Metabolism of porphyrins I- Biosynthesis of porphyrins and ... fileRegulation of hemoglobin...
Transcript of Metabolism of porphyrins I- Biosynthesis of porphyrins and ... fileRegulation of hemoglobin...
Metabolism of porphyrins I- Biosynthesis of porphyrins and heme:
Hemoglobin is synthesized in erythrocyte precursor cells in bonemarrow as well as
in spleen and in liver, The synthesis of protoporphyrin IX occurs in the following
steps:
i. Formation of δ-aminolevulonic acid (ALA):
The starting components for porphyrins biosynthesis are glycine and succinyl CoA
that is derived from the citric acid cycle in the mitochondria. Pyridoxal phosphate is
also necessary in this reaction to activate glycine. This step is the rate-controlling
and is catalyzed by ALA synthase enzyme that occurs in the mitochondria:
ii. Formation of porphobilinogen (PBG):
A cytoplasmic enzyme, porphobilinogen synthase (ALA-dehydratase), forms
porphobilinogen from 2 molecules of ALA. This enzyme is zinc-containing and is
sensitive to inhibition by lead:
iii. Tetrapyrrole synthesis (Uroporphyrinogen III):
The condensation of 4 PBG units to form uroporphyrinogen III
requires two proteins: a- The enzyme uroporphyrinogen I synthase on its own catalyzes the head-to-
tail condensation of PBG molecules to form uroporphyrinogen I which is of
no physiological importance as it cannot be transferred to heme.
b- A noncatatalytic protein, uroporphyrinogen III cosynthase, alters the
specificity of uroporphyrinogen I synthase, so that one of the PBG
molecules is condensed head-to-head to give uroporphyrinogen III:
iv. Conversion of uroporphyrinogen III to coproporphyrinogen III:
Uroporphyrinogen III is decarboxylated at its aceto side chains by a cytosolic
enzyme to form coproporphyrinogen III with methyl side chain:
v. Conversion of coproporphyrinogen III to protoporphyrinogen III:
Two mitochondrial enzymes oxidatively decarboxylate 2 side chains to form
protoporphyrinogen III:
vi. Formation of protoporphyrin IX and heme:
Two methylene bridges are oxidized and the formed protoporphyrin IX is
connected with Fe2+ by a mitochonrial enzyme to form heme (protoheme IX):
N.B. Coproporphyrinogen oxidase is only specific to type III coproporphyrinogen, and this
explain why protoporphyrin I has not been identified in natural materials.
Regulation of hemoglobin synthesis:
The regulation of heme synthesis occurs at the first step in the pathway, i.e, at the step
catalyzed by ALA synthase enzyme. This enzyme is allosterically inhibited by protohemin
IX (hemin), which is also corepressor of transcription of ALA synthase gene. Protohemin
IX (hemin) arises from protoheme IX (heme) by oxidation of Fe2+ to Fe3+. This occurs
when heme is in excess of globin because globin-bound heme is stabilized in the Fe2+ state,
but free heme is not. Hemin also regulates the synthesis of globin chains in erythropoietic
cells at the level of mRNA translation.
Abnormal porphyrin biosynthesis (Porphyrias):
Accumulation of porphyrins may occur under conditions of genetically defective syn-
thesis in the liver (hepatic porphyrias) or in the erythropoietic tissues (erythropoietic
porphyrias), as these systems are under seperate genetic control.
1- Acute intermittent hepatic porphyria:
a) It is due to deficiency of uroporphyrinogen I synthase.
Protoporphyrin IX + Fe2 HEME Heme synthase or Ferrocheltase
b) Excessive amounts of porphobilinogen and ALA are formed.
c) The symptoms are acute abdominal pain and neurologic symptoms.
2- Congenital erythropoietic porphyria:
a) It is due to a decrease of uroporphyrinogen III cosynthase to about one-tenth to one-
third of the normal level.
b) It is characterized by the urinary excretion of large amounts of uroporphrinogen I
c) It causes mutilating skin lesions and hemolytic anemia.
