Transcript The role of vitamin B12 within the body

The role of vitamin B12
within the body
Ahmad Sh. Silmi
Msc Haematology, FIBMS
Vitamin B12
The role of vitamin B12 within the
body
Vitamin B12 is required as coenzyme for
two metabolic reaction:
•
(1) Isomerization of L-methylmalonyl CoA to succinyl
CoA. This is important substrate in Hb synthesis.
•
(2) Methylation of homocystine to methionine. This
step is important in intracellular synthesis of folate
coenzyme.
COBALAMIN REACTIONS
Homocysteine Methionine
Methylmalonyl
CoA
Methyl
Cobalamin
N
H
THF
Adenosyl
Cobalamin
N
H
CH3
Succinyl CoA
Daily vitamin B12 requirement
• The only source available to man is dietary.
• The main dietary source is liver, kidney, red meat, eggs,
shellfish and dairy products.
• Normal mixed diet contains 5-30 μg /day.
• Vitamin B12 is relatively stable and little is lost
during cooking.
• Typical daily losses of vitamin B12 are between 1-4 μg.
• The vitamin is lost mainly in urine and faeces.
• Since normally there is no consumption of vitamin B12
within the body, the daily requirement matches daily
losses.
Vitamin B12 Stores
• Normal body stores of vitamin B12 about
3-4 mg, primarily in liver. This would be
sufficient for 3 years if dietary intake
ceased or if the ability to absorb the
vitamin was lost.
Vitamin B12 absorption
• VitaminB12 absorption is an active process, which
occurs in the ileum.
• Vitamin B12 is liberated from food by gastric and
duodenal enzymes and complexes in a 1:1 ratio with the
intrinsic factor.
• IF is a glycoproein, MW 45,ooo, which is synthesized
and secreted by gastric parietal cells.
• IF: B12 complex then progresses to the ileum where it
attaches to specific receptors on the ileal mucosal cells.
• This process requires the presence of calcium ions and
neutral pH.
• The vitamin is internalized from the complex and
released into the portal circulation after 6 hours.
Vitamin B12 Transport
• There are three vitamin B12 transport proteins normally
present in the plasma, which are known as
Transcobalamines (TCI-TCII-TCIII).
• The physiologically active is TCII which complex in a 1:1
ratio with vitamin B12.
• The complex is then binds to specific surface receptors
on developing blood cells in the bone marrow. Vitamin
B12 is then released by hydrolysis. The TCII is not
reutilized.
• The plasma half-life of TCII is 12 hours and congenital
absence of it causes megaloblastic anaemia within
weeks of birth.
• Transcobalamines I and III are -globulins synthesized
by granulocytes and known as R-binders that are found
in a wide range of body fluids. TCI&III do not readily
release vitamin B12 to the developing tissues. The
plasma half-life is 9-12 days and congenital absence of
them causes no physiological impairment.
GI ABSORPTION OF COBALAMIN
Cbl
R
Stomach
R-Cbl
R-Cbl
R
IF
TCI-Cbl
IF-Cbl
Cbl
TCII
Duodenum
IF
Cbl
TCII-Cbl
IF-Cbl
IF-Cbl TCII
Terminal Ileum
TCII-Cbl
Folates
Folates
• The parent of folate family compounds is
folic acid which has the following basic
structure. Humans are incapable to
synthesize it so the only source is diet.
Folic Acid
The role of folate within the
body
• The various form of folate function as a singlecarbon donor-acceptors in a variety of
biosynthetic reactions as shown below:
(1) Synthesis of methionine. By donation of methyle group from
N-5-methyl-tetrahydrofolate and requires vitamin B12 as a
coenzyme.
(2) Pyrimidine synthesis which is a rate limiting step in DNA
synthesis.
(3) Purine synthesis.
(4) Conversion of serine into glycin.
(5) Histidine catabolism.
Daily folate requirement and
storage
• Daily folate losses are about 100 g per day, mainly in
the faeces, urine, and sweat and skin cells.
• Faeces contain large amount of vitamin B12 and folic
acid, but these are due to the microbial flora activities
rather than losses from body stores.
• In order to maintain body stores, the total daily
requirement must match losses. Thus, the normal adult
daily requirement for folate is about 100 g.
• Folate is present mainly in liver, leafy vegetables, whole
grains and yeast. Folate is extremely sensitive to heat.
Cooking involves prolonged boiling result in sever loss.
