Heme Metabolism for Medical Students 2023 PDF

By:Abdisa Tufa,PhD Fellow Metabolism of heme Heme synthesis : Porphyria Heme degradations: Jaundice GLOBULAR HEME PROTEINS  Hemoproteins are a specialized group of proteins that contain heme as a Prosthetic group. Role of heme group is dictated by the environment.  Cytochro...

By:Abdisa Tufa,PhD Fellow Metabolism of heme Heme synthesis : Porphyria Heme degradations: Jaundice GLOBULAR HEME PROTEINS  Hemoproteins are a specialized group of proteins that contain heme as a Prosthetic group. Role of heme group is dictated by the environment.  Cytochromes  Catalase  Hemoglobin  Myoglobin  What is Porphyrin ? Porphyrin Pyrrole H H HC CH C C HC CH HC C C CH N N H HC C C CH NH HN HC C C CH N HC C C CH N C C H H Heme Heme = Porphyrin + iron  Synthesis of heme ★ The substrates mainly include succinyl-CoA, glycine, Fe2+. ★ Heme can be synthesized by almost all the tissues in the body which require hemoproteins. ★major sites of synthes is liver and bone marrow (erythroblasts:reticulocyte,prorubricyte) - cytochrome p450 in liver - hemoglobin in bone marrow - heme production equal to globin synthesis in marrow - variable in liver dependent on heme pool balance Inside RBCs, heme is synthesized in the normoblasts, but not in the matured ones.  Inside RBCs, heme is synthesized in the normoblasts, but not in the matured ones. VOMIT P Overview of Heme Synthesis Terminology in heme synthesis Synthesis of δ-aminolevulinic acid: Pyridoxal phosphate induced by: drugs (barbiturates), oral (vit. B6) contraceptive pills ALA Synthase committed step( rate limiting) Regulation occurs through control of gene transcription Heme functions as a feedback inhibitor. ,. Formation of porphobilinogen: inhibited by lead d- dehydratase contains zinc is very sensitive to lead and other heavy metals.  Inhibition by Pb++ results in elevated blood ALA, High ALA is thought to cause some of the neurological effects of lead poisoning, although Pb++ also may directly affect the nervous system. ALA is toxic to the brain, perhaps due to: COO COO Similar ALA & neurotransmitter GABA CH2 CH2 (g-aminobutyric acid) structures. ALA autoxidation generates reactive CH2 CH2 oxygen species (oxygen radicals). C O CH2 CH2 NH3+ NH3+ ALA GABA  COO COO CH2 CH2 CH2 Porphobilinogen (PBG) is the first H2 C N pathway N H intermediate that NH3 + H pyrrole includes a pyrrole Porphobilinogen (PBG) ring. The porphyrin ring is formed by condensation of 4 molecules of porphobilinogen. Porphobilinogen Deaminase catalyzes successive PBG condensations, initiated in each case by elimination of the amino group.  COO - CH 2 COO - Porphobilinogen CH 2 CH 2 Deaminase is organized in 3 - CH 2 COO - domains. OOC CH 2 CH 2 NH HN Predicted HO interdomain NH HN flexibility may CH 2 COO - accommodate CH 2 the growing CH 2 polypyrrole in CH 2 CH 2 the active site COO -COO - CH 2 cleft. hydroxymethylbilane COO - Hydrolysis of the link to the enzyme's dipyrromethane releases the tetrapyrrole hydroxymethylbilane. Recapitulation of Heme synthesis Mitochondri Cytosol Mitochondri a a  PORPHYRIA  Porphyria is a name given to a group of metabolic disorders. These disorders cause the individual to accumulate "porphyrins" or "porphyrin precursors" in their body. which in turn causes an abundance of the porphyrins.  In porphyria, the cells do not convert porphyrins to heme in a normal manner.  Porphyrias cont’d Porphyrias are also a genetic diseases in which activity of one of the enzymes involved in heme synthesis is decreased (e.g., PBG Synthase, Porphobilinogen Deaminase, etc…). Symptoms vary depending on ★ the enzyme ★ the severity of the deficiency ★ whether heme synthesis is affected primarily in liver or in developing erythrocytes.  common symptom of Porphyrias 1. Occasional episodes of severe neurological symptoms ◆had acute bouts of abdominal pain and mental confusion E.g. Elevated d-ALA, arising from derepression of ALA Synthase gene transcription, neurological symptoms. 2. Photosensitivity is another common symptom. ◆formation of superoxide radicals. ◆ Skin damage may result from exposure to light. This is attributable to elevated levels of light- absorbing pathway intermediates and their degradation products.  3. porphyrins build up in the body and are excreted in the urine and stool in excessive amounts. Urine=wine color if elevated. normal  Types of Porphyrias can be grouped into erythropoietic porphyria and hepatic porphyria - hepatic can be acute or chronic  Acute hepatic porphyrias Each acute hepatic porphyria is a result of a deficiency of one of the enzymes in the heme biosynthesis pathway. - similar symptoms - acute attacks of gastrointestinal pain, neurologic / psychologic, cardiovascular.  