Alcohol Metabolism Flashcards
1
Q
Ethanol Overview
A
- ethanol that we consume is converted to acetate primarily in the liver with the generation of NADH
- first the oxidation of ethanol to acetaldehyde, which is toxic, and then the oxidation of acetaldehyde to acetate
- the first step is catalyzed by alcohol dehydrogenase (ADH) in the cytosol and the second by acetaldehyde dehydrogenase (ALDH) in the mitochondria
- most of the acetate enters the blood and travels to muscle and other tissues where it is converted to acetyl CoA by acetyl CoA synthetase and is oxidizedd by the Krebs cycle
- 10-20% of ingested alcohol is oxidized through a liver microsomal alcohol oxidizing system (MEOS) comprised of cyt P450 enzymes (primarily CYP2E1)
2
Q
First steps of ethanol metabolism
A
- ethanol is a small molecule which has solubility in water and lipids. It is absorbed from the intestine by passive diffusion. A small % (0-5) enters gastric mucosal cells in the upper GI tract where it is metaboliszed
- the remainder enters the blood where 85-98% is metabolized in the liver and 2-10% is excreted through the lungs or kidneys
- the primary route of ethanol metabolism in the liver is through ADH, in the cytosol converting EtOH to acetaldehyde with production of NADH
- acetaldehyde which is not further metabolized can damage the liver and can also enter the blood exerting toxic effects on other tissues
- 90% of the acetaldehyde formed is further metaboized by low Km ALDH which converts acetaldehyde to acetate with production of NADH
- acetate is nontoxic and can enter the Krebs cycle or the pathway for fatty acid synthesis once converted to acetyl CoA in the liver
- most acetate enters the blood and is converted to acetyl CoA in muscle and other tissues
3
Q
MEOS
A
- microsomal alcohol oxidizing system
- the other route for alcohol metabolism in the liver, and located in the ER
- the primary cyt P450 mixed function oxidase isozyme involved is CYP2E1 which converts ethanol to acetaldehyde using NADPH as an additional electron donor and O2 as an electron acceptor
- the route is 10-20% of ethanol oxidation in a moderate drinker
4
Q
ADH Isozymes
A
- family of ADH Isozymes that differ in their specificity of chain length of the alcohol substrate
- at high concentrations ethanol can be metabolized by many members of the ADH family
- ADH have highest affinity (lowest Km) for ethanol are the ADH1- can form homodimers and heterodomers with each other but not with ADH2,3, or 4
- ADH1 family present in very high quantity in the liver (3% of soluble protein) and are referred to as liver ADH
- because of the large quality of liver ADH, and high affinity binding of ethanol, the liver is the major site of ethanol metabolism
- ADH4- upper GI tract. At high ethanol concentration in the upper GI tract (beer is 0.8 M) the conversion of ethanol to acetaldehyde by ADH4 (gastric ADH) may contribute to the risk of cancer for heavy drinkers
- ADH2- liver and lower GI
- ADH3- present in many tissues, in inactive toward ethanol, but active toward long chain alcohols
5
Q
Acetaldehyde Dehydrogenase
A
- acetaldehyde dehydrogenases catalyze the oxidation of acetaldehyde to acetate with generation of NADH
- > 80% of acetaldehyde oxidation in our liver is catalyzed by the mitochondria isozyme (ALDH2). ALDH2 has high affinity for acetaldehyde (Km= 0.2 uM)
- most of the remainder of acetaldehyde oxidation is carried out by a cytoplasmic isozyme (ALDH1)
- both ALDH2 (mitochondrial) and ALDH1 (cytoplasmic) isozyme are tetramers with individual subunits 500 aa
6
Q
Inactive ALDH2
A
- accumulation of acetaldehyde
- flushing, nausea, and vomiting and a distaste for alcoholic beverages. A single amino acid substitution (Glu to Lys) leads to an allelic variant designated ALDH2*2 which has 23 fold higher Km and 35 fold lower Vmax.
- homozygosity for this allele provides absolute protection against alcoholism
- alcoholics are frequently treated with ALDH inhibitors (disulfiram) which help them abstain. However, if they keep on drinking while on the drug the elevated acetaldehyde will damage the liver
7
Q
Acetyl CoA Synthase
A
- the metabolism of the acetate to acetyl CoA requires acetyl CoA synthetase (ACS)
- in liver the primary isoform is ACS I which is a cytosolic enzyme which generates acetyl CoA for the pathways of cholesterol and fatty acid synthesis
- acetate entry into these cytosolic pathways is regulated by cholesterol and insulin and, as a result, most of the acetate enters the blood
- acetate is taken up by other tissues( heart and skeletal muscle) which have a high concentration of the mitochondrial acetyl CoA synthetase )ACS II
- the acetyl CoA that is generated by ACS II can enter the TCA cycle and be oxidized to CO2
8
Q
Cytochrome P450
A
- ethanol is also oxidized in the liver by MEOS comprised of ER cytochrome P450 enzymes
- ethanol and NADPH both donate electrons in the reaction which reduces O2 to 2H2O
- the cytochrome P450 superfamily have 2 major catalytic components:
- cytochrome P450 reductase which transfers electrons (via FAD and FMN) from NADPH
- cytochrome P450 which contains the binding sites for O2 and substrate (ex. ethanol) and carries out the reaction
9
Q
CYP2E1
A
- one of over 100 cytochrome P450 mixed function oxidases
- this is the cytochrome P450 with the highest activity when ethanol is substrate, but other P450s are also involved. MEOS refers to the combined ethanol oxidizing activity of all P4502
- has a higher Km for ethanol than ADH1 and therefore becomes more involved in ethanol xidation when large quantities of ethanol are consumed
- the products of alcohol oxidation, but CYP2E1, include acetaldehyde and ROS resluting in oxidative stress and cellular damage
- chronic consumption of alcohol induces expression of CYP2E1 5-10 fold. Ethanol induction of CYP2E1 appears to act via stabilization of the protein and protection against degradation
