R. C. GuptaM.D. (Biochemistry)

Jaipur (Rajasthan), India

Porphyrins are formed by the union of fourpyrrole rings through methenyl bridges

They usually contain a metal ion linked tothe nitrogen atoms of the pyrrole rings

Biologically important porphyrins areusually present as conjugated proteins

Some important porphyrin-containing compounds are:

Haemoglobin

Myoglobin

Cytochromes

Catalase

Peroxidase

Tryptophan pyrrolase

Haemoglobin

Haemoglobin is a conjugated protein madeup of four subunits

Each subunit contains a haem group and apolypeptide chain

The haem group is made up of porphyrinand a ferrous ion

Haemoglobin can reversibly combine withoxygen, and transports oxygen in the body

Myoglobin

Structure is similar to that of haemoglobin

The difference is that it is a monomer

Myoglobin is present in muscles

It can reversibly combine with oxygen

Contain iron-porphyrin conjugated toproteins

Iron-porphyrin portion similar to that ofhaemoglobin

Components of respiratory chain inmitochondria; transport electrons

Some cytochromes perform other functionsas well e.g. microsomal hydroxylation

Cytochromes

An iron-porphyrin containing enzyme thatis present mainly in animals

Acts on, and detoxifies, hydrogen peroxide

Catalase

Another iron-porphyrin containing enzymethat acts on hydrogen peroxide

It occurs mainly in plants

Peroxidase

Iron-porphyrin containing enzyme

Acts on tryptophan

Tryptophan pyrrolase

Carbon atom numbers 1 to 8 have different substituents attached to them

The substituents may be acetate (A), propionate (P), methyl (M) and vinyl (V)

There are two types of porphyrins −porphyrin I and porphyrin III

Type III porphyrins are found more commonly than type I

Uro-porphyrins

Copro-porphyrins

Proto-porphyrins

The important porphyrins in human beings are:

Uroporphyrins are found in urine

Coproporphyrins are found in faeces

Protoporphyrins are found in blood andtissues

Synthesis of porphyrins begins with thecondensation of succinyl CoA with glycine

Succinyl CoA is an intermediate of citricacid cycle

This reaction is catalysed by d-amino-levulinic acid synthetase (ALA synthetase)

Synthesis

a-Amino-b-ketoadipic acid is decarbo-

xylated to d-aminolevulinic acid (ALA)

The reaction is catalysed by ALA synthetase

Pyridoxal phosphate (PLP) is required as acoenzyme

Two ALA molecules are condensed toform the first pyrrole compound, porpho-bilinogen

This reaction is catalysed by ALAdehydrase

Different porphyrins are formed fromporphobilinogen

The exact reactions leading to thesynthesis of porphyrins are not fullyunderstood

Four porphobilinogen molecules react toform hydroxymethylbilane

The reaction is catalysed by uropor-phyrinogen I synthetase

Hydroxymethylbilane can spontaneouslycyclize to form uroporphyrinogen I

It can be enzymatically cyclized to uro-porphyrinogen III

The enzymatic reaction is catalysed byuroporphyrinogen I synthetase and uro-porphyrinogen III cosynthetase

Uroporphyrinogens can be converted into coproporphyrinogens

Coproporphyrinogens can be converted into protoporphyrinogens

Protoporphyrin III is the most abundantporphyrin

Haem is synthesized from protopor-phyrin III

It is also the most important porphyrin

Haem synthesis is catalysed by haemsynthetase (ferro-chelatase)

