Killer clots: warfarin and vitamin K
Dani Bancroft explores the role of proteins in blood clotting
Blood contains a range of different cells, including white blood cells (leucocytes), red blood cells (erythrocytes) and platelets (thrombocytes). It also contains other proteins and chemicals suspended in the fluid (plasma). When the wall of a blood vessel is damaged, blood has to clot; otherwise, the organism could bleed to death. The different components of blood work together to ensure that clotting – which is also known as coagulation – takes place.
A damaged blood vessel causes a cascade of reactions in the circulatory system that is intended to maintain homeostasis (optimum internal conditions).
Platelets are specialised cell fragments found in the blood. When circulating platelets encounter chemical factors released by damaged tissues, they are triggered to change shape. This causes the platelets to aggregate and pull together to form a clot (thrombus) using the stringy protein thrombin. The thrombus can then plug the tear to stop further bleeding.
Rowan McOnegal/Wellcome Images
This process is mediated by protein-based clotting factors, of which there are 12. They are named factor I to factor XIII (there is no factor VI) and have a variety of unusual nicknames, such as ‘Christmas factor’.
These clotting factors are essential in the clotting process, which is also known as coagulation. The absence or deficiency of any one in the cascade leads to severe bleeding (haemorrhaging) and can even lead to death.
Once they have been made, some of the factors require extra processing so they are able to stick platelets together during the formation of a clot. Specifically, they require carboxylation (the chemical addition of a –COOH group onto the protein factor). This reaction is catalysed by the liver enzyme γ-glutamyl carboxylase.
The enzyme γ-glutamyl carboxylase requires the cofactor vitamin K to be able to catalyse this carboxylation reaction. A steady supply of vitamin K, which is found in high concentrations in green leafy vegetables, is needed in the diet for the liver to produce these clotting factors.
Warfarin in medicine
Warfarin is an anticoagulant (anti-clotting) drug from a class of compounds called coumadins, which are derived from the plant sweet clover. It was first discovered in the 1920s when a mysterious illness was noticed at a cattle ranch in the USA. After eating sweet clover fodder, the cows suffered fatal bleeding because they were unable to form clots in response to minor cuts or injuries.
Warfarin is now used as an anticoagulant drug to treat patients who have suffered tissue damage caused by the untimely activation of platelets to form a thrombus, or ‘embolism’. The tissue damage occurs because the clot causes a vascular blockage, leading to a temporary oxygen starvation in nearby tissues (ischaemia).
Thrombosis can occur at various different sites in the body’s circulatory system and has a specific medical name depending on its location. For example, in the lungs it is known as a pulmonary embolism; in the coronary arteries it is a myocardial infarction, or heart attack; in the lower leg, it is deep vein thrombosis (DVT); and in the carotid arteries leading to the brain, it is known as a cerebral infarction, or stroke.
Warfarin as a rodenticide
Warfarin disrupts the clotting cascade by preventing the carboxylation of certain clotting factors so that they do not function properly. This gives warfarin toxic characteristics when it is administered in high doses, which was exploited heavily during the 1950s when it was used as a pesticide to control rat populations.
Rats that ingested bait laced with warfarin suffered internal bleeding and death because the warfarin altered the active site of vitamin K reductase so that it was no longer complementary to vitamin K. This prevented the recycling of the enzyme γ-glutamyl carboxylase, which meant that clotting factors were not formed properly.
Eventually, however, resistance to warfarin developed within the rat population, meaning it is no longer effective. This has triggered demand for more effective and lethal coumadin-based replacements, such as brodifacoum, to control rat populations in urban environments.
Symptoms similar to the effects of warfarin are seen in the sex-linked genetic disease haemophilia A, in which there is a genetic deficiency in functional clotting factor VIII. Whenever someone with haemophilia A suffers a scrape or cut, a thrombus cannot form: without treatment, the person could bleed to death.Lead image:
Annie Cavanagh/Wellcome Images CC BY NC ND
- University of Bristol article on warfarin and rat poison
- New Scientist article on resistance to rat poison
- NHS Choices article on the uses of the drug warfarin
Questions for discussion
- What other diseases can be caused by clotting factor deficiency?
- Why is the resistance of rats to pesticides a concern for public health?