Vitamin B12

Question 1

B12 structure

Answer 1

Corrinoid ring, called Cobalamin

Question 2

Describe B12’s relationship with folate

Answer 2

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.

Question 3

Main function of B12

Answer 3

Protein metabolism, formation of RBCs and maintenance of CNS

Question 4

Source of B12

Answer 4

Made by bacteria.
Get it from contaminated animal products.
We can get it from fortified grains

Question 5

RDI for B12

Answer 5

2.4 micrograms per day

Question 6

People at risk of deficiencies

Answer 6

Vegan and vegeterians.

Question 7

Digestion of B12

Answer 7

1. Consume protein which has cobalamin attached
2. HCl and pepsin break down protein in stomach and cobalamin is released.
3. Cobalamin attached to R protein (hepatocorrin, from parietal cells)
4. Travels to duodenum
5. Pancreatic enzymes release cobalamin from hepatocorrin.

Question 8

Absorption of B12

Answer 8

1. In the duodenum cobalamin bind to Intrinsic Factor (made by parietal cells)
2. Bound Cbl and IF travel to terminal ilium where IF receptors are located
3. Complex absorbed into enterocytes and IF released from it
4. B12 then travels as transcobalamin where it can be taken up by all the cells for DNA synthesis.

Question 9

Where is B12 stored

Answer 9

In the liver in a hepatocorrin complex

Question 10

What does a deficiency in B12 cause?

Answer 10

Megoblastic or macrocytic anaemia

Question 11

Name neuropathy in B12

Answer 11

Subacute combined degeneration of the spinal cord

Question 12

What happens to reticulocytes with anaemia?

Answer 12

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

Question 13

Symptoms of severe anaemia

Answer 13

SOB

Tiredness

Question 14

Normal requirements for erythropoiesis

Answer 14

Iron, folate, B12

Question 15

Symptoms of B12 deficiency

Answer 15

Tingling in fingers

Concentration problems

Question 16

Describe pernicious anaemia

Answer 16

An autoimmune disorder that specifically effects the parietal cells and IF. Antibodies attach to the parietal cells or to IF and render them inactive.

Question 17

Ileil problems resulting in B12 deficiency

Answer 17

• Surgery on terminal ileum

- Inflammation on the terminal ileum (Chrons)

Question 18

Describe disadvantages of the Shillings test (not really used)

Answer 18

Involves radioisotopes
Involves 24 hr urine
Results difficult to interpret

Question 19

Test used to detect pernicious anaemia

Answer 19

• 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

Question 20

Effects of distal illeal resection

Answer 20

• 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.

Question 21

Why would having low gastrin effect B12 absorption?

Answer 21

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.

Question 22

Explain why there is reduced pancreatic enzymes if low gastrin and consequence of this.

Answer 22

Gastrin has a role in stimulating pancreatic secretion.

Pancreatic enzymes remove B12 from R binders (hepatocorrin) for binding to IF.

Question 23

Atrophic gastritis

Answer 23

Result of bile reflux, stomach becomes inflamed, body of stomach atrophies. Loss of parietal cells, loss of IF.