Vitamin B12 Flashcards
B12 structure
Corrinoid ring, called Cobalamin
Describe B12’s relationship with folate
Folate is in inactive form in cells
B12 removes and keeps folates methyl group, activating it.
Now both folate and B12 are active and available for DNA synthesis.
Main function of B12
Protein metabolism, formation of RBCs and maintenance of CNS
Source of B12
Made by bacteria.
Get it from contaminated animal products.
We can get it from fortified grains
RDI for B12
2.4 micrograms per day
People at risk of deficiencies
Vegan and vegeterians.
Digestion of B12
- Consume protein which has cobalamin attached
- HCl and pepsin break down protein in stomach and cobalamin is released.
- Cobalamin attached to R protein (hepatocorrin, from parietal cells)
- Travels to duodenum
- Pancreatic enzymes release cobalamin from hepatocorrin.
Absorption of B12
- In the duodenum cobalamin bind to Intrinsic Factor (made by parietal cells)
- Bound Cbl and IF travel to terminal ilium where IF receptors are located
- Complex absorbed into enterocytes and IF released from it
- B12 then travels as transcobalamin where it can be taken up by all the cells for DNA synthesis.
Where is B12 stored
In the liver in a hepatocorrin complex
What does a deficiency in B12 cause?
Megoblastic or macrocytic anaemia
Name neuropathy in B12
Subacute combined degeneration of the spinal cord
What happens to reticulocytes with anaemia?
Bone marrow makes more as a compensatory measure so the count should go up.
If there is an anaemia and no increase in reticulocytes there may be bone marrow pathology
Symptoms of severe anaemia
SOB
Tiredness
Normal requirements for erythropoiesis
Iron, folate, B12
Symptoms of B12 deficiency
Tingling in fingers
Concentration problems
Describe pernicious anaemia
An autoimmune disorder that specifically effects the parietal cells and IF. Antibodies attach to the parietal cells or to IF and render them inactive.
Ileil problems resulting in B12 deficiency
- Surgery on terminal ileum
- Inflammation on the terminal ileum (Chrons)
Describe disadvantages of the Shillings test (not really used)
Involves radioisotopes
Involves 24 hr urine
Results difficult to interpret
Test used to detect pernicious anaemia
- Antibody blood test to parietal cells (low sensitivity) and intrinsic factor (low specificity)
- Gastric biopsies looking for autoimmune gastritis
- Evidence of other autoimmune disease
- Check acid levels
Effects of distal illeal resection
- Failure to absorb B12 (where IF factors are present on enterocytes)
- Diarrhoea due to failure to absorb bile salts in terminal ileum, bile salts enter the colon which irritate the colon causing secretory diarrhoea.
- Fecal fat due to reduced bile salts (distal ileum is where bile salts are reabsorbed) and impaired absorption of fat.
Why would having low gastrin effect B12 absorption?
G cells release gastrin. Gastrin stimulates HCL cells and parietal cells to release histamine and gastric acid respectively. Where there is low acid the B12 is not adequately released from the protein food source.
Explain why there is reduced pancreatic enzymes if low gastrin and consequence of this.
Gastrin has a role in stimulating pancreatic secretion.
Pancreatic enzymes remove B12 from R binders (hepatocorrin) for binding to IF.
Atrophic gastritis
Result of bile reflux, stomach becomes inflamed, body of stomach atrophies. Loss of parietal cells, loss of IF.
