MICROSOMAL LIPOAMIDE REDUCTASE PROVIDES VITAMIN-K EPOXIDE REDUCTASE WITH REDUCING EQUIVALENTS

This study was undertaken to search for the endogenous dithiol cofacto r of the reductases of the vitamin K cycle. As a starting point, the r edox-active lipophilic endogenous compounds lipoic acid and lipoamide were looked at. The study shows that microsomes contain NADH-dependent lipoamide reductase activity. Reduced lipoamide stimulates microsomal vitamin K epoxide reduction with kinetics comparable with those for t he synthetic dithiol dithiothreitol (DTT). Reduced lipoic acid shows h igher (4-fold) K-m values. No reductase activity with lipoic acid was found to be present in microsomes or cytosol. The reduced-lipoamide-st imulated vitamin K epoxide reductase is as sensitive to warfarin and s alicylate inhibition as is the DTT-stimulated one. Both vitamin K epox ide reductase and lipoamide reductase activity are recovered in the ro ugh microsomes. NADH/lipoamide-stimulated vitamin K epoxide reduction is uncoupled by traces of Triton X-100, suggesting that microsomal lip oamide reductase and vitamin K epoxide reductase are associated. The r esults suggest that the vitamin K cycle obtains reducing equivalents f rom NADH through microsomal lipoamide reductase.