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Kinetic Characterization of Reduced Pyridine Nucleotide Dehydrogenases (Duroquinone-Dependent) in Cucurbita Microsomes ¹

Istituto Botanico, Universita' di Bologna, Italy, Biologisches Institut III, Universität Freiburg i.Br., Germany

Some properties of microsomal electron transfer chains, dependent for oxidase activity on addition of NADH or NADPH, duroquinone, and oxygen (L. De Luca et al., 1984, Plant Sci Lett 36: 93-98) are described. Activity is characterized by negatively cooperative kinetics toward reduced pyridine nucleotides, with limiting K_m of 10 to 50 micromolar at pH 7.0 (increasing at lower pH), as well as toward duroquinone with limiting K_m of 100 to 400 micromolar regardless of pH. Molecular oxygen is reduced by the enzyme complex with S_0.5 of about 30 micromolar and production of H₂O and H₂O₂, without superoxide involvement. The ratio NAD(P)H:O₂ averages 1.35 in the presence of KCN and 1.85 in its absence. The pyridine nucleotide specificity of the dehydrogenases has been investigated by kinetic competition experiments. Some enzyme heterogeneity was established for all preparations. At least two enzymes are detectable in plasma membrane-enriched fractions: a major NAD(P)H dehydrogenase having an acid pH optimum, and an NADPH dehydrogenase active around neutrality. Addition of Triton X-100 strongly enhances the activity over most of the pH scale, but depresses it increasingly at pH values higher than 8.0, to the effect that pH profile shows, under these conditions, a major peak at about pH 5.8 for both NADH and NADPH oxidase. Results with endoplasmic reticulum preparations are similar, except that they suggest the presence of still more activities at and above pH 7. The results are interpreted in terms of different complexes catalyzing electron transfer from NAD(P)H to O₂ without release of intermediates.