Vitamin‐K‐dependent proteins in microsomes of primary lewis lung tumors

Abstract Microsomes isolated from Lewis lung (LL) primary tumors raised in C57BL/6 mice have been shown to (i) contain a 4‐hydroxycoumarin (warfarin)‐sensitive cycle of vitamin K metabolism which is at least qualitatively similar to that of liver, and (ii) catalyze the incorporation of NaH 14 CO 3 into endogenous protein in a vitamin‐K hydroquinone‐dependent reaction to produce γ‐carboxyglutamate. As in liver microsomes, LL microsomal reduction of vitamin K 2,3‐epoxide to vitamin K was greatly enhanced by exogenous dithiols such as dithiothreitol, but under identical conditions the former was 10‐fold faster. The R(+) and S(−) warfarin enantiomers were highly and equally effective inhibitors of both the liver and tumor vitamin K 2,3‐epoxide reductases—the average I 50 against the tumor enzyme was 0.25 μ M . Partially purified reductases isolated by centrifugation of sodium‐cholate‐treated liver and LL tumor microsomes over a discontinuous sucrose gradient were also inhibited by the sulfhydryl reagent N‐ethylmaleimide following their reduction by dithiothreitol. Like the activity of the epoxide reductase, that of the γ‐carboxylase was much lower in tumor than in liver microsomes and was only detectable in microsomes isolated from tumor‐bearing mice previously administered S(−) warfarin. In view of the reported inhibition of LL tumor metastasis by warfarin and diet‐induced vitamin‐K deficiency, vitamin‐K‐dependent proteins may play a role in the spread and/or subsequent growth of LL cells.