2- Catabolism of heme and formation of bile pigments:
In human, under physiological conditions, about 2x108 erythrocytes are destroyed per
hour. Thus in one day, a 70 kg human turns over about 6 g of hemoglobin. Red cells
destruction occurs usually in the spleen, which is the major site of heme catabolism,
although some occurs in the liver. The catabolism occurs in the microsomal fractions of
the reticuloendothelial cells of spleen or liver by a complex system called heme oxygenase.
When heme reaches the oxygenase system, the iron has usually oxidized to ferric
form (hemin) that may be loosely bound to albumin as methemin. The hemin is reduced
with NADPH, and with the aid of more NADPH an oxygen is added to the α-methyl bridge
between pyrroles I & II. Oxidative opening occurs between rings I & II releasing CO gas
and forms biliverdin which is then reduced to the orange pigment bilirubin as follows:
Excretion of bilirubin:
1. The resulted bilirubin from heme catabolism is transfered from spleen to liver.
Bilirubin is only sparingly soluble in water, so in plasma it is bound to albumin.
Each mole of albumin appears to have one high-affinity and other low-affinity sites
for bilirubin. In 100 ml of plasma about 25 mg of bilirubin can be tightly bound to
albumin at its high-affinity site. Bilirubin in excess of this quantity can bound only
loosely and thus can be easily detached and diffused into tissues. A number of
compounds as some antibiotics competes with bilirubin for the high-affinity sites
on albumin and thus can displace bilirubin from albumin.
2. In the liver cell, it is transported through the cytoplasm to the endoplasmicreticulum
bound to a protein called ligand. Here it is conjugated to glucouronic acid by a
specific glucouronyl transferase that transfers the glucuronyl moiety from uridine
diphosphate glucuronate (UDP-glucuronate) to one of the propionyl side chains of
bilirubin. At the plasma membrane, the diglucuronide of bilirubin is formed by the
reaction of 2 moles of bilirubin monoglucuronide to form bilirubin diglucuronide
and free bilirubin. This last reaction is catalyzed by bilirubin glucuroniside
glucuronyl transferase. Then the water soluble bilirubin diglucuronide is secreted
in the bile.
N.B. Conjugated bilirubin (bilirubin diglucuronide) is usually called "direct bilirubin"
(DB) as being assayed directly by diazotization with sulphanilic acid (van den Berg
reaction) in aqueos solution. Free unconjugated bilirubin is called "indirect bilirubin"
(IB) as being diazotized only after methanol addition.
Under normal physiological conditions, most of (DB) in the liver is removed by bile
via the bile duct and excreted to small intestine. As (DB) reaches the terminal ileum and
the large intestine, the glucuronides are removed by specific bacterial enzymes (β-
glucuronidase). It is then reduced by the fecal bacterial flora to a group of colourless
tetrapyrrole compounds called urobilinogens which on air oxidation are oxidized to the
brown pigment urobilin. Some of urobilinogen in the intestine is absorbed to blood and
reexcreted via the liver (enterohepatic circulation) or excreted via kidney (urinary
urobilinogen).
UDP-Glucose UDP-Glucuronic acid
Dehydrogenase
NAD NADHH+
UDP-Glucuronic acid + BILIRUBIN
(IB)
BILIRUBIN monoglucuronide
+ UDP
UDP-Glucuronyl transferase
(endoplasmic reticulum)
2 BILIRUBIN
monoglucuronide BILIRUBIN diglucuronide (DB)
+ BILIRUBIN
Bilirubin glucuroniside
glucuronyl transferase
(plasma membrane)
When bilirubin in the blood exceeds 1 mg/dl (17.1 µmol/L), hyperbilirubinemia
exists. Hyperbilirubinemia may be due to the production of more bilirubin than the normal
liver can excrete, or it may result from the fai are of a damaged liver to excrete bilirubin
produced in normal amounts. In the absence of hepatic damage, obstruction to the excretory
ducts of the liver, by preventing the excretion of bilirubin, will also cause hyperbilirubi-
nemia. In all these situations, bilirubin accumulates in the blood, and when it reaches a
certain concentration (about 2-2.5 mg/dl), it diffuses into the tissues, which then become
yellow. The condition is called jaundice or icterus.
