We have used two hydroxylated naphthoquinol menaquinol analogues, reduced plumbagin (PBH2, 5-hydroxy-2-methyl-1,4-naphthoquinol) and reduced lapachol [LPCH2, 2-hydroxy-3-(3-methyl-2-butenyl)-1,4-naphthoquinol], as substrates for Escherichia colianaerobic reductases. These compounds have optical, solubility and redox properties that make them suitable for use in studies of the enzymology of menaquinol oxidation. Oxidized plumbagin and oxidized lapachol have well resolved absorbances at 419 nm (ε = 3.95 mM-1·cm-1) and 481 nm (ε = 2.66 mM-1·cm-1) respectively (in Mops/KOH buffer, pH 7.0). PBH2 is a good substrate for nitrate reductase A (Km = 282±28 µM, kcat = 120±6 s-1) and fumarate reductase (Km = 155±24 µM, kcat = 30±2 s-1), but not for DMSO reductase. LPCH2 is a good substrate for nitrate reductase A (Km = 57±35 µM, kcat = 68±13 s-1), fumarate reductase (Km = 85±27 µM, kcat = 74±6 s-1) and DMSO reductase (Km = 238±30 µM, kcat = 191±21 s-1). The sensitivity of enzymic LPCH2 and PBH2 oxidation to 2-n-heptyl-4-hydroxyquinoline N-oxide inhibition is consistent with their oxidation occurring at sites of physiological quinol binding.

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