Slide1: Heme & Porphyrin Metabolism


Slide2: heme Non-erythroid Myoglobin Cytochromes Many others Electron transfer and energy trapping Erythroid


Slide3: Porphyrins Cyclic compounds that bind metals usually iron Most common porphyrin in humans is heme - one ferrous (Fe+2 ) goup in tetrapyrole ring - prosthetic group for: Hemoglobin Myoglobin Cytochromes (CYP450 & Electron Transport chain) Catalase Tryptophan pyrrolase NOS (Nitric oxide synthase) Heme proteins are rapidly synthesized and degraded - 6 to 7 g per day hemoglobin turned over


Slide4: Structure of Porphyrins - cyclic, with 4 pyrrole rings attached by methenyl bridges side chains may vary - Uroporphyrin has Acetate and Propionate chains - Coproporphyrin has Methyl and Propionate chains order of chains define subgroups of porphyrins: only Type III porphyrins are normally important for humans


Slide5: Porphyrinogens - porphyrin precursors - chemically reduced - colorless - intermediate in heme synthesis Biosynthesis of Heme Major sites of synthesis are liver (CYP 450) which is variable dependent on heme pool balance And bone marrow erythroblasts (Hb) where is heme production equal to globin synthesis in marrow under the control of Erythropoietin


Slide6: Biosynthesis of Heme  -ALA synthesis and last 3 reactions take place in mitochondria - all others in cytoplasm - red blood cells have no mitochondria so can’t make heme. 1. Formation of -aminolevulinic acid - all carbons from glycine and succinyl CoA - catalyzed by ALA synthase - requires pyridoxal phosphate - rate controlling step hemin formed from oxidation of heme not taken up by protein & hemin inhibits ALA synthase synthesis


Slide7: Biosynthesis of Heme 2. Formation of porphobilinogen PBG by dehydration of 2 ALA molecules - by -aminolevulinic acid dehydrase - cytoplasmic - very sensitive to lead and other heavy metals accounts, in part, for lead poisoning - increased ALA, anemia


Slide8: Biosynthesis of Heme 3. Formation uroporphyrinogen III (UPG) by condensation of 4 porphobilinogens Two cytoplasmic enzymes share UPG III synthesis: hydroxymethylbilane synthase uroporphyrinogen III synthase 4. Formation of protoporphyrin IX by decarboxylation of side chains (A , P or M) followed by oxidation 5. Heme formation by incorporation of iron (Fe+2) - partly spontaneous (non – enzymatically) - ferrochelatase enhances rate - also inhibited by lead


Extravascular Pathway for RBC Destruction: Extravascular Pathway for RBC Destruction (Liver, Bone marrow, & Spleen) Hemoglobin Globin Amino acids Amino acid pool Heme Bilirubin Fe2+ Excreted Phagocytosis & Lysis Recycled


Slide10: DEGRADATION OF HEME TO BILIRUBIN P450 cytochrome  75% is derived from RBCs In normal adults this results in a daily load of 250-300 mg of bilirubin Normal plasma concentrations are less then 1 mg/dL Hydrophobic – transported by albumin to the liver for further metabolism prior to its excretion “unconjugated” bilirubin


NORMAL BILIRUBIN METABOLISM: NORMAL BILIRUBIN METABOLISM Uptake of bilirubin by the liver is mediated by a carrier protein (receptor) Uptake may be competitively inhibited by other organic anions On the smooth ER, bilirubin is conjugated with glucoronic acid, xylose, or ribose Glucoronic acid is the major conjugate - catalyzed by UDP glucuronyl tranferase “Conjugated” bilirubin is water soluble and is secreted by the hepatocytes into the biliary canaliculi Converted to stercobilinogen (urobilinogen) (colorless) by bacteria in the gut Oxidized to stercobilin which is colored Excreted in feces Some stercobilin may be re-adsorbed by the gut and re-excreted by either the liver or kidney


Slide12: Handling of Free (Intravascular) Hemoglobin Purposes: 1. Scavenge iron 2. Prevent major iron losses 3. Complex free heme (very toxic) Haptoglobin: hemoglobin-haptoglobin complex is readily metabolized in the liver and spleen forming an iron-globin complex and bilirubin. Prevents loss of iron in urine. Hemopexin: binds free heme. The heme-hemopexin complex is taken up by the liver and the iron is stored bound to ferritin. Methemalbumin: complex of oxidized heme and albumin.