Haem can combine with different poly-peptides to form various haemoproteins

Protoporphyrin III + Fe++ Haem synthetaseHaem

The major purpose of porphyrin synthesisis to form haem

The regulatory enzyme in the pathway isALA synthetase

The regulator is haem itself

Regulation occurs by repression andderepression

Regulation

When haem is not being utilized, itsconcentration increases

It combines with an aporepressor to formthe repressor

Repressor acts on ALA synthetase geneand represses synthesis of ALA synthetase

This decreases the synthesis of porphyrins

When haem begins to be utilized, itsconcentration decreases

The synthesis of ALA synthetase isderepressed

The enzyme concentration increasesand so does the porphyrin synthesis

Haemoglobin (Hb) is the most abundantporphyrin-containing compound

It is a tetramer made up of four subunits

Each subunit contains a haem group and apolypeptide chain

Haemoglobin

The polypeptide chains are of five typesviz. a, b, g, d and e

The a chain is made up of 141 amino acids

The b , g, d and e chains are made upof 146 amino acids each

The genes for polypeptide chains ofhaemoglobin are called globin genes

The globin genes are present in twoclusters on two different chromosomes

The a cluster is present on chromosome16 and the b cluster on chromosome 11

The a cluster contains a and z genes

The a gene locus has two genes, a1 anda2

The a1 and a2 genes are nearly identical

The b cluster contains b, d, e and g genes

There are two g genes – Ag and Gg

The Ag and Gg are nearly identical

Haemoglobin synthesis occurs in red bloodcell precursors

It begins in pro-erythroblasts

About 65% synthesis is completed byerythroblast stage

The remaining 35% is completed byreticulocyte stage

Part of haem synthesis occurs inmitochondria

The synthesis stops when mitochondriadisappear from red blood cells

Globin is synthesized on ribosomes incytosol

Synthesis of haem and globin issynchronous

Expression of globin genes isdevelopmentally regulated

Haemoglobin synthesis begins at threeweeks of gestation

The a gene is expressed throughout life

The e and z genes are expressed untilabout eighth week of gestation

The predominant haemoglobin at thisstage is a2e2, with some a2z2

After eighth week, the expression of e andz genes declines

Expression of g gene begins andpredominates until birth

The major haemoglobin at this stage isa2g2

After birth, the expression of g genedeclines, and that of b gene increases

The major haemoglobin after birth andthroughout life is a2b2, with some a2d2

The a2b2 haemoglobin is the normal adulthaemoglobin (HbA)

It accounts for 95-98% of the totalhaemoglobin in adults

A small amount of a2d2 haemoglobin(HbA2) is also found in adults

The a2e2 (and a2z2) form of Hb is known as

embryonic haemoglobin

The a2g2 form is foetal haemoglobin (HbF)

which is the predominant form in foetal life

HbF may also be present in adults in very

small amount

The major secondary structure in theglobin chains is a-helix

In b, g, d and e chains, there are eight a-helical regions named A through H

There are seven helices in a chain (thehelix D is missing)

Structure of haemoglobin

Helices in globin chain

A ferrous ion is present at the centre ofeach haem group

It has six electrons in its outermost orbit

Four of these link iron to the four nitrogenatoms of haem

One electron of iron links it to a histidineresidue of the polypeptide chain

This is His87 in the a chain and His92 in band other non-a chains

These histidine residues are present in thehelix F

The bond between iron and His87/His92 isknown as the proximal iron-histidine bond

Helix F

Proximal histidine

Haem

One other histidine residue in helix E is onthe opposite co-ordination position

This is His58 in the a chain and His63 in thenon-a chains

His58/His63 is known as the distal histidineresidue

Distal histidine residue prevents oxidation ofFe+2 by any oxidizing agent in the vicinity

On exposure to high oxygen tension,oxygen enters the space between the distalhistidine residue and Fe+2

Oxygen binds loosely to Fe+2, which isknown as oxygenation of haemoglobin

Haemoglobin can exist in two thermo-dynamic conformations

The conformations are known as Tense (T) and Relaxed (R)

Binding of oxygen changes the thermo-dynamic state of haemoglobin from T to R

During T→R transition, one pair of a and bsubunits rotates by 15° relative to the otherpair

The gap between the two b polypeptidechains becomes narrower when oxygenattaches to iron

2,3-Biphosphoglycerate (2,3-BPG) is animportant regulator of oxygenation anddeoxygenation of haemoglobin

It is formed in erythrocytes from 1,3-bi-phosphoglycerate (1,3-BPG) which is anintermediate of the glycolytic pathway

Central cavity

There is a central cavity in the Hb molecule

surrounded by the four polypeptide chains

2,3-BPG enters the central cavity when its

concentration is high

2,3-BPG

2,3-BPG binds to the two b chains by

salt bonds

2,3-BPG

b1 - Chain

b2 - Chain

Low availability of O2 in tissues increasesthe conversion of 1,3-BPG into 2,3-BPG

