Decyl-ubiquinone and decyl-plastoquinone were used as model compounds to test the potential effect of quinone derivatives on two enzymes of the vitamin K cycle in vitro. Substantial inhibition of γ-glutamate carboxylase was found, whereas vitamin K-epoxide reductase was inhibited to a much lesser extent. The inhibitory effect of both decylquinones was eliminated in a time-dependent way by solubilized microsomes, but not by purified carboxylase. Since a wide variety of prenylquinones occur as micronutrients, these results are of potential relevance for the effects of natural quinones in the human diet.