Vitamin-K-dependent proteins in microsomes of primary Lewis lung tumors

Vitamin K 2,3-epoxide reductase: the basis for stereoselectivity of 4-hydroxycoumarin anticoagulant activity

1. The administration of S-warfarin (1 mg kg-1 i.v.) to rats that were pre-loaded 48 h before with tracer doses (6 micrograms) of 14C-labelled R- or S-warfarin caused the plasma levels of these compounds to increase. This is due to the substitution of the microsomal (vitamin K 2,3-epoxide (K0) reductase) bound R- or S-[14C]-warfarin by the unlabelled 4-hydroxycoumarin administered. The rate of reappearance was 3-4 fold higher for R- than for S-warfarin; t1/2 of release: 1.2 +/- 0.04 and 3.7 +/- 0.6 h, respectively. 2. Liver microsomes prepared from rats pretreated with R- or S-[14C]-warfarin, released these compounds only in the presence of dithiothreitol (DTT; 10 mM). The rate of release was higher for R- than for S-warfarin-treated microsomes. 3. Liver microsomes treated in vitro with R- or S-acenocoumarol could be reactivated by DTT (10 mM). Reactivation was higher for the R- than for the S-acenocoumarol-treated microsomes. 4. The microsomal vitamin K0 reductase activity under 'normal' assay conditions ([DTT] = 2 mM) was as sensitive for R- as for S-4-hydroxycoumarins. At elevated DTT concentrations (= 42 mM) the rate of vitamin K0 conversion was about 1.5 fold higher in the presence of the R-isomers than in the presence of the S-isomers. For instance, at 2 mM DDT the reductase activities in the presence of 2.6 microM R- and S-warfarin were about 15% of control. At 42 mM DTT the activities were 90 and 65% of control, respectively. 5. In the in vitro experiments acenocoumarol appeared to be more potent than warfarin and phenprocoumon. 6. The following mechanism is proposed: vitamin K0 reductase becomes oxidized during substrate reduction. The oxidized (i.e. inactive) form binds equally to the R- and S-enantiomers of 4- hydroxycoumarins. The attached (covalently bound?) coumarin is released by the reactivation (i.e. reduction) of the enzyme. However, the rate of reactivation is strongly attenuated by the attached coumarin. This effect is more pronounced for the S-configuration of the 4-hydroxycoumarin anticoagulants.

Tissue distribution and warfarin sensitivity of vitamin K epoxide reductase

The distribution of vitamin K epoxide reductase activity and its sensitivity to warfarin have been examined in whole microsomes from tissues of both control and warfarin-resistant strain rats. The distribution of activity roughly paralleled that previously shown for the vitamin K-dependent carboxylase. Activity on a per gram tissue basis was highest in kidney, adrenal, spleen, lung, testes, and epididymis at a level about 1/20th of that present in liver microsomes. Vitamin K quinone formation by microsomes from warfarin-resistant rats was approximately half that of control strain samples. In addition, hydroxy vitamin K was formed by warfarin-resistant strain microsomes to about the same extent as vitamin K quinone in all tissues. The Km values for dithiothreitol (DTT) and vitamin K epoxide were similar in all tissues (range = 0.1-0.2 mM DTT at 40 microM vitamin K epoxide, and 10-30 microM vitamin K epoxide at 2 mM DTT). The sensitivities to warfarin were similar for all control strain rat tissues (I50 = 10-20 microM at 2 mM DTT and 40 microM vitamin K epoxide) and similarly elevated for all warfarin-resistant rat tissues (I50 = 30 to greater than 80 microM). These results suggest that the identical enzyme is expressed in all tissues and that tissue specific isozymes do not occur.

Warfarin: metabolism and mode of action

The various stages involved in the transport, pharmacological action and elimination of warfarin involve the specific binding of warfarin to a chiral macromolecular complex. However, it seems that the degree of stereoselectivity is variable, which presumably reflects the importance of the side-chain in binding to each type of macromolecule. It would appear that there is greater stereoselective control in the interaction of warfarin with cytochrome P-450 enzymes than that observed for interaction with the receptor, vitamin K1 epoxide reductase. Indeed, warfarin has been developed as a powerful stereochemical probe for in vitro studies of the terminal enzyme in the mixed-function oxidase system, cytochrome P-450. Warfarin undergoes hydroxylation in the 6, 7 and 8-positions of the aromatic ring which must interact with the active (haemoprotein) portion of the molecule, leaving the side-chain, which contains the chiral centre, free for recognition by the substrate binding site. In vitro studies indicate that the interaction of warfarin at its receptor, vitamin K1 epoxide reductase, is completely non-stereoselective. This suggests that only the 4-hydroxycoumarin ring portion of the drug binds to the enzyme. Consistent with this hypothesis, salicylate, which can mimic part of the 4-hydroxycoumarin ring system, produces hypothrombinaemia by inhibition of vitamin K1 epoxide reductase. These findings suggest that the coumarin ring system is largely responsible for the pharmacodynamic properties of warfarin, whereas the side-chain dictates the disposition and metabolism of the drug.