Binding of 2,3-BPG to haemoglobinchanges the R form into T form

This results in release of oxygen fromhaemoglobin

The reverse happens in lungs where theavailability of oxygen is high

Oxygen binding changes the T form intothe R form

This narrows the central cavity, leaving nospace for 2,3-BPG

Each subunit of Hb can bind one oxygenmolecule

There are four subunits in a molecule ofHb

So, one Hb molecule can bind four oxygenmolecules

Co-operative binding

Binding of one O2 molecule to haemo-globin facilitates the binding of other O2

molecules

This is known as co-operative binding andis responsible for the sigmoidal oxygendissociation/saturation curve

Co-operative binding is not shown bymyoglobin which is a monomer

Oxygenated myoglobin releases oxygenonly when oxygen tension is very low

Oxygen

saturation

(%)

pO2 (mmHg)

Haemoglobin

Myoglobin

Derivatives of haemoglobin

Haemoglobin can form the following derivatives:

Oxyhaemoglobin

Carboxyhaemoglobin

Methaemoglobin

Sulphaemoglobin

EMB-RCG

Oxyhaemoglobin

This is the oxygenated form of haemo-globin

It is bright red in colour

Oxygen is transported to tissues in theform of oxyhaemoglobin

Carboxyhaemoglobin

Haemoglobin combines with carbonmonoxide to form carboxyhaemoglobin

Affinity of haemoglobin for carbonmonoxide is 200 times that for oxygen

Carboxyhaemoglogin is cherry red incolour

Carboxyhaemoglobin is much more stableas compared to oxyhaemoglobin

Once it is formed, oxygen cannot displacecarbon monoxide from haemoglobin

Formation of carboxyHb decreases the O2

carrying capacity of the blood

Methaemoglobin

Some drugs and chemicals can oxidizethe ferrous ion of Hb to ferric ion

These include sulphonamides, antipyrine,nitrites, nitrobenzene etc

Hb is converted into methaemoglobin,which is brownish red in colour

Some methaemoglobin is formed normallyby endogenous oxidizing agents

However, RBCs possess methaemoglobinreductase and glutathione

These two continuously reduce met-haemoglobin to haemoglobin

Methaemoglobin cannot combine withoxygen

But methaemoglobin can combine withcyanide to form cyanmethaemoglobin

This property is used in the treatment ofcyanide poisoning

The patient is given sodium nitrite andsodium thiosulphate

Sodium nitrite converts haemoglobin intomethaemoglobin

Methaemoglobin combines with cyanideto form non-toxic cyanmethaemoglobin

Sodium thiosulphate reacts with cyanideto form non-toxic sodium thiocyanate

Sulphaemoglobin

Sulphonamides and H2S can converthaemoglobin into sulphaemoglobin

It is dirty brown in colour, and cannotcombine with oxygen

It persists in red blood cells throughouttheir remaining life span

Several abnormal haemoglobins result frommutations in the globin genes

Often, a single amino acid is substituted

Hundreds of mutant haemoglobins havebeen discovered

Abnormal haemoglobins

Most of mutant haemoglobins are capableof normal or near-normal functioning

Such mutants are known as haemoglobinvariants

In some mutants, amino acid substitutionoccurs in a critical region of the molecule

This impairs the functioning of haemoglobin

Such haemoglobins are known asabnormal haemoglobins

Diseases resulting from abnormal haemo-globins are called haemoglobinopathies

Some examples of abnormal haemoglobins and the diseases

resulting from them are:

Haemoglobin S

Haemoglobin M

Thalassaemia

HbS is formed when the glutamateresidue at position 6 in the b chain isreplaced by valine

This amino acid residue is present on thesurface of the haemoglobin molecule

Glutamate has a polar side chain whilevaline has a non-polar side chain

Haemoglobin S

Replacement of a polar residue by a non-polar residue alters the surface properties

Non-polar valine residue of one moleculeattracts the non-polar residue of another

