Heparin is a polymer classified as a mucopolysaccharide or a glycosoaminoglycan. It is biosynthesized and stored in mast cells of various mammalian tissues, particularly liver, lung and mucosa. Commercial heparin is chiefly isolated from beef lung or pork intestinal mucosa.

Heparin has been used for years as an anticoagulant. Heparin binds to antithrombin III, a naturally occurring plasma protease inhibitor, accelerating significantly the rate at which antithrombin III (AT-III) inhibits coagulation proteases (factor Xa and thrombin).

The activity of heparin as an anticoagulant has been shown to be related to the molecular weight; in the range of 6-12 kDa, heparin apparently binds to AT-III in a 1:1 stoichiometry. However, porcine heparin of MW 20 kDa can have two binding regions for AT-III, but the probability of a third region is negligible.7,8 There is a correlation between molecular weight and anticoagulant activity, but it is linear only over a narrow range (8-12 kDa).

The action of heparin is clinically monitored using an activated partial thromboplastin time (APTT); these two products are not tested using APTT activity. Low molecular weight heparins (below approximately 8000 Da) inhibit AT-III but have a higher ratio of anti-factor Xa to anti-AT-III activity than regular heparin. They have lowered effect on platelet aggregation than normal heparin, and no significant effect on blood coagulation tests such as APTT. Dosages of these products cannot be equated to those of "normal molecular weight heparin".

Merck 14,4653