathione and NADH con of concentration
• Gucosphaphate in regulating pentose phos pathway is high and flux.
• High energy required by high anabolism.
• Principal reaction ATP GTP.

Fuels use by cells

• After eating liver meals = glucose and galactose if ethanol
• Site for fuel is fatty acids
• Urea cycle to eliminate ammonia from amino
• Glucose and fructose flow to the liver and enters portal
• The location fructose liver metabolism

Carbohydrate and glucose metabolism

• Liver enters vein
• Km in glucose transport high enter post high carb increase in insulin leads glycogen
• Glycolysis activated
• The pathway is activity glucose

Lipid Metabolism

• Long chain FA are a fuel after they leave to 3 acyl glycerol and bund albumin
• On mito membrane are synthesized
• Can be chain specify beta oxidation

Beta oxidation Pathway

• Mito membrane transport carnitine
• Acyl transfer chain length in carboxylate can form very short and long chain lengths
• Long chain acid from meat and ingested tissue used beta oxidation

Metabolism Overview

• Peroxisomes (increase liver and in tissue) use cleavage of cholesterol chain nec to bile salts step in ethrachadnoinc pathway
• Catalyst capable of hydrogen perox Peroxisomes absents accumate fat in syrome
• Verc Long chain activation Cial
• Perixomal oxidase generates H Instead Fadh remsame
• Catalyst used bio path
• Octanoal passes though cardil
• Enters cial path

Peroxisome proliferation and activating receptors

• Role to play metabolim, Hypoagents and toxin, stimulate gener transcrition Major are involved fat beta oxia
• Fribrates are class bind pptos
• Binds eliciatate lipo change and levels reduce.
• Apo and cim also block apo particles causes uptake
• Alpha used and ligates increased transcipyion
• Gen lack cannot metabolize compate, if fats occur build in hepa.
• Liver enzymes fat acid

Reye's syndrome:

• vomits w the nervous sys inju Mito dysfun decreased enzyme acitiby, aspires cause In liver show up at autopsy (disrupt mito increased lipids and ren tubules

Xenobitoic as Fatty acid

• Livery use pathway to DETOXIFY Hydro molecules and fat like and contain carbs can metabolit to Benzo and salict present in soda preses. Alikes salct and Ben small or medium Fatty