U01.01.085 Vitamin K

Written by

in

 

Learning Objectives

  • Identify the clotting factors and proteins that require Vitamin K for maturation.
  • Explain the mechanism of gamma-carboxylation and its inhibition by Warfarin.
  • Recognize the clinical presentation and prevention of Hemorrhagic Disease of the Newborn.
  • Distinguish Vitamin K deficiency from other coagulation disorders using PT/aPTT labs.

1. Biochemical Function: The Carboxylation Step

Vitamin K (Phylloquinone/Menaquinone) acts as a critical cofactor for the \gamma-carboxylation of glutamic acid residues. This modification allows clotting factors to bind calcium and localize to phospholipid membranes during coagulation.

  • Activation: Vitamin K must be in its reduced form to act as a cofactor. It is activated by the enzyme Epoxide Reductase.
  • Warfarin Mechanism: Warfarin inhibits Epoxide Reductase, preventing the recycling of Vitamin K and thus halting the synthesis of functional clotting factors.

High-Yield Mnemonics:

  • K is for Koagulation.
  • Factors 10, 9, 7, 2 (1972) + Proteins C and S.

2. Neonatal Physiology & Deficiency

Neonates are uniquely predisposed to Vitamin K deficiency due to three main factors:

  1. Sterile Intestines: They lack the gut microbiota necessary to synthesize Vitamin K.
  2. Poor Placental Transfer: Very little Vitamin K crosses the placenta.
  3. Breast Milk: Human milk is a poor source of Vitamin K (and Vitamin D).
Memory Hook: Breast-fed infants don’t know about Vitamins D and K.

3. Clinical Findings & Lab Correlation

Deficiency leads to a bleeding diathesis known as Hemorrhagic Disease of the Newborn (or Vitamin K Deficiency Bleeding – VKDB).

Lab Parameter Result in Deficiency Reasoning
Prothrombin Time (PT) Increased (Prolonged) Factor VII has the shortest half-life and is affected first.
aPTT Increased (Prolonged) Factors IX, X, and II are part of the intrinsic/common pathways.
Bleeding Time Normal Platelet function and primary hemostasis are unaffected.

Secondary Causes: Prolonged use of broad-spectrum antibiotics (killing gut flora) or advanced liver disease (inability to utilize Vitamin K).

Activity

Activity