Vitamin K

Question 1

Vitamin K produced by bacteria

Answer 1

Colonic bacteria provide a large source of menaquinones but it is not enough to meet our needs

Question 2

Types of vitamin K found in diet

Answer 2

Phylloquinone - found in plants and oils

Menaquinones - some in animal products

Question 3

Absorption of vitamin K

Answer 3

Phylloquinone is absorbed mainly in the jejunum

It requires normal fat digestion and absorption

Question 4

Functions of Vitamin K

Answer 4

It is required for post translational modifications of specific proteins
-carboxylation of specific glutamic acid (GLU) residues to generate y-carboxyglutamate (GLA) residues– it becomes functional

Also essential for blood clotting and bone mineralization

Question 5

What is the active form of vitamin K?

Answer 5

hydroquinone (KH2)

Question 6

How is hydroquinone produced?

Answer 6

Through reductases that require dithiols or NADPH

Question 7

What requires hydroquinone?

Answer 7

K-dependent enzymes in order for carboxylation to occur

-epoxide is generated

Question 8

What does blood clotting require?

Answer 8

Requires that fibrinogen be converted to fibrin (active)
-Fibrin aggregates to form an insoluble polymer that is cross-linked with stabilizing factors to form a clot to stop bleeding

Question 9

Blood - cells =

Answer 9

Plasma

Question 10

Blood - cells - fibrinogen =

Answer 10

serum

Question 11

What catalyzes the reaction to form fibrin?

Answer 11

The reaction of prothrombin to thrombin (active)

Question 12

What two pathways convert pro-thrombin to thrombin?

Answer 12

Extrinsic (tissue factor exposed)

Intrinsic (collagen exposed)

Question 13

Vitamin K dependent carboxylated proteins that are involved in blood clotting

Answer 13

Factor VII - extrinsic pathway
Factor IX - intrinsic pathway
Factor X - Stuart-prower factor; both pathways
Factor II - thrombin, both pathways

All require GLA residues and CA to interact with phospholipids

Question 14

Vitamin K dependent anti-clotting factors

Answer 14

Protein C -associates with thrombomoudlin/calcium in the presense of thrombin & protein Ca
Protein S - interacts with Protein Ca and calcium to inactivate factors Va and VIIa
Protein X- unknowin function

Question 15

What are anticoagulants?

Answer 15

Coumarin (in licorice and lavender)
It is used for people at risk for thrombosis and embolism
Causes huge problems in pregnant women (stillbirth, abortion, skeletal fractures)

Question 16

Two Vitamin K dependent proteins in bone

Answer 16

Osteoclacin (bGP) - bone remodeling & Ca mobilization

Matrix GLA protein (MGP) - Ca mobilization and prevents calcification of cartilage

Question 17

How is the synthesis of the two vitamin k dependent proteins stimulated

Answer 17

by 1,25-(OH)2D3 (calcitriol) and retinoic acid

Question 18

Who is at risk for vitamin K deficiency?

Answer 18

-It is extremely unlikely in healthy individuals
-Populations at risk are:
GI malabsorptive disorders (biliary fistula, obstructive jaundice and liver disease)

Newborn infants - poor placental transfer and breast milk is low in vitamin K

Chronic antibody consumption

Question 19

How is vitamin K assessed?

Answer 19

Plasma level of phylloquinone
Blood clotting times
Measurement of undercarboxylated proteins (prothrombin or osteocalcin)

Question 20

What form of the vitamin do K-dependent enzymes require?

Answer 20

Hydroquinone

Carboxylation can then occur and epoxide is generated