Daily folate requirement and
storage
• Normal mixed diet may contain as much as 700
g of folate per day but improper food
preparation can reduce this amount close to the
minimum daily requirement.
• Typical body stores of folate in a normal, healthy
adult are about 10mg and are located in liver.
Thus, if dietary folate intake or intestinal
absorption ceased, the body stores would
become exhausted in about 3-4 months.
Folate absorption and transport
• Folates are absorbed maximally from the upper
jejunum.
• Folate polyglutate must be digested to form
monoglutamate before absorption.
• Absorbed folates are converted into N-5methyltetrahydrofolate and released into portal
blood stream.
• Plasma folates circulate freely or loosely bound
to a variety of specific plasma proteins.
• There is some evidence that a specific folate
transport protein exists and that its concentration
is increased by folate deficiency but its
physiological significance is unknown.
Megaloblastic Anaemia
• Megaloblastic anemia is referred to a
group of panhypoplastic disorders, which
are characterized by retardation of DNA
synthesis but RNA synthesis proceeds at
a normal rate.
• The resulting asynchrony between nuclear
and cytoplasm maturation in developing
cells is responsible for the distinctive
morphological and biochemical features of
the megaloblastic anaemias.
Megaloblastic Anaemias Causes
• Deficiency of either vitamin B12 or folic acid or
sometimes both.
• A number of uncommon exceptions exist where
the cause of the disorder is not attributable to
haematinic deficiency.
Causes of Vitamin B12
Deficiency
• 1- Inadequate dietary intake.
• 2- Intestinal malabsorption.
• 3- Increased requirements, which cannot
be met from the diet.
• 4- Failure of utilization of absorbed
vitamin.
Inadequate Dietary Intake
This is uncommon for three main reasons:
1- Vitamin B12 is present in a wide range of
readily available foodstuffs.
2- Vitamin B12 is relatively heat-stable.
3- Body stores of vitamin B12 are sufficient to
meet the requirements for at least three years
following complete cessation of dietary intake or
intestinal absorption.
Malabsorption of vitamin B12
• The most common cause of the deficiency,
which could be due to:
• Lack of intrinsic factor
• Gastrointestinal disease.
• Drug-induced Malabsorption .
1- Lack of intrinsic factor
(pernicious anaemia)
• Pernicious anemia is by far the most common
cause of B12 deficiency.
• This condition is especially common among the
elderly, with an observed prevalence of up to
1.9%.
• The disease is more common in women than in
men and is associated with blood group A.
pernicious anaemia
• Pernicious anemia is caused by intestinal
malabsorption due to atrophy of the gastric
mucosa and decreased secretion of intrinsic
factor.
• One recent hypothesis suggests an autoimmune
mechanism, as illustrated by a case of
spontaneous remission of pernicious anemia
after corticosteroid therapy.
• The question of a relationship between
pernicious anemia and Helicobacter pylori has
also been investigated, but evidence for this
theory has not been conclusive.
pernicious anaemia
• About 90% of patients have cytotoxic IgG directed
against gastric parietal cells or intrinsic factor
demonstrated in serum.
• In about 75% of these the antibody is demonstrated in
gastric juice. Polyclonal IgG or IgA are demonstrated in
serum and gastric juice in 50% of patients with
pernicious anaemia, this acts in one of two ways:
•
•
Either prevents the binding of vitamin B12 to intrinsic
factor. (Type 1 Ab)
OR inhibit the absorption of VitB12-IF complex.
(Type 2 Ab).
pernicious anaemia
• Pernicious anaemia is associated with an
increased incidence with congenital
deficiency of autoimmune thyriod disease,
rheumatoid arthritis and gastric carcinoma.
• A rare type of pernicious anaemia is
associated with congenital deficiency of
intrinsic factor or the synthesis of
dysfunctional variant of intrinsic factor.
Pernicious Anemia
Normal
Pernicious Anemia
Stomach
Stomach
Acid +
IF
Normal
gastric parietal
cells
Atrophic gastritis
Achlorhydria
No IF
Diagnosis of Pernicious Anemia
• The diagnosis is achieved by Schilling test
as:
Failure to absorb radiolabelled B12 on the initial assay,
followed by absorption when B12 is co-administered with
intrinsic factor, establishes the diagnosis.
Treatment of Pernicious
Anemia
It consists of intramuscular injections of
1000 mcg of vitamin B12 at weekly
intervals until B12 stores are replenished,
followed by monthly injections for life. Oral
and intranasal preparations of B12 have
been tried but without compelling success.
