Effect of Dietary Vitamin K on Avian Brain Thromboplastin Activity

IN the classical bioassay for vitamin K, plasma prothrombin times of chickens fed assay material are compared to those of chickens receiving graded levels of the vitamin. The assay is based on the assumption that the velocity of the clotting process is a function of the level of prothrombin (and factor VII) in the blood, and that the levels of these vitamin K-dependent factors reflect the amount of ingested vitamin K. For this bioassay the growing chicken is the animal of choice, since intestinal synthesis of vitamin K, of importance in some species, plays only a minimal role in the chicken’s vitamin K supply. In the one-stage prothrombin time determination (Quick, 1966), the clotting of decalcified plasma is timed after the simultaneous addition of Ca++ and a source of thromboplastic activity (“thromboplastin extract”), the latter supplying the phospholipid-protein complex necessary for the conversion of prothrombin to thrombin. Acetone-extracted chicken brain…