Porphyrins & Bile Pigments• Biomedical Importance

These topics are closely related, because heme is synthesizedfrom porphyrins and iron, and the products of degradation ofheme are the bile pigments and iron.Knowledge of the biochemistry of the porphyrins and of hemeis basic to understanding the varied functions of hemoproteinsin the body.The porphyrias are a group of diseases caused byabnormalities in the pathway of biosynthesis of the variousporphyrins. A much more prevalent clinical condition isjaundice, due to elevation of bilirubin in the plasma, due tooverproduction of bilirubin or to failure of its excretion and isseen in numerous diseases ranging from hemolytic anemias toviral hepatitis and to cancer of the pancreas.

Bio. 2. ASPU. Lectu.6. Prof. Dr. F. ALQuobaili

• Metalloporphyrins & Hemoproteins Are Important in NaturePorphyrins are cyclic compounds formed by the linkage of four pyrrole rings through methyne (==HC—) bridges. 

A characteristic property of the porphyrins is the formation ofcomplexes with metal ions bound to the nitrogen atom of thepyrrole rings. Examples are the iron porphyrins such as hemeof hemoglobin and the magnesium‐containing porphyrinchlorophyll, the photosynthetic pigment of plants.

• Natural Porphyrins Have Substituent Side Chains on the Porphin NucleusThe porphyrins found in nature are compounds in which variousside chains are substituted for the eight hydrogen atomsnumbered in the porphyrin nucleus. As a simple means ofshowing these substitutions, Fischer proposed a shorthandformula in which the methyne bridges are omitted and aporphyrin with this type of asymmetric substitution is classifiedas a type III porphyrin.

With the eight substituent positions numbered as indicated. Various porphyrins are represented these two figures.

Heme Is Synthesized from Succinyl-CoA & Glycine

Uroporphyrinogen III is converted to coproporphyrinogen III bydecarboxylation of all of the acetate (A) groups, which changesthem to methyl (M) substituents. The reaction is catalyzed byuroporphyrinogen decarboxylase, which is also capable ofconverting uroporphyrinogen I to coproporphyrinogen I.

• Formation of Heme Involves Incorporation of Iron into ProtoporphyrinThe final step in heme synthesis involves the incorporation offerrous iron into protoporphyrin in a reaction catalyzed byferrochelatase (heme synthase), another mitochondrialenzyme.

Coproporphyrinogen III then enters the mitochondria, whereit is converted to protoporphyrinogen III and then toprotoporphyrin III.

• ALA Synthase Is the Key Regulatory Enzyme inHepatic Biosynthesis of Heme

ALA synthase occurs in both hepatic (ALAS1) anderythroid (ALAS2) forms. The rate‐limiting reaction inthe synthesis of heme in liver is that catalyzed by ALAS1,a regulatory enzyme.The rate of synthesis of ALAS1 increases greatly in theabsence of heme and is diminished in its presence. Theturnover rate of ALAS1 in rat liver is normally rapid (half‐life about 1 h), a common feature of an enzymecatalyzing a rate‐limiting reaction.Heme also affects translation of the enzyme and itstransfer from the cytosol to the mitochondrion.

• Porphyrins Are Colored & Fluoresce

The various porphyrinogens are colorless, whereas thevarious porphyrins are all colored.

• Spectrophotometry Is Used to Test forPorphyrins & Their Precursors

Coproporphyrins and uroporphyrins when present inurine or feces (in the porphyrias), can be separatedfrom each other by extraction with appropriatesolvent mixtures. They can then be identified andquantified using spectrophotometric methods.

• ALA and PBG can also be measured in urine byappropriate colorimetric tests.

• The Porphyrias Are Genetic Disorders of HemeMetabolism

The porphyrias are a group of disorders due to abnormalitiesin the pathway of biosynthesis of heme; they can be geneticor acquired.

‐ Biochemistry Underlies the Causes, Diagnoses, &Treatments of the PorphyriasIn general, the porphyrias described are inherited in anautosomal dominant manner, with the exception ofcongenital erythropoietic porphyria, which is inherited in arecessive mode.Clinically, patients complain of abdominal pain andneuropsychiatric and photosensitivity symptoms.

The porphyrias can be classified on the basis of the organsor cells that are most affected, either erythropoietic orhepatic.Porphyrias can also be classified as acute or cutaneous onthe basis of their clinical features.The diagnosis of a specific type of porphyria can generallybe established by consideration of the clinical and familyhistory, the physical examination, and appropriatelaboratory tests.High levels of lead can affect heme metabolism bycombining with SH groups in enzymes such asferrochelatase and ALA dehydratase.It is hoped that treatment of the porphyrias at the genelevel will become possible.