This starts a chain reaction causingaggregation of several Hb molecules

Aggregated haemoglobin molecules

Aggregation results in the formation of afibrous structure

This distorts the erythrocyte into a sickle-shaped cell

Aggregated haemoglobin

molecules distort RBC

Normal

RBC

Sickled

RBC

Oxygenated haemoglobin exists in the Rstate

The non-polar valine residues are notexposed on the surface in R state

Therefore, there is no aggregation ofhaemoglobin molecules

Deoxygenated haemoglobin exists in theT state

In T state, the non-polar valine residuesare exposed on the surface

Therefore, deoxygenated haemoglobin Sgets aggregated

Haemoglobin is present in deoxygenatedform when oxygen tension is low

There is aggregation of haemoglobin Smolecules and sickling of RBCs at lowoxygen tension

Sickled erythrocytes are susceptible topremature destruction

Rapid destruction of erythrocytes causeshaemolytic anaemia

Inheritance of sickle cell anaemia isautosomal recessive

If the defect is inherited from one parentonly, it results in sickle cell trait

Sickle cell trait doesn’t cause any clinicalabnormality

If the defect is inherited from both theparents, it results in sickle cell disease

Sickle cell disease causes severehaemolytic anaemia

Presence of haemoglobin S gives someprotection against malaria

The malarial parasite inhabiting RBCs getskilled when the RBCs are sickled

Prevalence of HbS has been found to behigher where malaria is endemic

Formed by replacement of His58 in the achain by tyrosine due to a point mutation

Phenol group of tyrosine is bonded withiron

This converts Fe+2 into Fe+3 (formingmethaemoglobin)

Methaemoglobin cannot combine withoxygen

Haemoglobin MBoston

Thalassaemia results from a decrease in,or lack of, synthesis of either a chains or bchains

Defective synthesis of a chains leads to a-thalassaemia and that of b chains leads tob-thalassaemia

Thalassaemia

A variety of genetic defects can cause thalassaemia such as:

Deletion of a part or whole of a gene

Defective processing of the primary transcript

Defective transport or translation of mRNA

Premature termination

Decreased synthesis or lack of synthesisof one type of chain leads to an over-production of the unaffected chain

This results in the formation of ahaemoglobin having only a chains or onlyb chains

When the defect is transmitted by only oneparent, it results in thalassaemia minorwhich is symptomless

When the defect is transmitted by both theparents, it results in thalassaemia majorwhich is associated with severe anaemia

Porphyria is a group of disorders

Large quantities of porphyrins and/or theirprecursors are excreted in urine

Excessive excretion occurs due to a defectin the synthetic pathway

Porphyria

An enzyme in the synthetic pathway isabsent or deficient in porphyria

This leads to accumulation of inter-mediates proximal to the block

The urine is normal in colour when fresh butbecomes pink on exposure to light

The change in colour occurs due to oxidationof porphyrinogens

Skin photosensitivity is common in porphyria

Early intermediates bind to nervous tissue,and produce neuropsychiatric abnormalities

Thus, a defect early in the pathway is moreharmful than a defect in the later steps

The defective gene is present in all the

tissues but the expression is usually

confined to a particular tissue

Depending upon the site of expression of

genetic defect, porphyrias may be divided

into:

Erythropoieticporphyrias

Hepatic

porphyrias

Hepatic porphyrias include:

Acute intermittent porphyria

Porphyria cutanea tarda

Hereditary coproporphyria

Variegate porphyria

Erythropoietic porphyriasinclude:

Congenital erythropoieticporphyria

Protoporphyria

Acute intermittent porphyria

Mode of inheritance

Affected enzyme

Site of expression

Autosomal dominant

Uropor-phyriogen I synthetase

Liver cells

Porphyria cutanea tarda

Mode of inheritance

Affected enzyme

Site of expression

Autosomal dominant

Uropor-phyriogendecarboxylase

Liver cells

Hereditary coproporphyria

Mode of inheritance

Affected enzyme

Site of expression

Autosomal dominant

Copropor-phyriogenoxidase

Liver cells

Variegate coproporphyria

Mode of inheritance

Affected enzyme

Site of expression

Autosomal dominant

Protopor-phyriogenoxidase

Liver cells

Congenital erythropoietic porphyria

Mode of inheritance

Affected enzyme

Site of expression

Autosomal recessive

Uropor-phyriogen III cosynthetase

Erythroidcells

Protoporphyria

Mode of inheritance

Affected enzyme

Site of expression

Autosomal dominant

Ferro-chelatase

Liver cells

The clinical abnormalities are mainlyneuro-visceral in hepatic porphyrias andcutaneous in erythropoietic porphyrias

