Coumarin Derivatives A

Vitamin K promotes the hepatic Y-car-boxylation of glutamate residues on the precursors of factors II, VII, IX, and X, as well as that of other proteins, e.g., protein C, protein S, or osteocalcin. Carbox-yl groups are required for Ca2+-mediat-ed binding to phospholipid surfaces (p. 142). There are several vitamin K derivatives of different origins: K1 (phy-tomenadione) from chlorophyllous plants; K2 from gut bacteria; and K3 (menadione) synthesized chemically. All are hydrophobic and require bile acids for absorption.

Oral anticoagulants. Structurally related to vitamin K, 4-hydroxycouma-rins act as "false" vitamin K and prevent regeneration of reduced (active) vitamin K from vitamin K epoxide, hence the synthesis of vitamin K-dependent clotting factors.

Coumarins are well absorbed after oral administration. Their duration of action varies considerably. Synthesis of clotting factors depends on the intrahe-patocytic concentration ratio of cou-marins to vitamin K. The dose required for an adequate anticoagulant effect must be determined individually for each patient (one-stage prothrombin time). Subsequently, the patient must avoid changing dietary consumption of green vegetables (alteration in vitamin K levels), refrain from taking additional drugs likely to affect absorption or elimination of coumarins (alteration in cou-marin levels), and not risk inhibiting platelet function by ingesting acetylsali-cylic acid.

The most important adverse effect is bleeding. With coumarins, this can be counteracted by giving vitamin K1. Coagulability of blood returns to normal only after hours or days, when the liver has resumed synthesis and restored sufficient blood levels of clotting factors. In urgent cases, deficient factors must be replenished directly (e.g., by transfusion of whole blood or of pro-thrombin concentrate).

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