erythropoietic porphyrias - congenital erythropoietic porphyria (uroporphyrinogen III synthase) - erythropoietic protoporphyria (ferrochelatase) symptoms include: - skin rashes and blisters early in childhood - cholestatic liver cirrhosis and progressive liver failure  porphyria cutanea tarda - a chronic porphyria - liver and erythroid tissues - deficiencey in uroporphyrinogen decarboxylase - often no symptoms until 4th or 5th decade clinical expression determined by many factors: - hepatic iron overload - exposure to sunlight - hepatitis B or C - HIV symptoms include: - cutaneous rashes, blisters - urine that is red to brown in natural light, or pink to red in UV light Causes of Porphyria: caused by hereditary or acquired defects in heme synthesis — genetic diseases: the enzymes of heme synthesis — Liver desfunction, lead posioning Causes of Porphyrias Cont'd  Acquired Porphyrias - hexochlorobenzene used as a fungicide in Turkey in 1950s - thousands of children ate bread from treated wheat - they acquired porphyria cutanea tarda due to inhibition of uroporphyrinogen decarboxylase - due to hypertrichosis - referred to locally as the “monkey children”  Acquired Porphyrias lead poisoning -inhibition of ferrochelatase ,ALA dehydratase - displaces Zn+2 at enzyme active site children - developmental defects - drop in IQ - hyperactivity - insomnia - many other health problems adults - severe abdominal pain - mental confusion - many other symptoms Derangements in porphyrin metabolism: Disease state Genetics Tissue Organ pathology Acute intermittent dominant Liver Nervous system porphyria Hereditary dominant Liver Nervous system, skin coproporphyria Variegate porphyria dominant Liver Nervous system, skin Porphyria cutanea tarda dominant Liver Skin, induced by liver dis. Erythropoietic dominant Marrow Gall stones, liver dis., skin protoporphyria Congenital erythropoietic recessive Marrow Skin, RES porphyria Lead poisoning All tissues Nervous system, blood, others Recapitulation of Porphyria  The porphyrias Type Enzyme Major Laboratory tests Involved Symptoms Acute intermittent Uroporphyrinogen Abdominal pain urinary porphobilinogen  porphyria synthase Neuropsychiatric Congenital Uroporphyrinogen Photosensitivity urinary uroporphyrin  erythropoietic cosynthase porphyria porphobilinogen  Porphyria cutanea Decarboxylase Photosensitivity urinary uroporphyrin  tarda porphobilinogen  Variegate porphyria Oxidase Photosensitivity urinary uroporphyrin  Abdominal pain Neuropsychiatric fecal coproporphyrin  fecal protoporphyrin  Erythropoietic Ferrochelatase Photosensitivity fecal protoporphyrin  protoporphyria red cell protoporphyrin  56 Heme Degradation Jaundice (Icterus) 58  Conversion of heme to bilirubin: ER enzyme system Cytoprotective role: CO biliverdin the major source is Hg  Formation of bilirubin diglucuronide: increase the water solubility of bilirubin  BLOOD Stercobilin CELLS Urobilin excreted in feces Hemoglobin excreted in urine Globin Urobilinogen Heme O2 formed by bacteria KIDNEY reabsorbed Heme oxygenase INTESTINE into blood CO Biliverdin IX via bile duct to intestines NADPH Biliverdin Bilirubin diglucuronide reductase (water-soluble) NADP+ 2 UDP-glucuronic acid Bilirubin Bilirubin (water-insoluble) LIVER (water-insoluble) via blood unconjugated to the liver Catabolism of hemoglobin  Transport of Bilirubin in Plasma Bilirubin on release from macrophages circulates as unconjugated bilirubin in plasma tightly bound to albumin. Albumin + free Bilirubn Bilirubin ~ Albumin Complex Why bound to albumin? unconjugated bilirubin Significance: ★Increase the solubility of whole molecule ★ Prevent unconjugated bilirubin freely come into other tissue, cause damage.  blood membrance cytosol [bilirubin-albumin] [bilirubin-lipids] Other antibiotics [albumin]↑ [bilirubin] [bilirubin] Certain drugs as sulfonamides and salicylates compete with bilirubin for albumin binding and displace bilirubin to enter into the brain in neonates and increase the risk of kernicterus HEPATIC PHASE On coming in contact with the hepatocyte surface, unconjugated bilirubin is preferentially metabolized which involved 3 steps: Hepatic uptake Conjugation Secretion in bile  CONJUGATION OF BILIRUBIN UDPGT Bilirubin Bilirubin +UDP-gluconic acid monoglucuronides + UDP UDPGT UDP-gluconic acid Bilirubin diglucuronide + UDP Conjugation occur in endoplasmic reticulum UDP- glucuronosyl transferase, Product: mono or diglucuronides. The process of conjugation can be induced by drugs phenobarbital.  