• Catabolism of Heme Produces Bilirubin

When hemoglobin is destroyed in the body,globin is degraded to its constituent aminoacids, which are reused, and the iron of hemeenters the iron pool, also for reuse.The iron‐free porphyrin portion of heme is alsodegraded,mainly in the reticuloendothelial cellsof the liver, spleen, and bone marrow.

• The Liver Takes Up Bilirubin

Bilirubin is only sparingly soluble in water, but itssolubility in plasma is increased by noncovalentbinding to albumin.In the liver, the bilirubin is removed from albuminand taken up at the sinusoidal surface of thehepatocytes by a carrier‐mediated saturable system.This facilitated transport system has a very largecapacity.Once bilirubin enters the hepatocytes, it can bind tocertain cytosolic proteins, which help to keep itsolubilized prior to conjugation.

• Conjugation of Bilirubin with Glucuronic Acid Occurs in the Liver

This process is called conjugation and can employ polarmolecules other than glucuronic acid (eg, sulfate).The conjugation of bilirubin is catalyzed by a specificglucuronosyltransferase. The enzyme is mainly located inthe endoplasmic reticulum, uses UDP‐glucuronic acid asthe glucuronosyl donor, and is referred to as bilirubin‐UGT.Bilirubin monoglucuronide is an intermediate and issubsequently converted to the diglucuronide. Most of thebilirubin excreted in the bile of mammals is in the form ofbilirubin diglucuronide.

• Bilirubin Is Secreted into Bile

• Conjugated Bilirubin Is Reduced to Urobilinogen by Intestinal Bacteria

As the conjugated bilirubin reaches the terminal ileumand the large intestine, the glucuronides are removed byspecific bacterial enzymes (‐glucuronidases), and thepigment is subsequently reduced by the fecal flora to agroup of colorless tetrapyrrolic compounds calledurobilinogens. In the terminal ileum and large intestine, asmall fraction of the urobilinogens is reabsorbed andreexcreted through the liver to constitute theenterohepatic urobilinogen cycle.

• Hyperbilirubinemia Causes Jaundice• Elevated Amounts of Unconjugated Bilirubin in Blood

Occur in a Number of Conditions‐ Hemolytic Anemias‐ Neonatal "Physiologic Jaundice"‐ Crigler–Najjar Syndrome, Type I; Congenital Nonhemolytic

Jaundice (serum bilirubin usually exceeds 20 mg/dL) due tomutations in the gene encoding bilirubin‐UGT activity inhepatic tissues.

‐ Crigler–Najjar Syndrome, Type II. Also results frommutations in the gene encoding bilirubin‐UGT, but someactivity of the enzyme is retained and the condition has amore benign course than type I. Serum bilirubinconcentrations usually do not exceed 20 mg/dL.

‐ Gilbert SyndromeThis prevalent condition is caused by mutations in thegene encoding bilirubin‐UGT. It is more common amongmales. Approximately 30% of the enzyme's activity ispreserved and the condition is entirely harmless.

‐ Toxic HyperbilirubinemiaUnconjugated hyperbilirubinemia can result from toxin‐induced liver dysfunction such as that caused bychloroform, carbon tetrachloride, acetaminophen,hepatitis virus, cirrhosis, and Amanita mushroompoisoning.

• Obstruction in the Biliary Tree Is the MostCommon Cause of ConjugatedHyperbilirubinemia

‐ Obstruction of the Biliary TreeConjugated hyperbilirubinemia commonly results fromblockage of the hepatic or common bile ducts, most oftendue to a gallstone or to cancer of the head of thepancreas. Because of the obstruction, bilirubindiglucuronide cannot be excreted. It thus regurgitates intothe hepatic veins and lymphatics, and conjugated bilirubinappears in the blood and urine (choluric jaundice). Also,the stools are usually pale in color, and should beexamined routinely in any case of jaundice.

‐ Dubin–Johnson SyndromeThis benign autosomal recessive disorder consists of conjugatedhyperbilirubinemia in childhood or during adult life. ‐ Rotor SyndromeThis is a rare benign condition characterized by chronicconjugated hyperbilirubinemia and normal liver histology. Itsprecise cause has not been identified.• Some Conjugated Bilirubin Can Bind Covalently to Albumin• Urobilinogen & Bilirubin in Urine Are Clinical IndicatorsIn complete obstruction of the bile duct, no urobilinogen is found in the urine,.In jaundice secondary to hemolysis, the increased production of bilirubin leads to increased production of urobilinogen, which appears in the urine in large amounts.