title: Microsomal lipoamide reductase provides vitamin K epoxide reductase with reducing equivalents.
abstract: This study was undertaken to search for the endogenous dithiol cofactor of the reductases of the vitamin K cycle. As a starting point, the redox-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 the synthetic dithiol dithiothreitol (DTT). Reduced lipoic acid shows higher (4-fold) Km values. No reductase activity with lipoic acid was found to be present in microsomes or cytosol. The reduced-lipoamide-stimulated vitamin K epoxide reductase is as sensitive to warfarin and salicylate inhibition as is the DTT-stimulated one. Both vitamin K epoxide reductase and lipoamide reductase activity are recovered in the rough microsomes. NADH/lipoamide-stimulated vitamin K epoxide reduction is uncoupled by traces of Triton X-100, suggesting that microsomal lipoamide reductase and vitamin K epoxide reductase are associated. The results suggest that the vitamin K cycle obtains reducing equivalents from NADH through microsomal lipoamide reductase.
authors.affiliation: Department of Pharmacology, University of Limburg, Maastricht, The Netherlands.
authors.forename: H H
authors.initials: HH
authors.lastname: Thijssen
authors.forename: Y P
authors.initials: YP
authors.lastname: Janssen
authors.forename: L T
authors.initials: LT
authors.lastname: Vervoort
mesh_terms: D000818:Animals; D008058:Dihydrolipoamide Dehydrogenase / Q000378:metabolism*; D004229:Dithiothreitol / Q000378:metabolism; D008862:Microsomes, Liver / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009247:NADH, NADPH Oxidoreductases / Q000378:metabolism*; D051381:Rats; D012459:Salicylates / Q000494:pharmacology; D020156:Salicylic Acid; D008063:Thioctic Acid / Q000031:analogs & derivatives* / Q000378:metabolism*; D064417:Vitamin K Epoxide Reductases; D014859:Warfarin / Q000494:pharmacology
chemical_list: D012459:Salicylates; D014859:Warfarin; D008063:Thioctic Acid; C013091:lipoamide; D006899:Mixed Function Oxygenases; D064417:Vitamin K Epoxide Reductases; D009247:NADH, NADPH Oxidoreductases; D008058:Dihydrolipoamide Dehydrogenase; D020156:Salicylic Acid; D004229:Dithiothreitol