HOOC COOH CH2 CH2 M V M CH2 CH2 M MV O CH CH CH O N N N N H H 2 H H COOH OH O O O bilirubin diglucuronide OH OHHOOC C CO HO CH O2 CH2 O OH conjugated bilirubin OH M V M CH2 CH M M V 2 O CH CH2 CH O N N N N H H H H ★Increase the solubility of whole molecule ★ delete the toxicity of bilirubin. Unconjugated bilirubin: Bilirubin that are not conjugated with gluconic acid , also called hemobilirubin, indirected bilirubin. conjugated bilirubin: Bilirubin that are conjugated with gluconic acid, also called hepatic bilirubin, directed bilirubin. MAJOR DIFFERENCES BETWEEN UNCONJUGATED AND CONJUGATED BILIRUBIN FEATURE Unconjugated CONJUGATED bilirubin BILIRUBIN Normal serum level More Less (less than 0.25mg/dl) Water solubility Absent Present Affinity to lipids (alcohol Present Absent solubilty) Serum albumin binding High Low Van den Bergh reaction Indirect (Total minus Direct direct) Renal excretion Absent Present Affinity to brain tissue Present (kernicterus) Absent HYPERBILIRUBINEMIA  Elevated bilirubin levels in the blood (>10 mg/l); bilirubin may diffuse into peripheral tissues, giving them a yellow color (jaundice)  Cause: 1. Pre-hepatic: excessive formation of bilirubin by increased degradation of erythrocytes (icterus neonatus, hemolytic anemia) 2. Hepatic: insufficient processing of bilirubin as a result of liver defects (hepatitis, liver toxic damage, cirrhosis, hepatic failure) 3. Post-hepatic: by impaired excretion of gall (obstructive jaundice due to gallstones, inflammation of biliary tract)  Unconjugated bilirubin can cross the blood-brain barrier, leading to brain damage  Jaundice in neonates (increased bilirubin degradation+immaturity of the conjugation enzymes): phototerapy – isomerization of bilirubin to more soluble pigments  Genetic Disorders of Bilirubin Metabolism Clinical Condition Defect Bilirubin Findings Crigler-Najjar Severely defective Unconjugated Profound jaundice syndrome UDP-glucuronyltransferase bilirubin  Gilberts Reduced activity of Unconjugated Very mild jaundice syndrome UDP-glucuronyltransferase bilirubin  during illnesses Dubin-Johnson Abnormal transport of Conjugated Moderate jaundice syndrome conjugated bilirubin into bilirubin  the biliary system 70 JAUNDICE 71  TYPES HAEMOLYSIS A PREHEPATIC HEPATIC POSTHEPATIC OBSTRUCTIVE OR SURGICAL  CAUSES OF HEMOLYTIC JAUNDICE  Malaria  Side effects of certain drugs :antibiotic and anti- tuberculosis medicines, levodopa,  Certain drugs in combination with a hereditary enzyme deficiency known as glucose-6-phosphate dehydrogenase (G6PD)  Poisons Snake and spider venom, certain bacterial toxins, copper, and some organic industrial chemicals directly attack the membranes of red blood cells  Immune reactions to RBCs cancer  Transfusions  Kidney failure and other serious diseases  Erythroblastosis fetalis  HEPATOCELLULAR JAUNDICE Damage to liver cells( for example in patient with cirrhosis or hepatitis) causes a decrease in both bilirubin uptake and production of conjuagted bilirubin. Unconjugated bilirubin occur in the blood and increased urobilinogen in the urine. The urine is dark in color and stool are pale, clay color. Plasma level of AST and ALT are elevated and the patient experience nausea and anorexia.  OBSTRUCTIVE JAUNDICE In this instance jaundice is results from obstruction of the bile duct. For example, the presence of a hepatic tumor or bile stone may block the bile ducts, preventing passage of bilirubin into the intestine, patients with obstructive jaundice experience GI pain, nausea and produce stools that are a pale, clay color. Type Cause Example Frequence Prehepatic Hemolysis Autoimmune Rare Haemoglobinopathy According to the region Hepatic Infection Hepatitis A,B,C Very common Damage Alcohol, drugs Common Genetics Gilbert´s syndrome 1 in 20 Wilson´s disease 1 in 200 000 α1-Antitrypsin deficiency 1 in 1000 Autoimmune Chronic hepatitis Rare Newborn Physiologic Very common Posthepatic Intrahepatic Drugs Common bile ducts Primary biliary cirrhosis Rare Cholangitis Common Extrahepatic Gallstones Very common bile ducts Pancreatic cancer Rare  Bilirubin Urobilinogen blood urine deriv. in feces blood urine Prehepatic ↑↑(UC) N ↑ ↑ ↑ Intrahepatic ↑↑(both) ↑ N ↑↑ ↑↑ Posthepatic ↑↑(C) ↑ ↓ ↓ ↓ Summary: Synthesis of heme:  from glycine and succinate  induction by drugs and ↓ glucose; inhibition by lead  intermediates → porphyrinogens (porphyrins)  porphyrias Degradation of heme  to bilirubin (hydrophobic)  conjugation in liver  conversion to urobilinogen in intestine  hyperbilirubinemia (differential diagnosis)