Exists in several forms:
phylloquinone (K1) found in
plants and menaquinone (K2)
formed by bacteria.
Menadione (K3) is a synthetic
derivative for therapy, used in
the form of intramuscular or
intravenous injections
All neonates (sterile GIT) should
be given a single prophylactic
dose of vitamin K as
phylloquinone by mouth
Sometimes
vitamin K is
administrated
before
operations to
reduce bleeding
in surgery
Anticoagulants (dicumarol and warfarin) are
designed to work as vitamin K antagonists
Developed for clinical management
in patients at risk of thrombosis
Functions
Vitamin K-dependent
carboxylase activity is
necessary for
formation of:
γ-carboxyglutamate
residues
Enhances osteocalcin (protein in bone
matrix, containing γ-carboxyglutamate
residues) synthesis → functions as Ca
binding protein and modifies
crystallization of bone.
Synthesis of at least 4 blood clotting factors
(contain γ-carboxyglutamate residues),
among them prothrombin (factor II), the
zymogen of blood coagulation enzyme
thrombin → interaction with platelets →
blood clotting
Role of Vitamin K
in Blood
Coagulation
At least 13 proteins and Ca
ions are involved in blood
clotting
γ-carboxyglutamate residues in
blood clotting factors chelate Ca ions
and so permit their binding to lipid
membranes of platelets
In vitamin K deficiency or in the presence of its
antagonist → pre-prothrombin is released into
circulation
Pre-prothrombin is an abnormal precursor of
prothrombin containing few or no
γ-carboxyglutamate residues
Pre-prothrombin cannot chelate Ca ions or bind to
lipid membrane and so is unable to initiate blood
clotting
Deficiency
Deficiency causes increased chance of internal
bleeding (after a minor cut) and uncontrollable
haemorrhage (following a slight trauma).
The only known symptom is bleeding tendency with a prolonged PT (prothrombin time)
Combination of antibiotics, inadequate vitamin K intake,
and surgery raises a warning flag that clotting time should
be checked before surgery is performed !!
Toxicity
Excessive bleeding produced by the administration of
oral anticoagulant (overdose) can be corrected by
vitamin K administration
However, high doses of vitamin K can reduce the
effectiveness of anticoagulants used to prevent blood
clotting
Symptoms are hemolytic anemia
and jaundice in infants