Menadione reductase from Clostridium tyrobutyricum

Menadione reductase (EC 1.6.99.2) has been purified 67-fold from Clostridium tyrobutyricum extracts. The molecular weight was found to be 60 000 and the prosthetic group was identified as FMN on the basis of the enzymatic analysis data. The binding of FMN to the menadione dehydrogenase apoenzyme was relatively weak with an apparent Km value of 2.5 x 10(-6) M. The enzyme exhibited group substrate specificity for compounds with a quinoid structure; naphthoquinones and benzoquinones without long carbon chain substituents were the most active. The reactivity of the enzyme with vitamin K1, coenzyme Q6, and cytochrome c was negligible and, with 2,6-dichlorophenol indophenol, relatively low. It was shown that the enzymatic reduction of menadione with the participation of either NADH or NADPH takes place by a "Ping-Pong" mechanism. The enzyme catalyzed the oxidation of NADH and NADPH at equal rates and was inhibited by dicumarol and p-chloromercuribenzoate.