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Vitamin K-Dependent Reactions in Rat Liver: Role of Flavoproteins¹

Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin, Madison, WI 53706

The role of flavins in vitamin K function was assessed by examining blood coagulation and in vitro activities of hepatic vitamin K-dependent enzymes from control and riboflavin-deficient rats. One-stage prothrombin times and Factor VII activities were lower in flavin-deficient rats than in ad libitum or pair-fed controls. Fibrinogen, prothrombin, and Factor X activities were normal. Hepatic vitamin K-dependent carboxylase activity was severely depressed in flavin-deficient rats when assayed with [vitamin K + NADH] and somewhat depressed with reduced vitamin K (vitamin KH₂) as substrate. One-hour flavin repletion appreciably restored [vitamin K + NADH]-dependent activity, but vitamin KH₂-dependent activity was not restored even after 16 hours repletion. These results suggest that the carboxylating enzyme itself is not a flavoprotein, but that the microsomal NADH dehydrogenase required for [vitamin K + NADH]-dependent carboxylation is a flavoprotein. This dehydrogenase may differ from the cytosolic Warfarin-inhibitable ‘DT-diaphorase’ in that the activity of the latter, which is reduced 50% in flavin-deficient rats, is not at all restored by one-hour flavin repletion. Flavin status-dependent differences in NADH-dependent or vitamin KH₂-dependent epoxidation of vitamin K paralleled differences in the carboxylase. Flavin deficiency had no effect on vitamin K 2,3-epoxide reductase activity nor on its inhibition by Warfarin.

KEY WORDS: • dehydrogenases • flavoproteins • riboflavin • vitamin K

¹ Supported in part by a grant from the National Institutes of Health (AM-14881).

² National Institutes of Health Postdoctoral Fellow (HL-06136) and National Institutes of Health Postdoctoral Trainee (DE-07031).

³ To whom correspondence should be addressed.

Manuscript received 12 March 1981.