However, some overlapping of signs andsymptoms is not uncommon

Acute attacks of abdominal pain, nauseaand vomiting occur in hepatic porphyrias

Over-active sympathetic nervous systemcauses tachycardia, tremors andhypertension

Hepatic porphyrias

Anxiety, insomnia, disorientation anddepression are also common

Motor neuropathy may cause progressivemuscular weakness

Seizures can also occur

Cutaneous photosensitivity is an additional feature in:

Hereditary copro-

porphyria

Variegate porphyria

Porphyria cutanea

tarda

Severe cutaneous photosensitivity ispresent from a very early age

Porphyrin precursors are present in skin,and damage skin on exposure to sunlight

Multiple vesicles erupt on the skin

The skin is pigmented and fragile

Erythropoietic porphyrias

Denuded areas on skin are prone toinfections

Bones and teeth may be pigmented due todeposition of porphyrin precursors

Haemolysis may occur due to binding ofporphyrin precursors to haemoglobin

In protoporphyria, liver damage also occursin some patients

Cutaneous manifestations are produced byexposure to sunlight

Neuro-visceral symptoms are precipitatedby steroids, alcohol and some drugs

The drugs include barbiturates, mepro-bamate, carbamazepine, mephenytoin,sulphonamides, griseofulvin etc

When life-span of RBCs is over, they arebroken down in reticulo-endothelial system

Haem and globin are separated

Globin is broken down into amino acids

Catabolism of haemoglobin

Methenyl bridge between ring I and ring IIof haem is broken by haem oxygenase

This releases iron and converts haem intobiliverdin

Biliverdin is a green pigment

Biliverdin is reduced to bilirubin bybiliverdin reductase

Bilirubin is yellow in colour

This is the major bile pigment in humanbeings

Bilirubin formed from haem is insoluble inwater

It is known as unconjugated bilirubin

It has to be made water-soluble for itsexcretion

Conjugation with glucuronic acid makesbilirubin water-soluble

Conjugation of bilirubin occurs in liver

Bilirubin, released from reticulo-endothelialcells, has to be transported to liver

Being water-insoluble, it is transported inassociation with albumin

Albumin has two bilirubin-binding sites ̶ ahigh-affinity site and a low-affinity site

Bilirubin is first bound to the high-affinitysite

If high-affinity sites on all the albuminmolecules are saturated, bilirubin is boundto low-affinity site

The normal plasma albumin concentration is3.5-5.5 gm/dl

This is sufficient for binding of 20-25 mg ofbilirubin on the high-affinity sites of albumin

If unconjugated bilirubin level exceeds 20-25 mg/dl, it begins to bind to low-affinity site

Bilirubin is taken up by liver cells from thecirculating albumin

The uptake occurs with the help of acarrier-mediated active transport system

In hepatocytes, bilirubin is conjugated withglucuronic acid to make it water-soluble

Glucuronic acid is conjugated with thepropionate group

Since there are two propionate groups,two glucuronate moieties can be added

The conjugation reaction occurs in twosteps

Bilirubin

Bilirubin monoglucuronide

Bilirubin diglucuronide

UDP-glucuronic acid

UDP

UDP

UDP-glucuronic acid

Bilirubin UDP-glucuronyl

transferase

Bilirubin UDP-glucuronyl

transferase

Bilirubin diglucuronide may also be formedby a trans-esterification reaction

The reaction occurs between two bilirubinmonoglucuronide molecules

It is catalysed by bilirubin-glucuronideglucuronosyl transferase (dismutase)

Bilirubin

mono-glucuronide

Bilirubin

Bilirubindiglucuronide

Bilirubin

glucuronide

glucuronosyltransferase

Bilirubin

mono-glucuronide

Bilirubin diglucuronide is also known asconjugated bilirubin

It is excreted by liver into the intestinethrough bile

Excretion takes place through an activetransport mechanism

Bilirubin is freed from glucuronic acid in thelarge intestine

It is reduced by the enzymes of intestinalbacteria to urobilinogen

Most of the urobilinogen is excreted in thefaeces

A small portion of urobilinogen is absorbedinto portal circulation and is taken to liver