1- Hemolytic jaundice (prehepatic):
It is due to increased hemolysis of RBC's in a rate faster than normal liver can excrete.
This leads to accumulation of (IB) in serum which usually is abscent in urine being
insoluble. The increased bilirubin leads to increased urobilinogen which appears in large
amounts in urine.
2- Hepatic jaundice:
This type is caused by liver disfunction resulting from damage of parenchymal liver
cells as caused by some hepatotoxic agents e.g carbon tetrachloride and chloroform or
in case of hepatic viral infections. Here both serum (IB) and (DB) are increased. DB
being soluble appears in urine with urobilinogen.
3- Posthepatic or obstructive jaundice:
It is a conjugated hyperbilirubinemia (increased serum DB) results from the blockage
hepatic or bile ducts. (DB) formed in the liver normaly, canmnot be excreted as bile so
absorbed into hepatic vein appears in the circulation. In this case urobilinogen is not
formed and so it is abscent in feces and urine.
The following table summarizes the laboratory tests that are used to distinguish between
the different types of jaundice, prehepatic, hepatic and posthepatic:
Type Prehepatic Hepatic Poshepatic
Etiology
- Increased hemolysis
of RBC' s at a rate
faster than its
excretion by liver.
- Infective
erythropioesis
- Hepatitis e.g viral or
drug induced.
- Drugs e.g rifampicin
which interfere with
bilirubin conjugation.
- Intrahepatic
obstruction as in
cirrhosis.
- Gilbert's syndrome.
- Poisons as Ccl4, Pb,
As.
- Biliary obstruction
- Gallstone.
- Stricture.
- Carcinoma of
pancreas or
biliary tree.
Total bilirubin Increased Increased Increased
Unconjugated
bilirubin Increased Increased Normal
Conjugated
bilirubin Normal Increased Increased
Urinary
bilimbin Absent Present Present
Urinary
urobilinogen Increased Decreased Absent
Colour of
urine Dark yellow Cola colour Cola colour
Fecal
stercobilinoge
n
Increased Decreased Absent
Colour of stool Dark yellow Light brown Clay colour
Jaundice in the Newborn:
• Red-cell destruction, together with immature hepatic uptake, conjugation (due to
decreased activity of hepatic glucouronyl transferase) and secretion of bilirubin, may
cause a high plasma level of unconjugated bilirubin in the newborn; so-called
“physiological jaundice” is common.
• As a result of hemolytic disease, the plasma concentration of unconjugated bilirubin
may be as high as 30 mg/dl and exceed the plasma protein-binding capacity; free
unconjugated bilirubin may be deposited in the brain, causing kernicterus, a potential
very serious disorder of the central nervous system.
Treatment:
1. Phototherapy activates hepatic excretion of unconjugated bilirubin which may be due
to converting bilirubin to another geometric isomers that can be more easily excreted
in urine.
2. Administration of Phenobarbital that induces bilirubin-conjugating enzymes.
Some Inherited Hyperbilirubinaemias:
There is a group of inherited disorders in which either unconjugated or conjugated
hyperbilirubinemia is the only detectable abnormality.
1. Gilbert's syndrome is a relatively common, familial condition which may be
present at any age, but usually after the second decade. Plasma unconjugated
bilirubin concentrations are usually between 1.2 and 2.5 mg/dl and rarely exceed 5
mg/dl. They fluctuate and may rise during intercurrent illness or fasting. The
condition is harmless but must be differentiated from haemolysis and hepatitis. It
often becomes evident when plasma bilirubin concentrations fail to return to normal
after an attack of hepatitis, or during any mild illness which, because of the jaundice,
may be misdiagnosed as hepatitis.
2. Crigler-Najjar syndrome, due to deficiency of hepatic glucuronyl transferase, is
more serious. It usually presents at birth. The plasma unconjugated bilirubin
concentration may increase to concentrations that exceed the binding capacity of
albumin and so cause kernicterus.