Co-purification of Microsomal Epoxide Hydrolase with the Warfarin-Sensitive Vitamin K1 Oxide Reductase of the Vitamin K Cycle

Vitamin K-1 oxide reductase activity has been partially purified from rat liver microsomes. A three-step procedure produced a preparation in which warfarin-sensitive vitamin K-1 oxide reductase activity was 118-fold enriched over the activity in intact rat liver microsomes. A major component of the multi-protein mixture was identified as a 50 kDa protein that strongly cross-reacts with antiserum prepared against homogeneous rat liver microsomal epoxide hydrolase. The reductase preparation also had a high level of epoxide hydrolase activity against two xenobiotic epoxide substrates. The K-m values for hydrolysis by the reductase preparation were similar to those for homogeneous microsomal epoxide hydrolase itself, and the specific hydrolase activities of the reductase preparation were 25-35% of the specific activities measured for the homogeneous hydrolase preparation. Antibodies prepared against homogeneous microsomal epoxide hydrolase inhibited up to 80% of reductase activity of the reductase preparation. Homogeneous microsomal epoxide hydrolase had no vitamin K-1 oxide reductase activity. This evidence suggests that microsomal epoxide hydrolase, or a protein that is very similar to it, is a major functional component of a multi-protein complex that is responsible for vitamin K-1 oxide reduction ih rat liver microsomes. (C) 1998 Elsevier Science Inc.