Liver excretes most of it into the intestine(entero-hepatic circulation of urobilinogen)

A fraction enters the systemic circulation,and is excreted by the kidneys in urine

Haemoxygenase

Biliverdinreductase

Haem

Bilirubin UDP-gluc-uronyl transferaseBlood

vessel

Serum bilirubin ranges from 0.2-1.0 mg/dl in concentration

Jaundice

Unconjugated bilirubin

Conjugated bilirubin

This is total bilirubin which includes:

Concentration of unconjugated bilirubin inserum is 0.1-0.6 mg/dl

It is water-insoluble

It is also known as indirect reacting bilirubin

It reacts with Ehrlich’s diazo reagent onlyafter addition of methanol or ethanol

Concentration of conjugated bilirubin inserum is 0.1-0.4 mg/dl

It is water-soluble

It is also known as direct reacting bilirubin

It can react with Ehrlich’s diazo reagentwithout the addition of an organic solvent

A rise in serum bilirubin concentration isknown as hyperbilirubinaemia

When the level rises above 2 mg/dl,bilirubin gets deposited in tissues

The tissues are stained yellow

This is known as jaundice

The yellow staining can be seen in skinand mucous membranes

But is most clearly visible in the sclera

Jaundice can occur in a number of

diseases

Post-hepatic jaundice

Hepatic jaundice

Pre-hepatic jaundice

Depending upon the site of the defect,

jaundice can be divided into:

This is also known as haemolytic jaundice

It is due to an increased rate of haemolysis

Breakdown of haemoglobin is increased

Bilirubin is formed in large quantities

Pre-hepatic jaundice

Capacity of the liver cells to take up,conjugate and excrete bilirubin is exceeded

Concentration of unconjugated bilirubin inserum rises resulting in jaundice

Unconjugated bilirubin cannot be excretedby the kidneys

Therefore, urine doesn’t contain bilirubin

As the rate of formation of bilirubinincreases, so does the rate of formation ofurobilinogen

Therefore, urinary excretion of urobilinogenis increased

The laboratory findings in haemolytic jaundice are:

• Rise in unconjugated bilirubin in serum

• Absence of bilirubin from urine

• Increase in urobilinogen in urine

Haemolytic jaundice can occur in:

• Thalassaemia

• Sickle cell crisis

• Spherocytosis

• Glucose-6-phosphate dehydrogenase deficiency etc

A common cause of haemolytic jaundice is“physiological jaundice of neonates”

This occurs in some neonates between thethird and tenth days of life

Erythrocytes formed during foetal lifecontain HbF

These are rapidly destroyed after birth tobe replaced by erythrocytes containing HbA

Rate of formation of bilirubin is increased

The hepatic conjugating system is not fullydeveloped in the first two weeks of life

There is accumulation of unconjugatedbilirubin in blood causing jaundice

This is a transient and benign condition

A serious cause of haemolytic jaundice iserythroblastosis foetalis

It is also known as haemolytic disease ofthe newborns

This occurs when an Rh-negative motherconceives an Rh-positive baby

The Rh-antigen can be transferred acrossthe placenta from the foetus to the mother

Maternal immune system starts formingRh-antibodies

The antibodies are transferred across theplacenta to the foetus

The resulting Rh-incompatibility causessevere haemolysis in the foetus

Excessive haemolysis increases the levelof unconjugated bilirubin in serum

The baby is born with jaundice

The condition becomes serious if unconju-gated serum bilirubin exceeds 20-25 mg/dl

The excess bilirubin binds to low-affinitysite of albumin

This is off-loaded in the central nervoussystem which is rich in lipids

The lipids easily take up non-polar bilirubinfrom the low-affinity site of albumin

Bilirubin is attached to basal ganglia,hippocampus, cerebellum, medulla etc

Nervous tissue is stained yellow (knownas kernicterus)

Kernicterus is fatal or causes permanentneurological damage if the baby survives

Hepatic or hepatocellular jaundice is due torapid destruction of liver cells

This can be caused by hepatitis (viral oralcoholic), hepatotoxic drugs/chemicals,advanced cirrhosis and some inborn errors