- CYP2E1 induction increases ethanol clearance from the blood but produces acetaldehyde faster than it can be metabolized by ALDH resulting in damage to the liver and other tissues
10
Q
Variations in Ethanol Metabolism
A
- differences can determine whether an individual will become an alcoholic, develop alcohol induced liver disease or other diseases linked to alcohol consumption. The factors that determine individual differences are:
- genotypes: different polymorphic forms of ADHs and ALDHs can greatly effect ethanol oxidation and acetaldehyde accumulation. Different allelic variants of CYP2E1 can vary its activitity greater than 20 fold
- drinking history: the lelvel of gastric ADH decreases and CYP2E1 increases with progression from naive, to moderate, to heavy and chronic consumption of alcohol
- gender: blood levels of ethanol, after consuming a drink are higher in women than men because of lower gastric ADH activity, size and a 12% smaller water space
- quantity of ethanol an individual consumes- the higher the quantity the greater the involvement of CYP2E1 and the higher the levels of acetaldehyde and ROS
11
Q
Acute Effects on Lipid Metabolism in the Liver
A
- acute effects on liver metabolism include inhibition of fatty acid oxidation and stimulation of TAG synthesis leading to a fatty liver; and ketoacidosis or lactic acidosis causing hypo or hyperglycemia depending on the dietary state. These effects are considered reversible
- in contrast acetaldehyde and ROS generated from ethanol metabolism can result in alcohol induced hepatitis characterized by liver inflammation and necrotic cell death; or damage to hepatocytes leading to cirrhosis characterized by fibrosis (scarring), altered blood flow, and loss of liver function
- the acute effects of ethanol consumption on lipid metabolism are the result of increased NADH/NAD+ ratio
12
Q
Mechanisms of Acute Effects on Lipid Metabolism in the Liver
A
- ethanol oxidation by ADH and ALDH increaes NADH/NAD+
- high NADH/NAD+ ratio inhibits fatty acid oxidation and the TCA cycle leading to accumulation free fatty acids
- fatty acids are reesterified to G3P by acyltransferases in the ER. High NADH/NAD+ increases the levels of G3P and alcohol causes induction of the liver acyltransferases
- increased TAGs are converted to VLDL which accumulate in the liver (and enter the blood) leading to fatty liver (hepatic steatosis) and ethanol induced lipidemia
- fatty acids that are oxidized are converted to acetyl CoA and then to ketone bodies (acetoacetate and B-hydroxy-butyrate). High NADH/NAD+ inhibits the TCA cycle
- inhibition of the TCA cycle causes acetyl coA to enter the ketone body pathway raising their levels (ketoacidosis)
- very hight NADH/NAD+ is that there is increased production of lactate by lactate dehydrogenases- lactic acidosis
- increase of blood lactate decreases uric acid excretion by the kidney. Patients with gout therefore are advised not to drink excessively
- increased NADH/NAD+ shifts the lactate dehydrogenase equilibrium toward production of lactate at the expense of pyruvate. Therefore pyruvate formed from alanine is converted to lactate and cannot enter the gluconeogenesis. As a result drinking alcohol in the fasting state can cause hypoglycemia
- drinking alcohol with a meal can cause a transient hyperglycemia because the high NADH/NAD+ inhibits glycolysis at the G3P dehydrogenase step
13
Q
Effects of Chronic Alcohol Consumption
A
- chronic alcohol consumption leads to increased acetaldehyde and ROS in the liver and released into the blood
- the acetaldehyde form adducts with amino and sulfhydryl groups within amino acids and proteins and with nucleotides and phospholipids
- adduct formation with amino acids causes a decrease in protein synthesis
- this accumulation of proteins causes water to enter into hepatocytes with resulting swelling of the liver in portal hypertension and disruption of hepatic architecture. Cell damage leads to release of hepatic enzymes such as alanine aminotransferase (ALT) and aspartate amino transferase (AST)
- acetaldehyde forms an adduct with GSH and various antioxidant enzymes so that they cannot protect against the ROS which damage proteins, lipids and nucleotides
- induction of MEOS increases ROS production, lipid peroxidation and cell damage
- peroxidation of lipids in the mitochondrial inner membrane and oxidative damage to proteins inhibits electron transport and diminishes acetaldehyde conversion to acetate
- adduct formation with proteins can cause loss of function. Adduct formation with tubulin causes decreased secretion of plasma proteins and VLDL from liver. Many proteins that are normally secreted into the blood accumulate in the liver
14
Q
Fibrosis
A
- an insult to the liver (such as chronic alcohol consumption) causes a “wound-healing-like-reaction”
- this causes an overproduction of extracellular matrix which progresses to sclerosis which involves degeneration of the components of the extracellular matrix
- about 20% of heavy drinkers progress from fibrosis to Cirrhosis
- Fibrosis to sclerosis to cirrhosis
15
Q
Hepatic Cirrhosis
A
- liver damage is irreversible in this stage
- initially liver is enlarged, full of fat and crossed with collagen fibers (fibrosis) and have nodules of ballooning hepatocytes between the fibers
- as liver function is lost the liver becomes shrunken (Laennec Cirrhosis)
- loss of metabolic functions include protein synthesis and secretion, detoxification pathways, the ability to incorportate amino groups into urea leading to toxic levels of ammonia, and diminished conjugation and secretion of bilirubin occurs with increased accumulation of bilirubin in the blood
- the increased blood bilirubin is deposited in many tissues including skin and the sclera of the eyes. This causes the yellow color of the patient (jaundice)
