Vitamin K

Question 1

What are the forms of Vitamin K?

Answer 1

Vitamin K1 (Phylloquinone): Found in green vegetables
Vitamin K2 (Menaquinone): Synthesized by intestinal bacteria
Vitamin K3 (Menadione): Synthetic, water soluble form that was used previously therapeutically but is now banned in most countries due to its toxicity

Question 2

What role does Vitamin K play in coagulation?

Answer 2

Vitamin K participates in the maturation process (post-translational modification of various blood clotting factors)
Glutamic acid residues of Clotting factors (II, VII, IX & X, Protein C and Protein S) undergo Vitamin K dependent carboxylation and are converted to their active state - gamma carboxyglutamic acid (Gla)
Gla and its 2 adjacent negative charged carboxylate groups are calcium binding sites
Prothrombin-Calcium complex binds to the phospholipids on the platelet surface accelerating the clotting process

Question 3

What is an antivitamin?

Answer 3

Prevents a vitamin from exerting its typical biological effects
Structural analogues to the vitamin
Function as competitive antagonists

Question 4

What are some examples of antivitamin K?

Answer 4

Dicoumarol (a naturally occurring anticoagulant)
Warfarin (synthetic analog of vitamin K)

Question 5

How is active Vitamin K generated from inactive Vitamin K?

Answer 5

The hydroquinine form (active Vitamin K) has to be regenerated from the epoxide form (inactive Vitamin K)

Question 6

How does antivitamins inhibit Vitamin K action?

Answer 6

Prevent regeneration of active vitamin K and thus inhibit vitamin K action
Exert anticoagulant effects by inhibiting the maturation of Vitamin K dependent blood clotting proteins

Question 7

What are the functions of antivitamin K?

Answer 7

Reduce blood coagulation in patients at risk of thrombosis related diseases (e.g. deep vein thrombosis, pulmonary thromboembolism)

Question 8

What is fetal warfarin syndrome?

Answer 8

Treatment of pregnant women with warfarin may lead to the development of bone abnormalities in children

Question 9

How is bone development dependent on Vitamin K?

Answer 9

Two proteins in the bone matrix contain γ-carboxyglutamate – Osteocalcin & bone matrix Gla protein
1-2% of non collagenous protein of bone is Osteocalcin
Functions as a calcium binding protein which help the bones to retain calcium

Question 10

RDA of Vitamin K

Answer 10

1 µg/kg body wt (60-80 g/day)

Question 11

What are the food sources which contain Vitamin K?

Answer 11

Bacteria in the jejunum and ileum synthesize vitamin K
Green leafy vegetables (spinach, asparagus), cabbage family, carrots, liver, egg

Question 12

What are some causes fo Vitamin K deficiency?

Answer 12

Long term antibiotic treatment which destroys the normal intestinal flora
Fat malabsorption
Obstructive jaundice

Treatment with Warfarin or Dicoumarol and other anticoagulants (drug dosage needs to be carefully adjusted to prevent bleeding)
Vitamin K deficiency is rare

Question 13

Signs and symptoms of Vitamin K deficiency

Answer 13

Easy bruising and bleeding from minor injuries
Characterized by low plasma prothrombin activity

Question 14

Lab findings associated with Vitamin deficiency

Answer 14

Increased PT and PTT: Normal bleeding time

Question 15

Why are neonates given a Vitamin K injection immediately after birth?

Answer 15

Prevents hemorrhagic disease

Question 16

Why are newborns at risk of Vitamin K deficiency?

Answer 16

Have a sterile GI tract so no intestinal synthesis occurs
Do not possess adequate liver stores of vitamin K due to inefficient placental transfer
Human milk too is low in Vitamin K

Question 17

Why is Vitamin K injected before surgery?

Answer 17

It is risky to perform surgery on patients with coagulation defect
Prothrombin activity is usually checked and Vitamin K1 is given as a precaution

Question 18

What are the potential side-effects of menadione?

Answer 18

Hemolytic anemia
Neonatal brain damage
Neonatal liver damage
Neonatal death

Menadione is a synthetic, water soluble variant of Vitamin K