2-
Gastrointestinal Disease
• The most obvious follows surgical removal of the source of
intrinsic factor, or the site of absorption of the vitamin.
•
a) Total gastrectomy: the anaemia is developed after
depletion of the body stores, which is usually, occur within 5
years. This is sever when accompanied with iron deficiency
anaemia.
•
b) Partial gastrectomy: (stagnant or blind loop syndrome)
Partial removal of the stomach, and refashioning the junction
with the gut. The sterile duodenal part will colonized with
bacteria, which will consume huge amount of the vitamin.
•
c) Ileal resection or ileostomy: Involve removal of the vitB12
receptor.
Gastrointestinal Disease
• d) Crohn's disease: Granulomatous disease,
which most commonly affects the terminal ileum
and the ascending colon. It's manifested by
generalized Malabsorption of nutrients from the
diet.
• e) Infestation of the gut with the fish tapeworm
Diphyllobothrium lattum, which is capable of
extracting substantial quantities of vitamin B12
both complexed with intrinsic factor and free.
3- Drug-induced Malabsorption
•
•
•
•
A number of drugs have been reported to
impair vitamin B12 absorption such as:
Anticonvulsant, phenytoin
Antimicrobial, neomycin
Antigout, colchicine
Alcohol.
Increased Requirements
The requirements are increased during
pregnancy. The increase is not sufficient
to cause deficiency unless the pregnant
was previously borderline body stores of
the vitamin.
Failure of Vitamin B12 Transport
Congenital deficiency of transcobalamin II
develops megaloblastic anaemia in the
first weeks of life, despite the presence of
normal vitamin B12 concentration in the
serum. Early diagnosis prevents
neurological damages
Failure of Vitamin B12
Metabolism
• Rare number of congenital failure to convert the
absorbed vitamin B12 to it's active coenzyme
forms have been described, resulting in the
excretion of methylmalonic acid and
homocystine in the urine. These patients are
mentally retarded, but for unknown reasons,
rarely develop megaloblastic anaemia.
• Anaesthetic nitrous oxide inactivates vitamin
B12 coenzymes and induces megaloblastic
changes & mild neuropathy.
Folic acid deficiency
Causes:
Deficiency of folic acid can result from an
1- inadequate diet.
2- intestinal malabsorption.
3- increased requirement.
4- failure of utilization the absorbed vitamin.
Inadequate dietary Intake
This is common for 3 main reasons:
a) The ideal diet contains 700 g of folate of
which about half is absorbed.
b) Folate is very labile to heat; cooking can
destroy up to 90% of folate in it.
c) Body stores are only sufficient for 3
months when dietary intake stop.
Malabsorption
This can be due to several conditions like:
a) Coelic disease: villous atrophy, which
decreases iron and folate absorption.
b) Tropical sprue: similar to coelic disease.
c) Crohn disease: generalized
malabsorption in the intestine .
Increased requirements
This most commonly seen in:
• Pregnancy: the daily requirement for folate can
rise to 500μg in the 3ed trimester of pregnancy.
• More than 60% of pregnant women have
subnormal folate concentration.
• This recently, demonstrated to be associated
with neural tube defects.
• Prophylactic folic acid therapy is recommended
several months before conception.
Increased requirements
• The anaemia of chronic haemolytic
conditions such as sickle cell anaemia
frequently is exacerbated by folate
deficiency. Sever haemolytic conditions
increases the rate of haemopoiesis by a
factor of 10, which cannot be met by
dietary sources.
Drug-induced folate deficiency
• Some drugs are demonstrated to inhibit
folate absorption such as:
• Long-term therapy with anticonvulsant,
phenyton.
• Alcohol
• The cytotoxic drugs methotrexate, which
inhibit the enzyme dihydrofolate reducates
and cause depletion of thymidin and
purine nucleotides.
Failure of folate metabolism
A number of rare enzyme deficiencies have
been reported which cause impairment of
folate metabolism. Most of these are
associated with megaloblastic changes
and mental retardation.
Pathophysiology
Patients with megaloblastic anemia typically display :
• pallor, weakness, shortness of breath, and congestive
heart failure. In some cases, loss of appetite, weight loss
and gastrointestinal disturbances.