Hepatic jaundice

Due to destruction of liver cells, capacity ofliver to take up and conjugate bilirubin isdecreased

Concentration of unconjugated bilirubin inserum increases even though the rate offormation of bilirubin is normal

In viral hepatitis, the surviving liver cells areinflammed and swollen, and compress thebiliary canaliculi

This results in intra-heptatic biliaryobstruction

Due to obstruction, conjugated bilirubinregurgitates into systemic circulation

Hence, serum level of conjugated bilirubinis also raised in viral hepatitis

As conjugated bilirubin is water-soluble, itis excreted in urine

Therefore, urine contains bile pigments

The laboratory findings in viral hepatitis are:

• Unconjugated bilirubin is raised in serum

• Conjugated bilirubin is raised in serum

• Bilirubin is present in urine

• Urinary urobilinogen is usually normal

This is also known as obstructive jaundiceas it is due to an obstruction to the flow ofbile

The obstruction may be intra-hepatic orextra-hepatic

The commonest cause of biliary obstructionis presence of gall stones in the bile duct

Post-hepatic jaundice

The other causes of obstruction are:

• Cancer of pancreas

• Cancer of gall bladder/bile duct

• Stricture of bile duct

• Congenital atresia of bile duct

• Cholangitis

Conjugated bilirubin is regurgitated into circulation

As it is water-soluble, it is excreted in urine

Due to biliary obstruction, bilirubin conju-gated in liver

As bilirubin doesn’t reach the intestine,urobilinogen cannot be formed

Therefore, urobilinogen is absent fromurine

The laboratory findings in obstructive jaundice are:

• Rise in conjugated bilirubin in serum

• Presence of bilirubin in urine

• Absence of urobilinogen from urine

Jaundice also occurs in the following inherited disorders of

bilirubin metabolism:

• Gilbert’s syndrome

• Crigler-Najjar syndrome

• Lucey-Driscoll syndrome

• Rotor’s syndrome

• Dubin-Johnson syndrome

The active transport system for hepaticuptake of bilirubin is defective

Bilirubin UDP-glucuronyl transferaseactivity in liver cells is also sub-normal

The concentration of unconjugated biliburinis raised in serum

Gilbert’s syndrome

Inheritance of Gilbert’s syndrome isautosomal dominant

A mutation occurs in the promoterregion of the gene for bilirubin UDP-glucuronyl transferase

Expression of the gene is decreasedthough the enzyme is normal in function

Concentration of unconjugated bilirubin inserum is mildly raised

There is no clinical abnormality other thanthe permanent yellow discoloration

No treatment is required

Crigler-Najjar syndrome

This is an autosomal recessive disorder

Two types have been recognized:

Crigler-Najjarsyndrome, type I

Crigler-Najjarsyndrome, type II

In type I, a variety of mutations occur inthe gene for bilirubin UDP-glucuronyltransferase

The mutations may be deletions,insertions, mis-sense mutations andpremature stop codons

The result is a totally non-functionalenzyme

Concentration of unconjugated bilirubinis greatly elevated in serum

Kernicterus is common

Phototherapy and repeated plasma-pheresis can prevent kernicterus up topuberty but not later

In type II, there is a point mutation inone allele of the gene

Bilirubin UDP-glucuronyl transferaseactivity is sub-normal but not absent

Concentration of unconjugated bilirubinin serum is moderately increased

Prognosis is much better

This rare disorder is believed to be dueto an inhibitor of bilirubin UDP-glucuronyl transferase

The inhibitor is present in maternalblood for a short period only

Lucey-Driscoll syndrome

Severe unconjugated hyperbilirubinaemiadevelops in the newborn

The condition is transient

Phototherapy and, sometimes, exchangetransfusion may be required in the firstfour days of life

The active transport system for excretion ofconjugated bilirubin is defective

This leads to a moderate rise in conjugatedbilirubin in serum

The inheritance is autosomal recessive

Rotor’s syndrome

The inheritance is autosomal recessive

The nature of the defect is similar to that inRotor’s syndrome

In addition, porphyrins are deposited inliver giving it a black appearance

Dubin-Johnson syndrome

Concentration of conjugated bilirubin inplasma is raised

Apart from visible jaundice, there is nosign and symptom

No treatment is required