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Proficiency testing surveys in the state of New York indicate that despite increased sophistication in instrumentation, there has been no real improvement in interlaboratory reproducibility in prothrombin-time determinations over the last 10 years. This lack of improvement most pronounced in the therapeutic range of 20 to 30 sec. One reason may be that between types produced by the same manufacturer. While it has been possible in several laboratories to synthesize experimentally a thromboplastin with known content of lipid and active protein, no efforts have been made to make such a product by the same manufactures. While it has been made to make such a product commercially available. Blood levels of of warfarin were measured but cannot be reliably used to monitor anticoagulation. In a preliminary study, factor Xa activity was measured using chromogenic substrate S2222. Factor Xa activity gave a positive correlation with prothrombin times of patients receiving warfarin therapy. Chromogenic substrate factor assays may represent a future method of choice for controlling anticoagulant therapy.

The dithiothreitol-dependent vitamin K and vitamin K 2,3-epoxide hepatic microsomal reductase activities of warfarin-susceptible and warfarin-resistant rats were compared to gain insight into the role(s) of these activities in vitamin K metabolism and function. In microsomes from resistant rats, 3- to 4-fold more warfarin was required to produce 50% inhibition (I50) of vitamin K reduction to vitamin K hydroquinone than in microsomes from susceptible rats. For the reduction of vitamin K 2,3-epoxide to vitamin K a 6-fold higher warfarin concentration was required. In microsomes from resistant rats, the I50 warfarin concentration required to inhibit gamma-carboxylation of microsomal precursor protein was also 4-fold higher with vitamin K as substrate and was 6-fold higher with the epoxide as substrate than in microsomes from susceptible rats. Collectively, these data suggest that the vitamin K reductase contributes to the metabolism of vitamin K in intact rats and that warfarin inhibition of both the vitamin K and vitamin K 2,3-epoxide reductases is involved in its anticoagulant effect.

Mouse Lewis lung (LL) carcinoma cells possess a factor X activator (procoagulant) that is inhibited in vivo by warfarin treatment or diet-induced vitamin K deficiency. This inhibition suggests that vitamin-K-dependent proteins are involved in LL cell activation of factor X. A LL primary tumor clone (LL13) was isolated which contained a warfarin-sensitive vitamin-K cycle of metabolism and expressed factor X procoagulant activity. LL13 cells exposed to media containing warfarin or deficient in vitamin K grew as well as cells in normal media, and activated factor X to similar extents. In contrast, administration of warfarin to mice bearing LL13 cells inhibited factor X procoagulant activity as well as the vitamin K cycle of metabolism in the primary tumors. In relation to LL13 cells grown in media containing fetal bovine serum, those incubated for 20 hr in media containing mouse serum or the sera from LL13-bearing mice exhibited 9- to 10-times higher levels of factor X procoagulant activity. However, LL13 cells exposed to media containing the sera of warfarin-treated LL-13-bearing mice or to barium-sulfate-adsorbed normal mouse serum activated factor X much less efficiently. Collectively, these data suggest that inhibition of vitamin K function in LL cells does not affect the extent of factor X activation and thus the intrinsic factor X procoagulant is not a vitamin-K-dependent protein. They further suggest that both a warfarin-sensitive (vitamin-K-dependent) protein present in normal mouse serum and a LL13 cell component participate in factor X procoagulant activity.

A newly recognized metabolite of vitamin K1, vitamin K1 chromenol, is produced when the vitamin is added to the plasma or serum of a number of species. The metabolite was identified by comparison of its uv and mass spectra and high-performance liquid chromatographic retention times with those of the synthetic vitamin K1 chromenol. In aqueous solution vitamin K chromenol decomposed to a variety of products and reacted with nucleophilic substances. Optimal conditions for its formation and evidence that chromenol formation may be an enzyme catalyzed reaction are presented.

Vitamin K and 3- (and/or 2)-hydroxy-2,3-dihydro-2-methyl-3-phytyl-1,4-naphthoquinone (hydroxyvitamin K) have been identified as metabolites of vitamin K 2,3-epoxide incubated with hepatocytes isolated from normal and warfarin-resistant rats. Dithiothreitol added to the extracellular medium differentially enhanced the formation of both metabolites: hydroxyvitamin K formation, almost undetectable in the absence of dithiothreitol, was particularly affected. Addition of the vitamin K 2,3-epoxide reductase inhibitors warfarin (5 to 100 microM) and brodifacoum (1 to 5 microM) to normal rat hepatocyte cultures produced a slight increase in hydroxyvitamin K formation and a marked inhibition of vitamin K formation. Brodifacoum was a weak inhibitor of hydroxyvitamin K formation at higher concentrations. Hepatocytes from warfarin-resistant rats catalyzed hydroxyvitamin K formation 1.5 to 2 times faster and vitamin K formation 1.5 to 2 times slower than did normal rat hepatocytes. The addition of warfarin to these cultures had no effect on epoxide metabolism to hydroxyvitamin K and only partially diminished metabolism to vitamin K. In contrast, brodifacoum (1 microM) addition produced 50% inhibition of hydroxyvitamin K formation and almost complete inhibition of vitamin K formation. These data suggest that in resistant, but not in normal rat hepatocytes, the vitamin K 2,3-epoxide reductase makes a significant contribution to hydroxyvitamin K formation. A second sulfhydryl-dependent pathway, present in both strains, is also involved in the formation of this metabolite. They also suggest that in resistant rats, warfarin inhibition of the vitamin K 2,3-epoxide reductase, and presumably the sulfhydryl-dependent vitamin K reductase, is incomplete and independent of concentration.