B16 tumor cells contain a warfarin sensitive vitamin K1 2,3 epoxide reductase

Free radical generating mechanisms in the colon: their role in the induction and promotion of colorectal cancer?

A hypothesis is presented to account for the dietary induction and promotion of colorectal cancer. The principal agents are the secondary bile acids, lithocholic and deoxycholic acids, the vitamin K group and ferrous iron complexes. These metabolites may interact to subvert the normal free radical generating mechanisms involved in mucosal defence. Diets high in fat and red meat and low in fibre support a Bacteroides-dominated colonic microflora, which both synthesis and utilises vitamin K2 isoprenalogues or menaquinones as enzyme co-factors. Iron(II) complexes such as haemin from the breakdown of dietary haemoglobin and myoglobin also serve as growth factors for these bacteria and provide a rich source of haem-iron for intestinal uptake. Biliary secretion is stimulated by dietary fat and bile acids are essential for the intestinal uptake of vitamin K and possibly of iron complexes such as haemin. In the mature colonocyte, vitamin K and haemin may initiate redox cycling reactions which liberate superoxide (O2-.). Bile acids can activate the membrane bound phospholipase to liberate arachidonate and diacylglycerol. This leads in turn to the production of more O2-. which can enter the microcirculation and acts as a potent chemoattractant for the neutrophils that line the lamina propria. The released diacylglycerol can activate protein kinase C in the neutrophil membrane to switch on the respiratory burst oxidase system generating yet more O2-. and may stimulate the proliferation of transformed stem cells by a similar protein kinase C mediated mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

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.