It has been proposed that the same enzyme is responsible
for reduction of the quinone form as well as the epoxide form
of the vitamin (5), but no firm evidence has been presented to
support this hypothesis.
Vitamin K epoxide reductase is highly sensitive to inhibition
by coumarin anticoagulant drugs and is believed to be the target
for their anticoagulant effect (1, 2). We have shown in rat liver
that, in addition to the reductase, there are pyridine nucleotidedependent
dehydrogenases that also can reduce vitamin K quinone
(6). These enzymes are far less sensitive to coumarin drugs
and constitute an important pathway for vitamin K reduction
in cases of coumarin intoxication (7).
In order to understand the mechanism of anticoagulation
by coumarins, the system described above has been extensively
studied in various in vitro systems from rat liver (1, 2). Since
these drugs are used so frequently in patients, we decided to
study the system in human liver. Our goal was to determine
how closely the enzyme system in human liver corresponded to
the model established for the action of vitamin K and coumarins
in rat liver.