6. Vitamin B12 and Folate Flashcards
sources of vitamin B12
red meat, poultry, cheese, fish, not really vegetables
why do we need vitamin B12
key role in maintaining normal function of the brain and nervous system as well as the formation of blood cells
normally involved in the metabolism of almost all cells of the body
key role in DNA synthesis and regulation
absorption and transport of vitamin B12
-Intrinsic factor IF is produced by gastric parietal cells
-B12 binds to IF and the B12/IF complex moves to the ileum
binds to receptor on enterocyte and is then absorbed
-B12 is then released into circulation
-IF is not recycled
Post enterocyte circulation
-B12 binds to transcobalamin TC in circulation
Surface of receiving cell
-is the B12/TC bound complex
Once inside the cell we get the lysosomal degradation of TC and release of B12
vitamin B6
pyridoxine
essential co factor: conversion of succinyl coA and glycine into ALA (aminolevulinic acid) by ALA synthase
common in fruit, vegetables, cereals and meat
passive absorption in jejunum and ileum
recommended daily requirement: 1.5- 2mg
metabolic pathway of B6 and B12 in the mitochondrion
propionyl-coA to methylmalonyl-coA, this is converted to succinyl-coA by the actions of B12 and mm-mutase
succinyl-co A is then converted to ALA through the passive action of B6 and then ends as haem
If we see excess methylmalonyl coA we know there is a deficiency in B12 as methylmalonyl is not being converted
metabolic pathways of B12
vitamin B12 co factor for two biochemical reactions in the body:
- assists in the conversion of L- methylmalonyl co enzyme A to succinyl coenzyme A (vital in Haem synthesis)
- assists in the conversion of homocysteine to methionine which is needed for the production of cysteine and lots of proteins in the body
how much B12 do we need to absorb daily
to maintain body stores we need 1-3ug, the minimum to maintain sufficient health is 0.5ug but if increased demand we need stores
Only 1000-3000 units of IF needed (average basal IF secretion is 3000 units per hour, daily ~ 50,000 units)
storage and loss of vitamin B12
Storage
-Liver
-Healthy adult - total body content 3-5mg
Loss
-urine and faeces: desquamation of epithelial and excretion in bile
-Rate of loss: 0.05-0.1% of body content each day
folates
- Consist of a larger number of compounds
- Folic acid :also known as Vitamin B9
- Humans are incapable of synthesizing folate so all of folate intake comes from the dietary intake.
- Folates are found in both animal and plants.
vitamin B9 rich food
lentil kidney beans broccoli beets spinach cauliflower
B9 extra facts
Bakers yeast: 1930s led to the identification of folate as nutrient needed to prevent anaemia during pregnancy
Fortified breakfast cereals: 25% - 100% of the RDA for folic acid.
why do we need folate
- Synthesis of purine/pyrimidine precursors of DNA
- Especially important during periods of rapid cell division and growth –infancy / pregnancy
- Production of healthy red blood cells
activation, absorption and transport of folic acid
- folic acid (folate) not the active form, it needs to be reduced to tetrahydrofolate (THF)
absorption of folates takes place in the duodenum and jejunum
absorbed folates are converted into 5-methyltetrahydrofolate monoglutamate before entering the portal blood system
plasma folate circulates bound or unbound to plasma proteins, supports the blood system and allos movement from from intestines into circulation to the liver and around the body
causes of vitamin B12 deficiency
pernicious anaemia and veganism
pernicious anaemia: impairs absorption of vitamin B12 as a consequence of a reduction in intrinsic factor. you cant absorb B12 without IF
vegans: no B12 in plants . for vegans it is a deficiency due to diet
consequences of folate and vitamin B12 deficiency
closure of the neural tube occurs around the 28th day of pregnancy
the incidence of neural tube defects (spina bifida and anencephaly) reduced by 400ug folic acid supplements a day before conception and during the first month of pregnancy
-neurological complications: the progressive demyelination of nerve cells is thought to result from the increase in methymalonyl coS that results from vitamin B12 deficiency (tingling and numbness)
clinical features of folate and B12 deficiency
- Mild jaundice: ineffective erythropoiesis
- Neuropathy: affects the spinal cord and peripheral nerves
- Patients commonly feel tingling in their feet with difficulty in gait
- Glossitis (inflammation or infection of the tongue)
Haematological disorders due to B12 or Folate Deficiency
Megaloblastic anaemia due to folate deficiency
Pernicious anaemia is megaloblastic anaemia due to vitamin B12 deficiency
Why “Mega”loblastic anaemia?
- Deficiency of folate reduces conversion of dUMP to dTMP - rate limiting step in DNA synthesis
- DNA synthesis delayed and chromosomes breaks common
- RNA not affected to the same degree, so the protein synthesis in cytoplasm continues at a much faster rate than the DNA replication.
- Delays cell division
- Rapidly dividing cells need to replicate their DNA.
- We need to produce 2 x 1011 red cells per day to replace those destroyed.
- But DNA synthesis delayed, so cells keep getting bigger but division delayed resulting in MEGA blasts in the bone marrow and large red cells in blood
- large MCV. macrocytic but low rbc levels
haematological features of megaloblastic anaemia - what do we see in peripheral blood
- Macrocytosis (increased size of red blood cells)
- Increased MCV >90fl
- decrease in the number of cell divisions prior to the -loss of the nucleus in red cell precursors
- less rbc
- oval macrocytes - these large red cells tend to be oval in shape
- increased lobe number (>5) in nucleus of neutrophils - probably due to structural abnormalities with chromatin
haematological features of megaloblastic anaemia - what do we see in peripheral blood
- Macrocytosis (increased size of red blood cells)
- Increased MCV >90fl
- decrease in the number of cell divisions prior to the -loss of the nucleus in red cell precursors
- less rbc
- oval macrocytes - these large red cells tend to be oval in shape
- increased lobe number (>5) in nucleus of neutrophils - probably due to structural abnormalities with chromatin
pernicious anaemia
Megaloblastic anaemia caused by lack of intrinsic factor
Aetiology
- autoimmune attack on the gastric mucosa
- Stomach wall becomes thin, reduces gastric secretion of acid and intrinsic factor.
Antibodies
-Antibodies against gastric parietal cells are found in the serum in about 85% of patients
tests and treatment for B12 deficiency
if we find the MCV is increased, then we test for methylmalonyl coA
-historically absorption is measured indirectly by using the Schilling technique which used radioactive labels, no longer widely available
Treatment
vitamin B12 levels are replenished by intramuscular injection of the vitamin
not given orally as this bypasses the gi tract and goes directly into the circulation
megaloblastic anaemia treatment
folate deficiency
- oral dose of folic acid
- may aggravate neuropathy if B12 deficiency present
B12 deficiency
- intramuscular injection of hydroxycobalamin
diagnosis
in patients with megaloblastic anaemia, vitamin B12 deficiency must be ruled out before treating with folate
if vitamin B12 deficiency is present, folate supplementation can alleviate the anaemia but does not reverse neurologic deficits
full blood count/ blood film: may indicate megaloblastic anaemia indistinguishable from that of vitamin B12 deficiency
