Reactions of NADH Oxidation by Tetrazolium and Ubiquinone Catalyzed by Yeast Alcohol Dehydrogenase

Abstract

The processes of NADH oxidation by p-NTF violet and ubiquinone catalyzed by isolated yeast alcohol dehydrogenase in aqueous and water-alcohol buffer solutions were studied. In the presence of p-NTF in aqueous solution at a pH of 6–7, NADH oxidation was extremely slow due to inhibition of the enzyme by the remaining enzyme-bound hydrophobic product, formazan, which forms during the reduction of p-Nitrotetrazolium. However, when the medium was alkalinized to a pH of 8–9 or when alcohol (ethanol or isopropanol) was added, formazan was desorbed from the enzyme, leading to an increase in the NADH oxidation rate. It was assumed that this redox reaction can be used as the basis for colorimetric measurement of the activity of different alcohol dehydrogenases. Tetrazolium reduction by alcohol was not observed at any value within the entire pH range. NADH oxidation in the presence of the enzyme and ubiquinone was also slow, even with the addition of alcohol, but its rate increased when the medium was acidified to a pH of 5.5–6. When a Tris-phosphate buffer was replaced with HEPES, a quasi-vibrational process was observed: NADH oxidization with ubiquinone to NAD⁺ and its subsequent reverse recovery with alcohol to NADH.