HYPERBILIRUBINEMIA: HYPERBILIRUBINEMIA Increased plasma concentrations of bilirubin (> 3 mg/dL) occurs when there is an imbalance between its production and excretion Recognized clinically as jaundice


Slide14: Prehepatic (hemolytic) jaundice Results from excess production of bilirubin (beyond the livers ability to conjugate it) following hemolysis Excess RBC lysis is commonly the result of autoimmune disease; hemolytic disease of the newborn (Rh- or ABO- incompatibility); structurally abnormal RBCs (Sickle cell disease); or breakdown of extravasated blood High plasma concentrations of unconjugated bilirubin (normal concentration ~0.75 mg/dL)


Slide15: Intrahepatic jaundice Impaired uptake, conjugation, or secretion of bilirubin Reflects a generalized liver (hepatocyte) dysfunction In this case, hyperbilirubinemia is usually accompanied by other abnormalities in biochemical markers of liver function


Slide16: Posthepatic jaundice Caused by an obstruction of the biliary tree Plasma bilirubin is conjugated, and other biliary metabolites, such as bile acids accumulate in the plasma Characterized by pale colored stools (absence of fecal bilirubin or urobilin), and dark urine (increased conjugated bilirubin) In a complete obstruction, urobilin is absent from the urine


Diagnoses of Jaundice : Diagnoses of Jaundice


Neonatal Jaundice : Neonatal Jaundice Common, particularly in premature infants Transient (resolves in the first 10 – 14 days) Due to immaturity of the enzymes involved in bilirubin conjugation High levels of unconjugated bilirubin are toxic to the newborn – due to its hydrophobicity it can cross the blood-brain barrier and cause a type of mental retardation known as kernicterus If bilirubin levels are judged to be too high, then phototherapy with UV light is used to convert it to a water soluble, non-toxic form If necessary, exchange blood transfusion is used to remove excess bilirubin Phenobarbital is oftentimes administered to pregnant prior to an induced labor of a premature infant – crosses the placenta and induces the synthesis of UDP glucuronyl transferase Jaundice within the first 24 hrs of life or which takes longer then 14 days to resolve is usually pathological and needs to be further investigated.


Causes of Hyperbilirubinemia: Causes of Hyperbilirubinemia


Slide20: Benign liver disorder  Characterized by mild, fluctuating increases in unconjugated bilirubin caused by decreased ability of the liver to uptake bilirubin – often correlated with fasting.  Males more frequently affected then females Onset of symptoms in teens, early 20’s . Can be treated with small doses of phenobarbital to stimulate UDP glucuronyl transferase activity Gilbert’s Syndrome


Slide21: Autosomal recessive Extremely rare < 200 cases worldwide – gene frequency is < 1:1000 Characterized by a complete absence (type I) or marked reduction (type II) in bilirubin conjugation Present with a severe unconjugated hyperbilirubinemia that usually presents at birth Affected individuals are at a high risk for kernicterus Condition is fatal when the enzyme is completely absent Treated by phototherapy (10-12 hrs/day) and liver transplant by age 5 Crigler-Najjar Syndrome


Slide22:  Characterized by impaired biliary secretion (drainage) of conjugated bilirubin  Present with a conjugated hyperbilirubinemia that is usually mild Dubin-Johnson and Rotor’s Syndromes


Slide23: ALA PBG Uroporphyrinogen C7-porphyrinogen C6 -porphyrinogen C5 -porphyrinogen Coproporphyrinogen Protoporphyrinogen Protoporphyrin Haem Porphyrins are the obligate intermediates on the haem synthetic pathway Each step is catalysed by an enzyme Where an enzyme is defective, porphyrins accumulate, and a porphyria results.


Slide27: 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


Slide29: Acute hepatic porphyrias - acute intermittent porphyria (hydroxymethylbilane synthase) - hereditary coproporphyria (coprophyrinogen oxidase) - variagate porphyria (protoporphyrinogen oxidase) similar symptoms - acute attacks of gastrointestinal pain, neurologic/psychologic, cardiovascular - often precipitated by drugs, particularly barbiturates, infection, starvation, alcohol - activators of p450 system - uses up heme, increases ALA synthase, increases pathologic metabolites


Slide30: 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 Treatment for Porphyrias - medical support for vomiting and pain - hemin, decreases ALA synthase synthesis - avoidance of sunlight and precipitating drugs, factors