• In addition to these non-specific changes, a range of
signs which are specific to megaloblastic patients, such
as those affecting tissue, where the tissue divide most
rapidly associated with impaired mitotic function and
premature cell death. These can be described under
three headings:
General tissue manifestations
• Deficiency of vitamin B12 or folic acid
affects all dividing cells but the effects are
manifest most clearly in rapidly dividing
tissues such as bone marrow and
epithelial cells. Disturbances in the
epithelial cells causes:
1- Angular stomatitis (lesions at the
corner of the mouth)
2- Glossitis (inflammation of the tongue)
Neurological manifestations
• Degeneration of the dorsal and lateral columns
of the spinal cord are typical findings in sever
megaloblastic anemia due to deficiency of
vitamin B12. The mechanism is not known yet.
• Folic acid deficiency in pregnancy is associated
with the incidence of neural tube defects such as
spina bifida and is also believed to lead to mild
dementia and impairment of intellectual
function.
Haematological manifestations
• The megaloblastic bone marrow is hypercellular, with an increased
in erythropoietic activity. There is immature forms of all cell lines,
and premature death of cells in the process of development. This is
known as ineffective haemopoiesis, and responsible for the
pancytopenia, which characterizes this condition.
• The increased cell turnover leads to increase in the concentration of
unconjugated bilirubin, LDH and lysozyme.
• Peripheral blood shows macrocytosis and the presence of the late
megaloblast. Reticulocytopenia is present.
• Megaloblastic leucopoiesis is reflected by the appearance of bizarre,
giant metamylocyte in the bone marrow, and an increased in the
circulating hypersegmented granulocytes.
• Morphological changes in megaloblastic megakaryocytes include an
increase in cell size and failure of cytoplasmic granulation. However,
these changes often are indistinct.
Summary
Practical approach to Macrocytosis
• History for alcohol, liver
• CBC and blood film for evidence of
marrow disease
• Reticulocyte count
• B12/Folate levels
• Liver function, TSH
• Bone marrow exam if cause in doubt and
you really want to know
Low Hb=Anemia
MCV
Low=microcytic
High=macrocytic
Normal=normocytic
Ferritin
Fe deficient
Establish
cause
Measure B12 + folate
Fe normal
Normal
Anemia of
chronic disease or
hemoglobinopathy
Obvious
cause
Low Establish cause
Cause not obvious
Consider bone marrow
Reticulocyte count
Hemolysis
or blood
loss
high
low
Anemia of chronic disease
Renal failure
Marrow failure
Normal
oval macrocytes
hypersegmented
polymorph
Normal marrow cells
Megaloblastic marrow cells
Megaloblastic marrow cells
giant metamyelocyte
Vitamin B12 deficiency causes demyelination in
the spinal cord and peripheral nerves
Vitamin B12 and folate
All you need to know
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Both vitamin B12 and folate deficiency cause an identical megaloblastic anemia
Vitamin B12 deficiency causes demyelination in the spinal cord and peripheral
nerves
It takes about 3 months to run out of folic acid, and 3 years to run out of vitamin
B12
Folate is in meat and vegetables (foliage) and is absorbed from the jejunum
Vitamin B12 is only in foods of animal origin; its absorption from the terminal ileum
requires a specific binding protein called intrinsic factor
Folate deficiency results from poor diet, malabsorption or increased requirements
Vitamin B12 deficiency is commonly the result of Pernicious Anemia
Pernicious Anemia results from an autoimmune attack on gastric parietal cells
causing achlorhydria and Intrinsic Factor deficiency
Low vitamin B12 levels are common in the elderly and usually do not cause
anemia: they should be managed with oral vitamin B12 replacement
Deficiency of vitamin B12 and folic acid can be diagnosed by measuring serum
levels; homocysteine and methylmalonic acid levels may help
Folate can be replaced orally. Vitamin B12 is traditionally given parenterally, but
high dose oral therapy also works for Pernicious Anemia
Increasing dietary folate intake in a population lowers its average level of serum
homocysteine: the daily dose for a maximum effect is 400 mcg
Neural tube defects can be prevented by using folate supplements in women
intending to become pregnant
FOLATE/COBALAMIN
Properties
Property
Folic Acid
Cobalamin
Food Source
Almost all foods
Animal protein only
Water soluble
Yes
Yes
Site of absorption
Duodenum/Jejunum Ileum
Mech of absorption Deconjugation of
Uptake of IF-Cbl
poly-Glu
complex
Metabolic Function One Carbon
Unknown
transfers
Body stores
4-5 months
2-12 years
Dietary deficiency Common
Rare
Deficiency states
Megaloblastic
Yes
Yes
anemia
Neurologic
No
Yes
disease