7

Question 1

BIOTIN, Vitamin B7 Sources

Answer 1

dietary & bacterial.
1. Diet: Widely distributed in plants (vegetables, legumes & grains) & animal
tissues (liver, kidney, milk, milk products and egg-yolk).
- Molasses & royal jelly (bee) contain a large amount of biotin. 2. Synthesized by intestinal flora.

Question 2

Absorption. Storage BIOTIN, Vitamin B7:

Answer 2

Absorption: Dietary protein-bounded biotin→ free biotin before absorption. § It is absorbed from jejunum & ilium by a sodium-dependent transporter.
Storage: small amounts in muscle, liver & brain.

Question 3

Functions of Biotin (Vitamin B
7

Answer 3

Biotin acts as a coenzyme for enzymes involved in carboxylation reactions, by moving a carboxyl group from one compound to
another.
These Enzymes include:
o Pyruvate carboxylase : converts pyruvate → oxaloacetate which is
essential for Krebs’s cycle & gluconeogenesis. o Acetyl-CoA carboxylase: converts acetyl CoA → malonyl CoA for FA
synthesis.

Question 4

Vitamin B7, Biotin Deficiency:

Answer 4

oMay occur during pregnancy.
oExcessive alcoholism.
oProlonged consumption of increased amount of raw egg white. Avidin found in raw egg white binds tightly to biotin and prevents its
intestinal absorption,
oLong-term total parenteral nutrition.

Question 5

Symptoms of biotin deficiency:

Answer 5

Dermatitis, glossitis, loss of appetite, nausea, alopecia, depression & hallucinations.
§ Biotin deficiency is teratogenic in several mammals.

Question 6

Folic Acid or Folate; Vitamin B9:
The active forms are :

Answer 6

• DHF, - THF, - 5-Methyl THF. - 5,10 Methylene THF.

Question 7

Functions of Folic Acid or Folate:

Answer 7

Folic acid or folate, plays a key role in “one-carbon metabolism”.
Folic acid acts as a coenzyme for:
o Synthesis of purine.
oSerine synthesis from glycine.
oRe-methylation of homocysteine to methionine.
(both require folic acid & vit B12)

Question 8

Folic Acid or Folate Deficiency:
Causes of deficiency:

Answer 8

1. Alcoholism: Chronic alcoholic patients are frequently deficient in one or
   more vitamins. The deficiencies commonly involve folate, vitamin B6,
   thiamine and vitamin A.
2. Increased needs (pregnancy & lactation).
3. Extensive small bowel disease.
4. Treatment with drugs which are dihydrofolate reductase inhibitors, as
   methotrexate.
5. Deficient intake or poor absorption.

Question 9

Folic Acid or Folate Deficiency: Clinical Aspects:

Answer 9

Folate & neural tube defects (NTD) in fetus: Anencephaly & spina bifida are the most common NTD.

Megaloblastic anemia : due to ↓ synthesis of purines (A & G) and thymidine, so cells can not divide as they are unable to make DNA.
v Atherosclerosis & ischemic heart disease:
Fo late ↓→↑ Homocysteine blood level→ ↑
risk of Atherosclerosis & ischemic heart disease.

Question 10

Excretion of folate:

Toxicity of folate:

Answer 10

urine & in feces. as a part of bile, but it is reabsorbed.
Toxicity of folate: § Rare; in reported cases one can have insomnia, irritability and GI
distress.

Question 11

Vitamin B12, Cobalamin:
Active forms of vitamin B12

Vitamin B 12 is composed of a

Answer 11

• Cyano-cobalamin.
• Methyl-cobalamin.
• 5’-Deoxy-adenosyl-cobalamin.

is composed of a corrin ring with a cobalt ion in
the center.

Question 12

Sources of Vitamin B
12, Cobalamin:

Answer 12

Vitamin B12is synthesized only by micro- organisms; so it is not found in plants.
only in animal foods

Question 13

Vitamin B12Absorption & Storage:

Answer 13

§B12 is released from food in acidic pH of stomach.
§ Free B12 then binds R-protein (a glycoprotein) & the
complex moves into the intestine.
§B12is released from R-protein by pancreatic enzymes & binds another glycoprotein, intrinsic factor (IF).
§ Cobalamin–IF complex travels through the intestine & binds to specific receptors on surface of mucosal cells in ileum.

Cobalamin is transported into mucosal cell → into the blood, carried byB 12-binding protein.
§B12is taken up and stored in the liver particularly.

Question 14

Vitamin B
12 Functions:

Answer 14

1. Remethylation of homocysteine → methionine.
2. Conversion of methyl-malonyl CoA→ succinyl-CoA
   plays a vital role in DNA synthesis, protein synthesis & fatty
   acid metabolism. needed for folate regeneration, so it is important for DNA synthesis

Question 15

cobalamin ↓ →

Answer 15

↓ of the tetrahydrofolate forms needed in
purine & thymine synthesis → megaloblastic anemia.

N 5-methyl form, which accumulates

Question 16

Vitamin B12Deficiency:

Answer 16

1-Megaloblastic anemia: due to Vitamin B12 ↓.
§ Accumulation of 5-methyl-THF & impaired DNA synthesis → continuing cell growth without division → megaloblastic anemia.
§ Risk for coronary heart disease due to homocysteine production.
2-Pernicious anemia: autoimmune condition and is due to the lack of intrinsic factor (IF) → inability to absorb vitamin B12.
anemic, but later on they show neuropsychiatric symptoms.

Question 17

Sy mptoms of vitamin B 12
deficiency are:

Answer 17

1. Impaired DNA synthesis, cell division & function.
2. Megaloblastic anemia.
3. Dementia & memory loss.
4. Numbness & peripheral neuropathies.
5. Spinal cord degeneration.

Question 18

Causes of Vitamin B12
Deficiency

Answer 18

§ Most causes of impaired absorption:
o Deficiency of intrinsic factor.
oMalabsorption (long term vegetarians; vegetarian children & infants).
oGIT disease (inflammatory bowel disease, gastrectomy…….etc.)
oProlonged use of H-blockers & proton pump inhibitors